Fusion power is a theoretical form of power
generation in which energy will be generated
by using nuclear fusion reactions to produce
heat for electricity generation. In a fusion
process, two lighter atomic nuclei combine
to form a heavier nucleus, and at the same
time, they release energy. This is the same
process that powers stars like our Sun. Devices
designed to harness this energy are known
as fusion reactors.
Fusion processes require fuel and a highly
confined environment with a high temperature
and pressure, to create a plasma in which
fusion can occur. In stars, the most common
fuel is hydrogen, and gravity creates the
high temperature and confinement needed for
fusion. Fusion reactors generally use hydrogen
isotopes such as deuterium and tritium, which
react more easily, and create a confined plasma
of millions of degrees using inertial methods
(laser) or magnetic methods (tokamak and similar),
although many other concepts have been attempted.
The major challenges in realising fusion power
are to engineer a system that can confine
the plasma long enough at high enough temperature
and density, for a long term reaction to occur,
and for the most common reactions, managing
neutrons that are released during the reaction,
which over time can degrade many common materials
used within the reaction chamber.
As a source of power, nuclear fusion is expected
to have several theoretical advantages over
fission. These include reduced radioactivity
in operation and little nuclear waste, ample
fuel supplies, and increased safety. However,
controlled fusion has proven to be extremely
difficult to produce in a practical and economical
manner. Research into fusion reactors began
in the 1940s, but to date, no design has produced
more fusion power output than the electrical
power input; therefore, all existing designs
have had a negative power balance.Over the
years, fusion researchers have investigated
various confinement concepts. The early emphasis
was on three main systems: z-pinch, stellarator
and magnetic mirror. The current leading designs
are the tokamak and inertial confinement (ICF)
by laser. Both designs are being built at
very large scales, most notably the ITER tokamak
in France, and the National Ignition Facility
laser in the United States. Researchers are
also studying other designs that may offer
cheaper approaches. Among these alternatives
there is increasing interest in magnetized
target fusion and inertial electrostatic confinement.
== Background ==
=== Mechanism ===
Fusion reactions occur when two or more atomic
nuclei come close enough for long enough that
the nuclear force pulling them together exceeds
the electrostatic force pushing them apart,
fusing them into heavier nuclei. For nuclei
lighter than iron-56, the reaction is exothermic,
releasing energy. For nuclei heavier than
iron-56, the reaction is endothermic, requiring
an external source of energy. Hence, nuclei
smaller than iron-56 are more likely to fuse
while those heavier than iron-56 are more
likely to break apart.
The strong force acts only over short distances.
The repulsive electrostatic force acts over
longer distances. In order to undergo fusion,
the fuel atoms need to be given enough energy
to approach each other close enough for the
strong force to become active. The amount
of kinetic energy needed to bring the fuel
atoms close enough is known as the "Coulomb
barrier". Ways of providing this energy include
speeding up atoms in a particle accelerator,
or heating them to high temperatures.
Once an atom is heated above its ionization
energy, its electrons are stripped away (it
is ionized), leaving just the bare nucleus
(the ion). The result is a hot cloud of ions
and the electrons formerly attached to them.
This cloud is known as plasma. Because the
charges are separated, plasmas are electrically
conductive and magnetically controllable.
Many fusion devices take advantage of this
to control the particles as they are heated.
=== Cross Section ===
A reaction's cross section, denoted σ, is
the measure of the probability that a fusion
reaction will happen. This depends on the
relative velocity of the two nuclei. Higher
relative velocities generally increase the
probability, but the probability begins to
decrease again at very high energies. Cross
sections for many fusion reactions were measured
(mainly in the 1970s) using particle beams.In
a plasma, particle velocity can be characterized
using a probability distribution. If the plasma
is thermalized, the distribution looks like
a bell curve, or maxwellian distribution.
In this case, it is useful to use the average
particle cross section over the velocity distribution.
This is entered into the volumetric fusion
rate:
P
fusion
=
n
A
n
B
⟨
σ
v
A
,
B
⟩
E
fusion
{\displaystyle P_{\text{fusion}}=n_{A}n_{B}\langle
\sigma v_{A,B}\rangle E_{\text{fusion}}}
where:
P
fusion
{\displaystyle P_{\text{fusion}}}
is the energy made by fusion, per time and
volume
n is the number density of species A or B,
of the particles in the volume
⟨
σ
v
A
,
B
⟩
{\displaystyle \langle \sigma v_{A,B}\rangle
}
is the cross section of that reaction, average
over all the velocities of the two species
v
E
fusion
{\displaystyle E_{\text{fusion}}}
is the energy released by that fusion reaction.
=== Lawson Criterion ===
The Lawson Criterion shows how energy output
varies with temperature, density, speed of
collision, and fuel. This equation was central
to John Lawson's analysis of fusion working
with a hot plasma. Lawson assumed an energy
balance, shown below.
P
out
=
η
capture
(
P
fusion
−
P
conduction
−
P
radiation
)
{\displaystyle P_{\text{out}}=\eta _{\text{capture}}\left(P_{\text{fusion}}-P_{\text{conduction}}-P_{\text{radiation}}\right)}
η, efficiency
P
conduction
{\displaystyle P_{\text{conduction}}}
, conduction losses as energy laden mass leaves
P
radiation
{\displaystyle P_{\text{radiation}}}
, radiation losses as energy leaves as light
P
out
{\displaystyle P_{\text{out}}}
, net power from fusion
P
fusion
{\displaystyle P_{\text{fusion}}}
, is rate of energy generated by the fusion
reactions.Plasma clouds lose energy through
conduction and radiation. Conduction occurs
when ions, electrons or neutrals impact other
substances, typically a surface of the device,
and transfer a portion of their kinetic energy
to the other atoms. Radiation is energy that
leaves the cloud as light in the visible,
UV, IR, or X-ray spectra. Radiation increases
with temperature. Fusion power technologies
must overcome these losses.
=== Triple product: density, temperature,
time ===
The Lawson criterion argues that a machine
holding a thermalized and quasi-neutral plasma
has to meet basic criteria to overcome radiation
losses, conduction losses and reach efficiency
of 30 percent. This became known as the "triple
product": the plasma density, temperature
and confinement time. Attempts to increase
the triple product led to targeting larger
plants. Larger plants move structural materials
further away from the centre of the plasma,
which reduces conduction and radiation losses
since more of the radiation is internally
reflected. This emphasis on
(
n
T
τ
)
{\displaystyle (nT\tau )}
as a metric of success has impacted other
considerations such as cost, size, complexity
and efficiency. This has led to larger, more
complicated and more expensive machines such
as ITER and NIF.
=== Plasma behavior ===
Plasma is an ionized gas that conducts electricity.
In bulk, it is modeled using magnetohydrodynamics,
which is a combination of the Navier-Stokes
equations governing fluids and Maxwell's equations
governing how magnetic and electric fields
behave. Fusion exploits several plasma properties,
including:
Self-organizing plasma conducts electric and
magnetic fields. Its motions can generate
fields that can in turn contain it.
Diamagnetic plasma can generate its own internal
magnetic field. This can reject an externally
applied magnetic field, making it diamagnetic.
Magnetic mirrors can reflect plasma when it
moves from a low to high density field.
=== Energy capture ===
Multiple approaches have been proposed for
energy capture. The simplest is to heat a
fluid. Most designs concentrate on the D-T
reaction, which releases much of its energy
in a neutron. Electrically neutral, the neutron
escapes the confinement. In most such designs,
it is ultimately captured in a thick "blanket"
of lithium surrounding the reactor core. When
struck by a high-energy neutron, the lithium
can produce tritium, which is then fed back
into the reactor. The energy of this reaction
also heats the blanket, which is then actively
cooled with a working fluid and then that
fluid is used to drive conventional turbomachinery.
It has also been proposed to use the neutrons
to breed additional fission fuel in a blanket
of nuclear waste, a concept known as a fission-fusion
hybrid. In these systems, the power output
is enhanced by the fission events, and power
is extracted using systems like those in conventional
fission reactors.Designs that use other fuels,
notably the p-B reaction, release much more
of their energy in the form of charged particles.
In these cases, alternate power extraction
systems based on the movement of these charges
are possible. Direct energy conversion was
developed at LLNL in the 1980s as a method
to maintain a voltage using the fusion reaction
products. This has demonstrated energy capture
efficiency of 48 percent.
== Approaches ==
=== 
Magnetic confinement ===
Tokamak: the most well-developed and well-funded
approach to fusion energy. This method races
hot plasma around in a magnetically confined,
donut-shaped ring, with an internal current.
When completed, ITER will be the world's largest
tokamak. As of April 2012 an estimated 215
experimental tokamaks were either planned,
decommissioned or currently operating (35)
worldwide.
Spherical tokamak: also known as spherical
torus. A variation on the tokamak with a spherical
shape.
Stellarator: Twisted rings of hot plasma.
The stellarator attempts to create a natural
twisted plasma path, using external magnets,
while tokamaks create those magnetic fields
using an internal current. Stellarators were
developed by Lyman Spitzer in 1950 and have
four designs: Torsatron, Heliotron, Heliac
and Helias. One example is Wendelstein 7-X,
a German fusion device that produced its first
plasma on December 10, 2015. It is the world's
largest stellarator, designed to investigate
the suitability of this type of device for
a power station.
Levitated Dipole Experiment (LDX): These use
a solid superconducting torus. This is magnetically
levitated inside the reactor chamber. The
superconductor forms an axisymmetric magnetic
field that contains the plasma. The LDX was
developed by MIT and Columbia University after
2000 by Jay Kesner and Michael E. Mauel.
Magnetic mirror: Developed by Richard F. Post
and teams at LLNL in the 1960s. Magnetic mirrors
reflected hot plasma back and forth in a line.
Variations included the Tandem Mirror, magnetic
bottle and the biconic cusp. A series of well-funded,
large, mirror machines were built by the US
government in the 1970s and 1980s, principally
at Lawrence Livermore National Laboratory.
Bumpy torus: A number of magnetic mirrors
are arranged end-to-end in a toroidal ring.
Any fuel ions that leak out of one are confined
in a neighboring mirror, permitting the plasma
pressure to be raised arbitrarily high without
loss. An experimental facility, the ELMO Bumpy
Torus or EBT was built and tested at Oak Ridge
National Laboratory in the 1970s.
Field-reversed configuration: This device
traps plasma in a self-organized quasi-stable
structure; where the particle motion makes
an internal magnetic field which then traps
itself.
Spheromak: Very similar to a field reversed
configuration, a semi-stable plasma structure
made by using the plasmas' own self-generated
magnetic field. A spheromak has both a toroidal
and poloidal fields, while a Field Reversed
Configuration only has no toroidal field.
Reversed field pinch: Here the plasma moves
inside a ring. It has an internal magnetic
field. Moving out from the center of this
ring, the magnetic field reverses direction.
=== Inertial confinement ===
Direct drive: In this technique, lasers directly
blast a pellet of fuel. The goal is to ignite
a fusion chain reaction. Ignition was first
suggested by John Nuckolls, in 1972. Notable
direct drive experiments have been conducted
at the Laboratory for Laser Energetics, Laser
Mégajoule and the GEKKO XII facilities. Good
implosions require fuel pellets with close
to a perfect shape in order to generate a
symmetrical inward shock wave that produces
the high-density plasma.
Fast ignition: This method uses two laser
blasts. The first blast compresses the fusion
fuel, while the second high energy pulse ignites
it. Experiments have been conducted at the
Laboratory for Laser Energetics using the
Omega and Omega EP systems and at the GEKKO
XII laser at the Institute for Laser Engineering
in Osaka Japan.
Indirect drive: In this technique, lasers
blasts a structure around the pellet of fuel.
This structure is known as a Hohlraum. As
it disintegrates the pellet is bathed in a
more uniform x-ray light, creating better
compression. The largest system using this
method is the National Ignition Facility.
Magneto-inertial fusion or Magnetized Liner
Inertial Fusion: This combines a laser pulse
with a magnetic pinch. The pinch community
refers to it as magnetized liner Inertial
fusion while the ICF community refers to it
as magneto-inertial fusion.
Heavy Ion Beams There are also proposals to
do inertial confinement fusion with ion beams
instead of laser beams. The main difference
is the mass of the beam has momentum, whereas
lasers do not.
=== Magnetic or electric pinches ===
Z-Pinch: This method sends a strong current
(in the z-direction) through the plasma. The
current generates a magnetic field that squeezes
the plasma to fusion conditions. Pinches were
the first method for man-made controlled fusion.
Some examples include the Dense plasma focus
(DPF) and the Z machine at Sandia National
Laboratories. In DPF the focus consists of
two coaxial cylindrical electrodes made from
copper or beryllium and housed in a vacuum
chamber containing a low-pressure fusible
gas. An electrical pulse is applied across
the electrodes, heating the gas into a plasma.
The current forms into a minuscule vortex
along the axis of the machine, which then
kinks into a cage of current with an associated
magnetic field. The cage of current and magnetic-field-entrapped
plasma is called a plasmoid. The acceleration
of the electrons about the magnetic field
lines heats the nuclei within the plasmoid
to fusion temperatures.
Theta-Pinch: This method sends a current inside
a plasma, in the theta direction.
Screw Pinch: This method combines a theta
and z-pinch for improved stabilization.
=== Inertial electrostatic confinement ===
Fusor: This method uses an electric field
to heat ions to fusion conditions. The machine
typically uses two spherical cages, a cathode
inside the anode, inside a vacuum. These machines
are not considered a viable approach to net
power because of their high conduction and
radiation losses. They are simple enough to
build that amateurs have fused atoms using
them.
Polywell: This design attempts to combine
magnetic confinement with electrostatic fields,
to avoid the conduction losses generated by
the cage.
=== Other ===
Magnetized target fusion: This method confines
hot plasma using a magnetic field and squeezes
it using inertia. Examples include LANL FRX-L
machine, General Fusion and the plasma liner
experiment.
Cluster Impact Fusion Microscopic droplets
of heavy water are accelerated at great velocity
into a target or into one another. Researchers
at Brookhaven reported positive results which
were later refuted by further experimentation.
Fusion effects were actually produced because
of contamination of the droplets.
Uncontrolled: Fusion has been initiated by
man, using uncontrolled fission explosions
to ignite so-called Hydrogen Bombs. Early
proposals for fusion power included using
bombs to initiate reactions.
Beam fusion: A beam of high energy particles
can be fired at another beam or target and
fusion will occur. This was used in the 1970s
and 1980s to study the cross sections of high
energy fusion reactions.
Bubble fusion: This was a fusion reaction
that was supposed to occur inside extraordinarily
large collapsing gas bubbles, created during
acoustic liquid cavitation. This approach
was discredited.
Cold fusion: This is a hypothetical type of
nuclear reaction that would occur at, or near,
room temperature. Cold fusion is discredited
and gained a reputation as pathological science.
Muon-catalyzed fusion: This approach replaces
electrons in the plasma by muons - far more
massive particles with the same electric charge.
Their greater mass allows nuclei to get much
closer and collide more easily, so it greatly
reduces the kinetic energy (heat and pressure)
required to initiate fusion. A problem is
that muons require more energy to produce
than can be obtained from muon-catalysed fusion,
making this approach impractical for power
generation.
Space-Based Solar Power argues that a majority
of available fusion fuels exists within the
sphere of the Sun where it is gravitationally
confined, and that a tractable way to accomplish
large-scale fusion power is to build very
large space-borne platforms that capture energy
via photons rather than via a carnot cycle.
The theoretical limit of producing power by
such means is a type-2 civilization using
a Dyson Sphere.
== Common tools ==
=== 
Heating ===
Gas is heated to form a plasma hot enough
to start fusion reactions. A number of heating
schemes have been explored:
Radiofrequency Heating A radio wave is applied
to the plasma, causing it to oscillate. This
is basically the same concept as a microwave
oven. This is also known as electron cyclotron
resonance heating or Dielectric heating.Electrostatic
Heating An electric field can do work on charged
ions or electrons, heating them.Neutral Beam
Injection An external source of hydrogen is
ionized and accelerated by an electric field
to form a charged beam which is shone through
a source of neutral hydrogen gas towards the
plasma which itself is ionized and contained
in the reactor by a magnetic field. Some of
the intermediate hydrogen gas is accelerated
towards the plasma by collisions with the
charged beam while remaining neutral: this
neutral beam is thus unaffected by the magnetic
field and so shines through it into the plasma.
Once inside the plasma the neutral beam transmits
energy to the plasma by collisions as a result
of which it becomes ionized and thus contained
by the magnetic field thereby both heating
and refuelling the reactor in one operation.
The remainder of the charged beam is diverted
by magnetic fields onto cooled beam dumps.
Antiproton annihilation Theoretically a quantity
of antiprotons injected into a mass of fusion
fuel can induce thermonuclear reactions. This
possibility as a method of spacecraft propulsion,
known as Antimatter-catalyzed nuclear pulse
propulsion, was investigated at Pennsylvania
State University in connection with the proposed
AIMStar project.
Magnetic Oscillations
=== Measurement ===
Thomson Scattering Light scatters from plasma.
This light can be detected and used to reconstruct
the plasmas' behavior. This technique can
be used to find its density and temperature.
It is common in Inertial confinement fusion,
Tokamaks and fusors. In ICF systems, this
can be done by firing a second beam into a
gold foil adjacent to the target. This makes
x-rays that scatter or traverse the plasma.
In Tokamaks, this can be done using mirrors
and detectors to reflect light across a plane
(two dimensions) or in a line (one dimension).
Langmuir probe This is a metal object placed
in a plasma. A potential is applied to it,
giving it a positive or negative voltage against
the surrounding plasma. The metal collects
charged particles, drawing a current. As the
voltage changes, the current changes. This
makes a IV Curve. The IV-curve can be used
to determine the local plasma density, potential
and temperature.Neutron detectors Deuterium
or tritium fusion produces neutrons. Neutrons
interact with surrounding matter in ways that
can be detected. Several types of neutron
detectors exist which can record the rate
at which neutrons are produced during fusion
reactions. They are an essential tool for
demonstrating success.
Flux loop A loop of wire is inserted into
the magnetic field. As the field passes through
the loop, a current is made. The current is
measured and used to find the total magnetic
flux through that loop. This has been used
on the National Compact Stellarator Experiment,
the polywell and the LDX machines.
X-ray detector All plasma loses energy by
emitting light. This covers the whole spectrum:
visible, IR, UV, and X-rays. This occurs anytime
a particle changes speed, for any reason.
If the reason is deflection by a magnetic
field, the radiation is Cyclotron radiation
at low speeds and Synchrotron radiation at
high speeds. If the reason is deflection by
another particle, plasma radiates X-rays,
known as Bremsstrahlung radiation. X-rays
are termed in both hard and soft, based on
their energy.
=== Power production ===
It has been proposed that steam turbines be
used to convert the heat from the fusion chamber
into electricity. The heat is transferred
into a working fluid that turns into steam,
driving electric generators.
Neutron blankets Deuterium and tritium fusion
generates neutrons. This varies by technique
(NIF has a record of 3E14 neutrons per second
while a typical fusor produces 1E5–1E9 neutrons
per second). It has been proposed to use these
neutrons as a way to regenerate spent fission
fuel or as a way to breed tritium using a
breeder blanket consisting of liquid lithium
or, as in more recent reactor designs, a helium
cooled pebble bed consisting of lithium bearing
ceramic pebbles fabricated from materials
such as Lithium titanate, lithium orthosilicate
or mixtures of these phases.Direct conversion
This is a method where the kinetic energy
of a particle is converted into voltage. It
was first suggested by Richard F. Post in
conjunction with magnetic mirrors, in the
late sixties. It has also been suggested for
Field-Reversed Configurations. The process
takes the plasma, expands it, and converts
a large fraction of the random energy of the
fusion products into directed motion. The
particles are then collected on electrodes
at various large electrical potentials. This
method has demonstrated an experimental efficiency
of 48 percent.
== Records ==
Fusion records have been set by a number of
devices. Here are some:
=== Q ===
The ratio of energy extracted against the
amount of energy supplied. This record is
considered to be set by the Joint European
Torus (JET) in 1997 when the device extracted
16 MW of power. However, this ratio can be
seen three different ways.
0.69 is the actual point in time ratio between
”fusion power” and actual input power
in the plasma (23 MW).
0.069 is the ratio between the “fusion”
power and the power required to produce the
23MW input power (essentially it takes into
account the efficiency of the NB system).
0.0069 is the ratio between the “fusion”
power and the total peak power required for
a JET pulse. This takes into account all the
power from the grid plus the one from the
two large JET flywheel generators.
=== Runtime ===
In Field Reversed Configurations, the longest
run time is 300 ms, set by the Princeton Field
Reversed Configuration in August 2016. However
this involved no fusion.
=== Beta ===
The fusion power trends as the plasma confinement
raised to the fourth power. Hence, getting
a strong plasma trap is of real value to a
fusion power plant. Plasma has a very good
electrical conductivity. This opens the possibility
of confining the plasma with magnetic field,
generally known as magnetic confinement. The
field puts a magnetic pressure on the plasma,
which holds it in. A widely used measure of
magnetic trapping in fusion is the beta ratio:
β
=
p
p
m
a
g
=
n
k
B
T
(
B
2
/
2
μ
0
)
{\displaystyle \beta ={\frac {p}{p_{mag}}}={\frac
{nk_{B}T}{(B^{2}/2\mu _{0})}}}
This is the ratio of the externally applied
field to the internal pressure of the plasma.
A value of 1 is ideal trapping. Some examples
of beta values include:
The START machine: 0.32
The Levitated dipole experiment: 0.26
Spheromaks: ≈ 0.1, Maximum 0.2 based on
Mercier limit.
The DIII-D machine: 0.126
The Gas Dynamic Trap a magnetic mirror: 0.6
for 5E-3 seconds.
The Sustained Spheromak Plasma Experiment
at Los Alamos National labs < 0.05 for 4E-6
seconds.
== Confinement ==
Confinement refers to all the conditions necessary
to keep a plasma dense and hot long enough
to undergo fusion. Here are some general principles.
Equilibrium: The forces acting on the plasma
must be balanced for containment. One exception
is inertial confinement, where the relevant
physics must occur faster than the disassembly
time.
Stability: The plasma must be so constructed
so that disturbances will not lead to the
plasma disassembling.
Transport or conduction: The loss of material
must be sufficiently slow. The plasma carries
off energy with it, so rapid loss of material
will disrupt any machines power balance. Material
can be lost by transport into different regions
or conduction through a solid or liquid.To
produce self-sustaining fusion, the energy
released by the reaction (or at least a fraction
of it) must be used to heat new reactant nuclei
and keep them hot long enough that they also
undergo fusion reactions.
=== Unconfined ===
The first human-made, large-scale fusion reaction
was the test of the hydrogen bomb, Ivy Mike,
in 1952. As part of the PACER project, it
was once proposed to use hydrogen bombs as
a source of power by detonating them in underground
caverns and then generating electricity from
the heat produced, but such a power station
is unlikely ever to be constructed.
=== Magnetic confinement ===
Magnetic Mirror One example of magnetic confinement
is with the magnetic mirror effect. If a particle
follows the field line and enters a region
of higher field strength, the particles can
be reflected. There are several devices that
try to use this effect. The most famous was
the magnetic mirror machines, which was a
series of large, expensive devices built at
the Lawrence Livermore National Laboratory
from the 1960s to mid 1980s. Some other examples
include the magnetic bottles and Biconic cusp.
Because the mirror machines were straight,
they had some advantages over a ring shape.
First, mirrors were easier to construct and
maintain and second direct conversion energy
capture, was easier to implement. As the confinement
achieved in experiments was poor, this approach
was abandoned.Magnetic Loops Another example
of magnetic confinement is to bend the field
lines back on themselves, either in circles
or more commonly in nested toroidal surfaces.
The most highly developed system of this type
is the tokamak, with the stellarator being
next most advanced, followed by the Reversed
field pinch. Compact toroids, especially the
Field-Reversed Configuration and the spheromak,
attempt to combine the advantages of toroidal
magnetic surfaces with those of a simply connected
(non-toroidal) machine, resulting in a mechanically
simpler and smaller confinement area.
=== Inertial confinement ===
Inertial confinement is the use of rapidly
imploding shell to heat and confine plasma.
The shell is imploded using a direct laser
blast (direct drive) or a secondary x-ray
blast (indirect drive) or heavy ion beams.
Theoretically, fusion using lasers would be
done using tiny pellets of fuel that explode
several times a second. To induce the explosion,
the pellet must be compressed to about 30
times solid density with energetic beams.
If direct drive is used—the beams are focused
directly on the pellet—it can in principle
be very efficient, but in practice is difficult
to obtain the needed uniformity. The alternative
approach, indirect drive, uses beams to heat
a shell, and then the shell radiates x-rays,
which then implode the pellet. The beams are
commonly laser beams, but heavy and light
ion beams and electron beams have all been
investigated.
=== Electrostatic confinement ===
There are also electrostatic confinement fusion
devices. These devices confine ions using
electrostatic fields. The best known is the
Fusor. This device has a cathode inside an
anode wire cage. Positive ions fly towards
the negative inner cage, and are heated by
the electric field in the process. If they
miss the inner cage they can collide and fuse.
Ions typically hit the cathode, however, creating
prohibitory high conduction losses. Also,
fusion rates in fusors are very low because
of competing physical effects, such as energy
loss in the form of light radiation. Designs
have been proposed to avoid the problems associated
with the cage, by generating the field using
a non-neutral cloud. These include a plasma
oscillating device, a magnetically-shielded-grid,
a penning trap, the polywell and the F1 cathode
driver concept. The technology is relatively
immature, however, and many scientific and
engineering questions remain.
== History of research ==
=== 1920s ===
Research into nuclear fusion started in the
early part of the 20th century. In 1920 the
British physicist Francis William Aston discovered
that the total mass equivalent of four hydrogen
atoms (two protons and two neutrons) are heavier
than the total mass of one helium atom (He-4),
which implied that net energy can be released
by combining hydrogen atoms together to form
helium, and provided the first hints of a
mechanism by which stars could produce energy
in the quantities being measured. Through
the 1920s, Arthur Stanley Eddington became
a major proponent of the proton–proton chain
reaction (PP reaction) as the primary system
running the Sun.
=== 1930s ===
Neutrons from fusion was first detected by
staff members of Ernest Rutherfords' at the
University of Cambridge, in 1933. The experiment
was developed by Mark Oliphant and involved
the acceleration of protons towards a target
at energies of up to 600,000 electron volts.
In 1933, the Cavendish Laboratory received
a gift from the American physical chemist
Gilbert N. Lewis of a few drops of heavy water.
The accelerator was used to fire heavy hydrogen
nuclei deuterons at various targets. Working
with Rutherford and others, Oliphant discovered
the nuclei of Helium-3 (helions) and tritium
(tritons).A theory was verified by Hans Bethe
in 1939 showing that beta decay and quantum
tunneling in the Sun's core might convert
one of the protons into a neutron and thereby
producing deuterium rather than a diproton.
The deuterium would then fuse through other
reactions to further increase the energy output.
For this work, Bethe won the Nobel Prize in
Physics.
=== 1940s ===
The first patent related to a fusion reactor
was registered in 1946 by the United Kingdom
Atomic Energy Authority. The inventors were
Sir George Paget Thomson and Moses Blackman.
This was the first detailed examination of
the Z-pinch concept. Starting in 1947, two
UK teams carried out small experiments based
on this concept and began building a series
of ever-larger experiments.
=== 1950s ===
The first successful man-made fusion device
was the boosted fission weapon tested in 1951
in the Greenhouse Item test. This was followed
by true fusion weapons in 1952's Ivy Mike,
and the first practical examples in 1954's
Castle Bravo. This was uncontrolled fusion.
In these devices, the energy released by the
fission explosion is used to compress and
heat fusion fuel, starting a fusion reaction.
Fusion releases neutrons. These neutrons hit
the surrounding fission fuel, causing the
atoms to split apart much faster than normal
fission processes—almost instantly by comparison.
This increases the effectiveness of bombs:
normal fission weapons blow themselves apart
before all their fuel is used; fusion/fission
weapons do not have this practical upper limit.
In 1949 an expatriate German, Ronald Richter,
proposed the Huemul Project in Argentina,
announcing positive results in 1951. These
turned out to be fake, but it prompted considerable
interest in the concept as a whole. In particular,
it prompted Lyman Spitzer to begin considering
ways to solve some of the more obvious problems
involved in confining a hot plasma, and, unaware
of the z-pinch efforts, he developed a new
solution to the problem known as the stellarator.
Spitzer applied to the US Atomic Energy Commission
for funding to build a test device. During
this period, James L. Tuck who had worked
with the UK teams on z-pinch had been introducing
the concept to his new coworkers at the Los
Alamos National Laboratory (LANL). When he
heard of Spitzer's pitch for funding, he applied
to build a machine of his own, the Perhapsatron.
Spitzer's idea won funding and he began work
on the stellarator under the code name Project
Matterhorn. His work led to the creation of
the Princeton Plasma Physics Laboratory. Tuck
returned to LANL and arranged local funding
to build his machine. By this time, however,
it was clear that all of the pinch machines
were suffering from the same issues involving
instability, and progress stalled. In 1953,
Tuck and others suggested a number of solutions
to the stability problems. This led to the
design of a second series of pinch machines,
led by the UK ZETA and Sceptre devices.
Spitzer had planned an aggressive development
project of four machines, A, B, C, and D.
A and B were small research devices, C would
be the prototype of a power-producing machine,
and D would be the prototype of a commercial
device. A worked without issue, but even by
the time B was being used it was clear the
stellarator was also suffering from instabilities
and plasma leakage. Progress on C slowed as
attempts were made to correct for these problems.
By the mid-1950s it was clear that the simple
theoretical tools being used to calculate
the performance of all fusion machines were
simply not predicting their actual behavior.
Machines invariably leaked their plasma from
their confinement area at rates far higher
than predicted. In 1954, Edward Teller held
a gathering of fusion researchers at the Princeton
Gun Club, near the Project Matterhorn (now
known as Project Sherwood) grounds. Teller
started by pointing out the problems that
everyone was having, and suggested that any
system where the plasma was confined within
concave fields was doomed to fail. Attendees
remember him saying something to the effect
that the fields were like rubber bands, and
they would attempt to snap back to a straight
configuration whenever the power was increased,
ejecting the plasma. He went on to say that
it appeared the only way to confine the plasma
in a stable configuration would be to use
convex fields, a "cusp" configuration.When
the meeting concluded, most of the researchers
quickly turned out papers saying why Teller's
concerns did not apply to their particular
device. The pinch machines did not use magnetic
fields in this way at all, while the mirror
and stellarator seemed to have various ways
out. This was soon followed by a paper by
Martin David Kruskal and Martin Schwarzschild
discussing pinch machines, however, which
demonstrated instabilities in those devices
were inherent to the design.
The largest "classic" pinch device was the
ZETA, including all of these suggested upgrades,
starting operations in the UK in 1957. In
early 1958, John Cockcroft announced that
fusion had been achieved in the ZETA, an announcement
that made headlines around the world. When
physicists in the US expressed concerns about
the claims they were initially dismissed.
US experiments soon demonstrated the same
neutrons, although temperature measurements
suggested these could not be from fusion reactions.
The neutrons seen in the UK were later demonstrated
to be from different versions of the same
instability processes that plagued earlier
machines. Cockcroft was forced to retract
the fusion claims, and the entire field was
tainted for years. ZETA ended its experiments
in 1968.
The first experiment to achieve controlled
thermonuclear fusion was accomplished using
Scylla I at the Los Alamos National Laboratory
in 1958. Scylla I was a θ-pinch machine,
with a cylinder full of deuterium. Electric
current shot down the sides of the cylinder.
The current made magnetic fields that pinched
the plasma, raising temperatures to 15 million
degrees Celsius, for long enough that atoms
fused and produce neutrons. The sherwood program
sponsored a series of Scylla machines at Los
Alamos. The program began with 5 researchers
and 100,000 in US funding in January 1952.
By 1965, a total of 21 million had been spent
on the program and staffing never reached
above 65.
In 1950–1951 I.E. Tamm and A.D. Sakharov
in the Soviet Union, first discussed a tokamak-like
approach. Experimental research on those designs
began in 1956 at the Kurchatov Institute in
Moscow by a group of Soviet scientists led
by Lev Artsimovich. The tokamak essentially
combined a low-power pinch device with a low-power
simple stellarator. The key was to combine
the fields in such a way that the particles
orbited within the reactor a particular number
of times, today known as the "safety factor".
The combination of these fields dramatically
improved confinement times and densities,
resulting in huge improvements over existing
devices.
=== 1960s ===
A key plasma physics text was published by
Lyman Spitzer at Princeton in 1963. Spitzer
took the ideal gas laws and adapted them to
an ionized plasma, developing many of the
fundamental equations used to model a plasma.
Laser fusion was suggested in 1962 by scientists
at Lawrence Livermore National Laboratory,
shortly after the invention of the laser itself
in 1960. At the time, Lasers were low power
machines, but low-level research began as
early as 1965. Laser fusion, formally known
as inertial confinement fusion, involves imploding
a target by using laser beams. There are two
ways to do this: indirect drive and direct
drive. In direct drive, the laser blasts a
pellet of fuel. In indirect drive, the lasers
blast a structure around the fuel. This makes
x-rays that squeeze the fuel. Both methods
compress the fuel so that fusion can take
place.
At the 1964 World's Fair, the public was given
its first demonstration of nuclear fusion.
The device was a θ-pinch from General Electric.
This was similar to the Scylla machine developed
earlier at Los Alamos.
The magnetic mirror was first published in
1967 by Richard F. Post and many others at
the Lawrence Livermore National Laboratory.
The mirror consisted of two large magnets
arranged so they had strong fields within
them, and a weaker, but connected, field between
them. Plasma introduced in the area between
the two magnets would "bounce back" from the
stronger fields in the middle.
The A.D. Sakharov group constructed the first
tokamaks, the most successful being the T-3
and its larger version T-4. T-4 was tested
in 1968 in Novosibirsk, producing the world's
first quasistationary fusion reaction. When
this were first announced, the international
community was highly skeptical. A British
team was invited to see T-3, however, and
after measuring it in depth they released
their results that confirmed the Soviet claims.
A burst of activity followed as many planned
devices were abandoned and new tokamaks were
introduced in their place — the C model
stellarator, then under construction after
many redesigns, was quickly converted to the
Symmetrical Tokamak.
In his work with vacuum tubes, Philo Farnsworth
observed that electric charge would accumulate
in regions of the tube. Today, this effect
is known as the Multipactor effect. Farnsworth
reasoned that if ions were concentrated high
enough they could collide and fuse. In 1962,
he filed a patent on a design using a positive
inner cage to concentrate plasma, in order
to achieve nuclear fusion. During this time,
Robert L. Hirsch joined the Farnsworth Television
labs and began work on what became the fusor.
Hirsch patented the design in 1966 and published
the design in 1967.
=== 1970s ===
In 1972, John Nuckolls outlined the idea of
ignition. This is a fusion chain reaction.
Hot helium made during fusion reheats the
fuel and starts more reactions. John argued
that ignition would require lasers of about
1 kJ. This turned out to be wrong. Nuckolls's
paper started a major development effort.
Several laser systems were built at LLNL.
These included the argus, the Cyclops, the
Janus, the long path, the Shiva laser and
the Nova in 1984. This prompted the UK to
build the Central Laser Facility in 1976.During
this time, great strides in understanding
the tokamak system were made. A number of
improvements to the design are now part of
the "advanced tokamak" concept, which includes
non-circular plasma, internal diverters and
limiters, often superconducting magnets, and
operate in the so-called "H-mode" island of
increased stability. Two other designs have
also become fairly well studied; the compact
tokamak is wired with the magnets on the inside
of the vacuum chamber, while the spherical
tokamak reduces its cross section as much
as possible.
In 1974 a study of the ZETA results demonstrated
an interesting side-effect; after an experimental
run ended, the plasma would enter a short
period of stability. This led to the reversed
field pinch concept, which has seen some level
of development since. On May 1, 1974, the
KMS fusion company (founded by Kip Siegel)
achieves the world's first laser induced fusion
in a deuterium-tritium pellet.In the mid-1970s,
Project PACER, carried out at Los Alamos National
Laboratory (LANL) explored the possibility
of a fusion power system that would involve
exploding small hydrogen bombs (fusion bombs)
inside an underground cavity. As an energy
source, the system is the only fusion power
system that could be demonstrated to work
using existing technology. It would also require
a large, continuous supply of nuclear bombs,
however, making the economics of such a system
rather questionable.
In 1976, the two beam Argus laser becomes
operational at livermore. In 1977, The 20
beam Shiva laser at Livermore is completed,
capable of delivering 10.2 kilojoules of infrared
energy on target. At a price of $25 million
and a size approaching that of a football
field, Shiva is the first of the megalasers.
That same year, the JET project is approved
by the European Commission and a site is selected.
=== 1980s ===
As a result of advocacy, the cold war, and
the 1970s energy crisis a massive magnetic
mirror program was funded by the US federal
government in the late 1970s and early 1980s.
This program resulted in a series of large
magnetic mirror devices including: 2X, Baseball
I, Baseball II, the Tandem Mirror Experiment,
the Tandem mirror experiment upgrade, the
Mirror Fusion Test Facility and the MFTF-B.
These machines were built and tested at Livermore
from the late 1960s to the mid 1980s. A number
of institutions collaborated on these machines,
conducting experiments. These included the
Institute for Advanced Study and the University
of Wisconsin–Madison. The last machine,
the Mirror Fusion Test Facility cost 372 million
dollars and was, at that time, the most expensive
project in Livermore history. It opened on
February 21, 1986 and was promptly shut down.
The reason given was to balance the United
States federal budget. This program was supported
from within the Carter and early Reagan administrations
by Edwin E. Kintner, a US Navy captain, under
Alvin Trivelpiece.In Laser fusion progressed:
in 1983, the NOVETTE laser was completed.
The following December 1984, the ten beam
NOVA laser was finished. Five years later,
NOVA would produce a maximum of 120 kilojoules
of infrared light, during a nanosecond pulse.
Meanwhile, efforts focused on either fast
delivery or beam smoothness. Both tried to
deliver the energy uniformly to implode the
target. One early problem was that the light
in the infrared wavelength, lost lots of energy
before hitting the fuel. Breakthroughs were
made at the Laboratory for Laser Energetics
at the University of Rochester. Rochester
scientists used frequency-tripling crystals
to transform the infrared laser beams into
ultraviolet beams. In 1985, Donna Strickland
and Gérard Mourou invented a method to amplify
lasers pulses by "chirping". This method changes
a single wavelength into a full spectrum.
The system then amplifies the laser at each
wavelength and then reconstitutes the beam
into one color. Chirp pulsed amplification
became instrumental in building the National
Ignition Facility and the Omega EP system.
Most research into ICF was towards weapons
research, because the implosion is relevant
to nuclear weapons.
During this time Los Alamos National Laboratory
constructed a series of laser facilities.
This included Gemini (a two beam system),
Helios (eight beams), Antares (24 beams) and
Aurora (96 beams). The program ended in the
early nineties with a cost on the order of
one billion dollars.In 1987, Akira Hasegawa
noticed that in a dipolar magnetic field,
fluctuations tended to compress the plasma
without energy loss. This effect was noticed
in data taken by Voyager 2, when it encountered
Uranus. This observation would become the
basis for a fusion approach known as the Levitated
dipole.
In Tokamaks, the Tore Supra was under construction
over the middle of the eighties (1983 to 1988).
This was a Tokamak built in Cadarache, France.
In 1983, the JET was completed and first plasmas
achieved. In 1985, the Japanese tokamak, JT-60
was completed. In 1988, the T-15 a Soviet
tokamak was completed. It was the first industrial
fusion reactor to use superconducting magnets
to control the plasma. These were Helium cooled.
In 1989, Pons and Fleischmann submitted papers
to the Journal of Electroanalytical Chemistry
claiming that they had observed fusion in
a room temperature device and disclosing their
work in a press release. Some scientists reported
excess heat, neutrons, tritium, helium and
other nuclear effects in so-called cold fusion
systems, which for a time gained interest
as showing promise. Hopes fell when replication
failures were weighed in view of several reasons
cold fusion is not likely to occur, the discovery
of possible sources of experimental error,
and finally the discovery that Fleischmann
and Pons had not actually detected nuclear
reaction byproducts. By late 1989, most scientists
considered cold fusion claims dead, and cold
fusion subsequently gained a reputation as
pathological science. However, a small community
of researchers continues to investigate cold
fusion claiming to replicate Fleishmann and
Pons' results including nuclear reaction byproducts.
Claims related to cold fusion are largely
disbelieved in the mainstream scientific community.
In 1989, the majority of a review panel organized
by the US Department of Energy (DOE) found
that the evidence for the discovery of a new
nuclear process was not persuasive. A second
DOE review, convened in 2004 to look at new
research, reached conclusions similar to the
first.In 1984, Martin Peng of ORNL proposed
an alternate arrangement of the magnet coils
that would greatly reduce the aspect ratio
while avoiding the erosion issues of the compact
tokamak: a Spherical tokamak. Instead of wiring
each magnet coil separately, he proposed using
a single large conductor in the center, and
wiring the magnets as half-rings off of this
conductor. What was once a series of individual
rings passing through the hole in the center
of the reactor was reduced to a single post,
allowing for aspect ratios as low as 1.2.
The ST concept appeared to represent an enormous
advance in tokamak design. However, it was
being proposed during a period when US fusion
research budgets were being dramatically scaled
back. ORNL was provided with funds to develop
a suitable central column built out of a high-strength
copper alloy called "Glidcop". However, they
were unable to secure funding to build a demonstration
machine, "STX". Failing to build an ST at
ORNL, Peng began a worldwide effort to interest
other teams in the ST concept and get a test
machine built. One way to do this quickly
would be to convert a spheromak machine to
the Spherical tokamak layout. Peng's advocacy
also caught the interest of Derek Robinson,
of the United Kingdom Atomic Energy Authority
fusion center at Culham. Robinson was able
to gather together a team and secure funding
on the order of 100,000 pounds to build an
experimental machine, the Small Tight Aspect
Ratio Tokamak, or START. Several parts of
the machine were recycled from earlier projects,
while others were loaned from other labs,
including a 40 keV neutral beam injector from
ORNL. Construction of START began in 1990,
it was assembled rapidly and started operation
in January 1991.
=== 1990s ===
In 1991 the Preliminary Tritium Experiment
at the Joint European Torus in England achieved
the world's first controlled release of fusion
power.In 1992, a major article was published
in Physics Today by Robert McCory at the Laboratory
for laser energetics outlying the current
state of ICF and advocating for a national
ignition facility. This was followed up by
a major review article, from John Lindl in
1995, advocating for NIF. During this time
a number of ICF subsystems were developing,
including target manufacturing, cryogenic
handling systems, new laser designs (notably
the NIKE laser at NRL) and improved diagnostics
like time of flight analyzers and Thomson
scattering. This work was done at the NOVA
laser system, General Atomics, Laser Mégajoule
and the GEKKO XII system in Japan. Through
this work and lobbying by groups like the
fusion power associates and John Sethian at
NRL, a vote was made in congress, authorizing
funding for the NIF project in the late nineties.
In the early nineties, theory and experimental
work regarding fusors and polywells was published.
In response, Todd Rider at MIT developed general
models of these devices. Rider argued that
all plasma systems at thermodynamic equilibrium
were fundamentally limited. In 1995, William
Nevins published a criticism arguing that
the particles inside fusors and polywells
would build up angular momentum, causing the
dense core to degrade.
In 1995, the University of Wisconsin–Madison
built a large fusor, known as HOMER, which
is still in operation. Meanwhile, Dr George
H. Miley at Illinois, built a small fusor
that has produced neutrons using deuterium
gas and discovered the "star mode" of fusor
operation. The following year, the first "US-Japan
Workshop on IEC Fusion", was conducted. At
this time in Europe, an IEC device was developed
as a commercial neutron source by Daimler-Chrysler
and NSD Fusion.In 1996, the Z-machine was
upgraded and opened to the public by the US
Army in August 1998 in Scientific American.
The key attributes of Sandia's Z machine are
its 18 million amperes and a discharge time
of less than 100 nanoseconds. This generates
a magnetic pulse, inside a large oil tank,
this strikes an array of tungsten wires called
a liner. Firing the Z-machine has become a
way to test very high energy, high temperature
(2 billion degrees) conditions. In 1996, the
Tore Supra creates a plasma for two minutes
with a current of almost 1 million amperes
driven non-inductively by 2.3 MW of lower
hybrid frequency waves. This is 280 MJ of
injected and extracted energy. This result
was possible because of the actively cooled
plasma-facing componentsIn 1997, JET produced
a peak of 16.1MW of fusion power (65% of heat
to plasma), with fusion power of over 10MW
sustained for over 0.5 sec. Its successor,
the International Thermonuclear Experimental
Reactor (ITER), was officially announced as
part of a seven-party consortium (six countries
and the EU). ITER is designed to produce ten
times more fusion power than the power put
into the plasma. ITER is currently under construction
in Cadarache, France.
In the late nineties, a team at Columbia University
and MIT developed the Levitated dipole a fusion
device which consisted of a superconducting
electromagnet, floating in a saucer shaped
vacuum chamber. Plasma swirled around this
donut and fused along the center axis.
=== 2000s ===
In the March 8, 2002 issue of the peer-reviewed
journal Science, Rusi P. Taleyarkhan and colleagues
at the Oak Ridge National Laboratory (ORNL)
reported that acoustic cavitation experiments
conducted with deuterated acetone (C3D6O)
showed measurements of tritium and neutron
output consistent with the occurrence of fusion.
Taleyarkhan was later found guilty of misconduct,
the Office of Naval Research debarred him
for 28 months from receiving Federal Funding,
and his name was listed in the 'Excluded Parties
List'."Fast ignition" was developed in the
late nineties, and was part of a push by the
Laboratory for Laser Energetics for building
the Omega EP system. This system was finished
in 2008. Fast ignition showed such dramatic
power savings that ICF appears to be a useful
technique for energy production. There are
even proposals to build an experimental facility
dedicated to the fast ignition approach, known
as HiPER.
In April 2005, a team from UCLA announced
it had devised a way of producing fusion using
a machine that "fits on a lab bench", using
lithium tantalate to generate enough voltage
to smash deuterium atoms together. The process,
however, does not generate net power (see
Pyroelectric fusion). Such a device would
be useful in the same sort of roles as the
fusor. In 2006, China's EAST test reactor
is completed. This was the first tokamak to
use superconducting magnets to generate both
the toroidal and poloidal fields.
In the early 2000s, Researchers at LANL reasoned
that a plasma oscillating could be at local
thermodynamic equilibrium. This prompted the
POPS and Penning trap designs. At this time,
researchers at MIT became interested in fusors
for space propulsion and powering space vehicles.
Specifically, researchers developed fusors
with multiple inner cages. Greg Piefer graduated
from Madison and founded Phoenix Nuclear Labs,
a company that developed the fusor into a
neutron source for the mass production of
medical isotopes. Robert Bussard began speaking
openly about the Polywell in 2006. He attempted
to generate interest in the research, before
his death. In 2008, Taylor Wilson achieved
notoriety for achieving nuclear fusion at
14, with a homemade fusor.In March 2009, a
high-energy laser system, the National Ignition
Facility (NIF), located at the Lawrence Livermore
National Laboratory, became operational.The
early 2000s saw the founding of a number of
privately backed fusion companies pursuing
innovative approaches with the stated goal
of developing commercially viable fusion power
plants. Secretive startup Tri Alpha Energy,
founded in 1998, began exploring a field-reversed
configuration approach. In 2002, Canadian
company General Fusion began proof-of-concept
experiments based on a hybrid magneto-inertial
approach called Magnetized Target Fusion.
These companies are funded by private investors
including Jeff Bezos (General Fusion) and
Paul Allen (Tri Alpha Energy). Toward the
end of the decade, UK-based fusion company
Tokamak Energy started exploring spherical
tokamak devices.
=== 2010s ===
NIF, the French Laser Mégajoule and the planned
European Union High Power laser Energy Research
(HiPER) facility continued researching inertial
(laser) confinement.
In 2010, NIF researchers conducted a series
of "tuning" shots to determine the optimal
target design and laser parameters for high-energy
ignition experiments with fusion fuel. Firing
tests were performed on October 31, 2010 and
November 2, 2010. In early 2012, NIF director
Mike Dunne expected the laser system to generate
fusion with net energy gain by the end of
2012. However, that did not happen until August
2013. The facility reported that their next
step involved improving the system to prevent
the hohlraum from either breaking up asymmetrically
or too soon.A 2012 paper demonstrated that
a dense plasma focus had achieved temperatures
of 1.8 billion degrees Celsius, sufficient
for boron fusion, and that fusion reactions
were occurring primarily within the contained
plasmoid, a necessary condition for net power.In
April 2014, Lawrence Livermore National Laboratory
ended the Laser Inertial Fusion Energy (LIFE)
program and redirected their efforts towards
NIF. In August 2014, Phoenix Nuclear Labs
announced the sale of a high-yield neutron
generator that could sustain 5×1011 deuterium
fusion reactions per second over a 24-hour
period.In October 2014, Lockheed Martin's
Skunk Works announced the development of a
high beta fusion reactor, the Compact Fusion
Reactor, intending on making a 100-megawatt
prototype by 2017 and beginning regular operation
by 2022. Although the original concept was
to build a 20-ton, container-sized unit, the
team later conceded that the minimum scale
would be 2,000 tons.In January 2015, the polywell
was presented at Microsoft Research.In August,
2015, MIT announced a tokamak it named ARC
fusion reactor using rare-earth barium-copper
oxide (REBCO) superconducting tapes to produce
high-magnetic field coils that it claimed
produce comparable magnetic field strength
in a smaller configuration than other designs.In
October 2015, researchers at the Max Planck
Institute of Plasma Physics completed building
the largest stellarator to date, named Wendelstein
7-X. On December 10, they successfully produced
the first helium plasma, and on February 3,
2016 produced the device's first hydrogen
plasma. With plasma discharges lasting up
to 30 minutes, Wendelstein 7-X is attempting
to demonstrate the essential stellarator attribute:
continuous operation of a high-temperature
hydrogen plasma.
General Fusion developed its plasma injector
technology and Tri Alpha Energy constructed
and operated its C-2U device.In 2017 Helion
Energy's fifth-generation plasma machine went
into operation, seeking to achieve plasma
density of 20 Tesla and fusion temperatures.
In 2018 General Fusion was developing a 70%
scale demo system to be completed around 2023.In
2018, energy corporation Eni announced a $50
million investment in the newly founded Commonwealth
Fusion Systems, to attempt to commercialize
ARC technology using a test reactor (SPARC)
in collaboration with MIT.
== Fuels ==
By firing particle beams at targets, many
fusion reactions have been tested, while the
fuels considered for power have all been light
elements like the isotopes of hydrogen—protium,
deuterium, and tritium. The deuterium and
helium-3 reaction requires helium-3, an isotope
of helium so scarce on Earth that it would
have to be mined extraterrestrially or produced
by other nuclear reactions. Finally, researchers
hope to perform the protium and boron-11 reaction,
because it does not directly produce neutrons,
though side reactions can.
=== Deuterium, tritium ===
The easiest nuclear reaction, at the lowest
energy, is:
21D + 31T → 42He (3.5 MeV) + 10n (14.1 MeV)This
reaction is common in research, industrial
and military applications, usually as a convenient
source of neutrons. Deuterium is a naturally
occurring isotope of hydrogen and is commonly
available. The large mass ratio of the hydrogen
isotopes makes their separation easy compared
to the difficult uranium enrichment process.
Tritium is a natural isotope of hydrogen,
but because it has a short half-life of 12.32
years, it is hard to find, store, produce,
and is expensive. Consequently, the deuterium-tritium
fuel cycle requires the breeding of tritium
from lithium using one of the following reactions:
10n + 63Li → 31T + 42He
10n + 73Li → 31T + 42He + 10nThe reactant
neutron is supplied by the D-T fusion reaction
shown above, and the one that has the greatest
yield of energy. The reaction with 6Li is
exothermic, providing a small energy gain
for the reactor. The reaction with 7Li is
endothermic but does not consume the neutron.
At least some neutron multiplication reactions
are required to replace the neutrons lost
to absorption by other elements. Leading candidate
neutron multiplication materials are beryllium
and lead however the 7Li reaction above also
helps to keep the neutron population high.
Natural lithium is mainly 7Li however this
has a low tritium production cross section
compared to 6Li so most reactor designs use
breeder blankets with enriched 6Li.
Several drawbacks are commonly attributed
to D-T fusion power:
It produces substantial amounts of neutrons
that result in the neutron activation of the
reactor materials.
Only about 20% of the fusion energy yield
appears in the form of charged particles with
the remainder carried off by neutrons, which
limits the extent to which direct energy conversion
techniques might be applied.
It requires the handling of the radioisotope
tritium. Similar to hydrogen, tritium is difficult
to contain and may leak from reactors in some
quantity. Some estimates suggest that this
would represent a fairly large environmental
release of radioactivity.The neutron flux
expected in a commercial D-T fusion reactor
is about 100 times that of current fission
power reactors, posing problems for material
design. After a series of D-T tests at JET,
the vacuum vessel was sufficiently radioactive
that remote handling was required for the
year following the tests.In a production setting,
the neutrons would be used to react with lithium
in the context of a breeder blanket comprising
lithium ceramic pebbles or liquid lithium,
in order to create more tritium. This also
deposits the energy of the neutrons in the
lithium, which would then be transferred to
drive electrical production. The lithium neutron
absorption reaction protects the outer portions
of the reactor from the neutron flux. Newer
designs, the advanced tokamak in particular,
also use lithium inside the reactor core as
a key element of the design. The plasma interacts
directly with the lithium, preventing a problem
known as "recycling". The advantage of this
design was demonstrated in the Lithium Tokamak
Experiment.
=== Deuterium ===
This is the second easiest fusion reaction,
fusing two deuterium nuclei. The reaction
has two branches that occur with nearly equal
probability:
This reaction is also common in research.
The optimum energy to initiate this reaction
is 15 keV, only slightly higher than the optimum
for the D-T reaction. The first branch does
not produce neutrons, but it does produce
tritium, so that a D-D reactor will not be
completely tritium-free, even though it does
not require an input of tritium or lithium.
Unless the tritons can be quickly removed,
most of the tritium produced would be burned
before leaving the reactor, which would reduce
the handling of tritium, but would produce
more neutrons, some of which are very energetic.
The neutron from the second branch has an
energy of only 2.45 MeV (0.393 pJ), whereas
the neutron from the D-T reaction has an energy
of 14.1 MeV (2.26 pJ), resulting in a wider
range of isotope production and material damage.
When the tritons are removed quickly while
allowing the 3He to react, the fuel cycle
is called "tritium suppressed fusion" The
removed tritium decays to 3He with a 12.5
year half life. By recycling the 3He produced
from the decay of tritium back into the fusion
reactor, the fusion reactor does not require
materials resistant to fast 14.1 MeV (2.26
pJ) neutrons.
Assuming complete tritium burn-up, the reduction
in the fraction of fusion energy carried by
neutrons would be only about 18%, so that
the primary advantage of the D-D fuel cycle
is that tritium breeding would not be required.
Other advantages are independence from scarce
lithium resources and a somewhat softer neutron
spectrum. The disadvantage of D-D compared
to D-T is that the energy confinement time
(at a given pressure) must be 30 times longer
and the power produced (at a given pressure
and volume) would be 68 times less.Assuming
complete removal of tritium and recycling
of 3He, only 6% of the fusion energy is carried
by neutrons. The tritium-suppressed D-D fusion
requires an energy confinement that is 10
times longer compared to D-T and a plasma
temperature that is twice as high.
=== Deuterium, helium-3 ===
A second-generation approach to controlled
fusion power involves combining helium-3 (3He)
and deuterium (2H):
This reaction produces a helium-4 nucleus
(4He) and a high-energy proton. As with the
p-11B aneutronic fusion fuel cycle, most of
the reaction energy is released as charged
particles, reducing activation of the reactor
housing and potentially allowing more efficient
energy harvesting (via any of several speculative
technologies). In practice, D-D side reactions
produce a significant number of neutrons,
resulting in p-11B being the preferred cycle
for aneutronic fusion.
=== Protium, boron-11 ===
If aneutronic fusion is the goal, then the
most promising candidate may be the hydrogen-1
(protium) and boron reaction, which releases
alpha (helium) particles, but does not rely
on neutron scattering for energy transfer.
1H + 11B → 3 4HeUnder reasonable assumptions,
side reactions will result in about 0.1% of
the fusion power being carried by neutrons.
At 123 keV, the optimum temperature for this
reaction is nearly ten times higher than that
for the pure hydrogen reactions, the energy
confinement must be 500 times better than
that required for the D-T reaction, and the
power density will be 2500 times lower than
for D-T.
Because the confinement properties of conventional
approaches to fusion such as the tokamak and
laser pellet fusion are marginal, most proposals
for aneutronic fusion are based on radically
different confinement concepts, such as the
Polywell and the Dense Plasma Focus. Results
have been extremely promising:
"In the October 2013 edition of Nature Communications,
a research team led by Christine Labaune at
École Polytechnique in Palaiseau, France,
reported a new record fusion rate: an estimated
80 million fusion reactions during the 1.5
nanoseconds that the laser fired, which is
at least 100 times more than any previous
proton-boron experiment. "
== Material selection ==
=== 
Considerations ===
Even on smaller plasma production scales,
the material of the containment apparatus
will be intensely blasted with matter and
energy. Designs for plasma containment must
consider:
A heating and cooling cycle, up to a 10 MW/m²
thermal load.
Neutron radiation, which over time leads to
neutron activation and embrittlement.
High energy ions leaving at tens to hundreds
of electronvolts.
Alpha particles leaving at millions of electronvolts.
Electrons leaving at high energy.
Light radiation (IR, visible, UV, X-ray).Depending
on the approach, these effects may be higher
or lower than typical fission reactors like
the pressurized water reactor (PWR). One estimate
put the radiation at 100 times that of a typical
PWR. Materials need to be selected or developed
that can withstand these basic conditions.
Depending on the approach, however, there
may be other considerations such as electrical
conductivity, magnetic permeability and mechanical
strength. There is also a need for materials
whose primary components and impurities do
not result in long-lived radioactive wastes.
=== Durability ===
For long term use, each atom in the wall is
expected to be hit by a neutron and displaced
about a hundred times before the material
is replaced. High-energy neutrons will produce
hydrogen and helium by way of various nuclear
reactions that tends to form bubbles at grain
boundaries and result in swelling, blistering
or embrittlement.
=== Selection ===
One can choose either a low-Z material, such
as graphite or beryllium, or a high-Z material,
usually tungsten with molybdenum as a second
choice. Use of liquid metals (lithium, gallium,
tin) has also been proposed, e.g., by injection
of 1–5 mm thick streams flowing at 10 m/s
on solid substrates.If graphite is used, the
gross erosion rates due to physical and chemical
sputtering would be many meters per year,
so one must rely on redeposition of the sputtered
material. The location of the redeposition
will not exactly coincide with the location
of the sputtering, so one is still left with
erosion rates that may be prohibitive. An
even larger problem is the tritium co-deposited
with the redeposited graphite. The tritium
inventory in graphite layers and dust in a
reactor could quickly build up to many kilograms,
representing a waste of resources and a serious
radiological hazard in case of an accident.
The consensus of the fusion community seems
to be that graphite, although a very attractive
material for fusion experiments, cannot be
the primary plasma-facing material (PFM) in
a commercial reactor.
The sputtering rate of tungsten by the plasma
fuel ions is orders of magnitude smaller than
that of carbon, and tritium is much less incorporated
into redeposited tungsten, making this a more
attractive choice. On the other hand, tungsten
impurities in a plasma are much more damaging
than carbon impurities, and self-sputtering
of tungsten can be high, so it will be necessary
to ensure that the plasma in contact with
the tungsten is not too hot (a few tens of
eV rather than hundreds of eV). Tungsten also
has disadvantages in terms of eddy currents
and melting in off-normal events, as well
as some radiological issues.
== Safety and the environment ==
=== 
Accident potential ===
Unlike nuclear fission, fusion requires extremely
precise and controlled temperature, pressure
and magnetic field parameters for any net
energy to be produced. If a reactor suffers
damage or loses even a small degree of required
control, fusion reactions and heat generation
would rapidly cease. Additionally, fusion
reactors contain only small amounts of fuel,
enough to "burn" for minutes, or in some cases,
microseconds. Unless they are actively refueled,
the reactions will quickly end. Therefore,
fusion reactors are considered immune from
catastrophic meltdown.For similar reasons,
runaway reactions cannot occur in a fusion
reactor. The plasma is burnt at optimal conditions,
and any significant change will simply quench
the reactions. The reaction process is so
delicate that this level of safety is inherent.
Although the plasma in a fusion power station
is expected to have a volume of 1,000 cubic
metres (35,000 cu ft) or more, the plasma
density is low and typically contains only
a few grams of fuel in use. If the fuel supply
is closed, the reaction stops within seconds.
In comparison, a fission reactor is typically
loaded with enough fuel for several months
or years, and no additional fuel is necessary
to continue the reaction. It is this large
amount of fuel that gives rise to the possibility
of a meltdown; nothing like this exists in
a fusion reactor.In the magnetic approach,
strong fields are developed in coils that
are held in place mechanically by the reactor
structure. Failure of this structure could
release this tension and allow the magnet
to "explode" outward. The severity of this
event would be similar to any other industrial
accident or an MRI machine quench/explosion,
and could be effectively stopped with a containment
building similar to those used in existing
(fission) nuclear generators. The laser-driven
inertial approach is generally lower-stress
because of the increased size of the reaction
chamber. Although failure of the reaction
chamber is possible, simply stopping fuel
delivery would prevent any sort of catastrophic
failure.Most reactor designs rely on liquid
hydrogen as both a coolant and a method for
converting stray neutrons from the reaction
into tritium, which is fed back into the reactor
as fuel. Hydrogen is highly flammable, and
in the case of a fire it is possible that
the hydrogen stored on-site could be burned
up and escape. In this case, the tritium contents
of the hydrogen would be released into the
atmosphere, posing a radiation risk. Calculations
suggest that at about 1 kilogram (2.2 lb),
the total amount of tritium and other radioactive
gases in a typical power station would be
so small that they would have diluted to legally
acceptable limits by the time they blew as
far as the station's perimeter fence.The likelihood
of small industrial accidents, including the
local release of radioactivity and injury
to staff, cannot be estimated yet. These would
include accidental releases of lithium or
tritium or mishandling of decommissioned radioactive
components of the reactor itself.
=== Magnet quench ===
A quench is an abnormal termination of magnet
operation that occurs when part of the superconducting
coil enters the normal (resistive) state.
This can occur because the field inside the
magnet is too large, the rate of change of
field is too large (causing eddy currents
and resultant heating in the copper support
matrix), or a combination of the two.
More rarely a defect in the magnet can cause
a quench. When this happens, that particular
spot is subject to rapid Joule heating from
the enormous current, which raises the temperature
of the surrounding regions. This pushes those
regions into the normal state as well, which
leads to more heating in a chain reaction.
The entire magnet rapidly becomes normal (this
can take several seconds, depending on the
size of the superconducting coil). This is
accompanied by a loud bang as the energy in
the magnetic field is converted to heat, and
rapid boil-off of the cryogenic fluid. The
abrupt decrease of current can result in kilovolt
inductive voltage spikes and arcing. Permanent
damage to the magnet is rare, but components
can be damaged by localized heating, high
voltages, or large mechanical forces.
In practice, magnets usually have safety devices
to stop or limit the current when the beginning
of a quench is detected. If a large magnet
undergoes a quench, the inert vapor formed
by the evaporating cryogenic fluid can present
a significant asphyxiation hazard to operators
by displacing breathable air.
A large section of the superconducting magnets
in CERN's Large Hadron Collider unexpectedly
quenched during start-up operations in 2008,
necessitating the replacement of a number
of magnets. In order to mitigate against potentially
destructive quenches, the superconducting
magnets that form the LHC are equipped with
fast-ramping heaters which are activated once
a quench event is detected by the complex
quench protection system. As the dipole bending
magnets are connected in series, each power
circuit includes 154 individual magnets, and
should a quench event occur, the entire combined
stored energy of these magnets must be dumped
at once. This energy is transferred into dumps
that are massive blocks of metal which heat
up to several hundreds of degrees Celsius—because
of resistive heating—in a matter of seconds.
Although undesirable, a magnet quench is a
"fairly routine event" during the operation
of a particle accelerator.
=== Effluents ===
The natural product of the fusion reaction
is a small amount of helium, which is completely
harmless to life. Of more concern is tritium,
which, like other isotopes of hydrogen, is
difficult to retain completely. During normal
operation, some amount of tritium will be
continually released.Although tritium is volatile
and biologically active, the health risk posed
by a release is much lower than that of most
radioactive contaminants, because of tritium's
short half-life (12.32 years) and very low
decay energy (~14.95 keV), and because it
does not bioaccumulate (instead being cycled
out of the body as water, with a biological
half-life of 7 to 14 days). Current ITER designs
are investigating total containment facilities
for any tritium.
=== Waste management ===
The large flux of high-energy neutrons in
a reactor will make the structural materials
radioactive. The radioactive inventory at
shut-down may be comparable to that of a fission
reactor, but there are important differences.
The half-life of the radioisotopes produced
by fusion tends to be less than those from
fission, so that the inventory decreases more
rapidly. Unlike fission reactors, whose waste
remains radioactive for thousands of years,
most of the radioactive material in a fusion
reactor would be the reactor core itself,
which would be dangerous for about 50 years,
and low-level waste for another 100. Although
this waste will be considerably more radioactive
during those 50 years than fission waste,
the very short half-life makes the process
very attractive, as the waste management is
fairly straightforward. By 500 years the material
would have the same radiotoxicity as coal
ash.Additionally, the choice of materials
used in a fusion reactor is less constrained
than in a fission design, where many materials
are required for their specific neutron cross-sections.
This allows a fusion reactor to be designed
using materials that are selected specifically
to be "low activation", materials that do
not easily become radioactive. Vanadium, for
example, would become much less radioactive
than stainless steel. Carbon fiber materials
are also low-activation, as well as being
strong and light, and are a promising area
of study for laser-inertial reactors where
a magnetic field is not required.
In general terms, fusion reactors would create
far less radioactive material than a fission
reactor, the material it would create is less
damaging biologically, and the radioactivity
"burns off" within a time period that is well
within existing engineering capabilities for
safe long-term waste storage.
=== Nuclear proliferation ===
Although fusion power uses nuclear technology,
the overlap with nuclear weapons would be
limited. A huge amount of tritium could be
produced by a fusion power station; tritium
is used in the trigger of hydrogen bombs and
in a modern boosted fission weapon, but it
can also be produced by nuclear fission. The
energetic neutrons from a fusion reactor could
be used to breed weapons-grade plutonium or
uranium for an atomic bomb (for example by
transmutation of U238 to Pu239, or Th232 to
U233).
A study conducted 2011 assessed the risk of
three scenarios:
Use in small-scale fusion station: As a result
of much higher power consumption, heat dissipation
and a more recognizable design compared to
enrichment gas centrifuges this choice would
be much easier to detect and therefore implausible.
Modifications to produce weapon-usable material
in a commercial facility: The production potential
is significant. But no fertile or fissile
substances necessary for the production of
weapon-usable materials needs to be present
at a civil fusion system at all. If not shielded,
a detection of these materials can be done
by their characteristic gamma radiation. The
underlying redesign could be detected by regular
design information verifications. In the (technically
more feasible) case of solid breeder blanket
modules, it would be necessary for incoming
components to be inspected for the presence
of fertile material, otherwise plutonium for
several weapons could be produced each year.
Prioritizing a fast production of weapon-grade
material regardless of secrecy: The fastest
way to produce weapon usable material was
seen in modifying a prior civil fusion power
station. Unlike in some nuclear power stations,
there is no weapon compatible material during
civil use. Even without the need for covert
action this modification would still take
about 2 months to start the production and
at least an additional week to generate a
significant amount for weapon production.
This was seen as enough time to detect a military
use and to react with diplomatic or military
means. To stop the production, a military
destruction of inevitable parts of the facility
leaving out the reactor itself would be sufficient.
This, together with the intrinsic safety of
fusion power would only bear a low risk of
radioactive contamination.Another study concludes
that "[..]large fusion reactors – even if
not designed for fissile material breeding
– could easily produce several hundred kg
Pu per year with high weapon quality and very
low source material requirements." It was
emphasized that the implementation of features
for intrinsic proliferation resistance might
only be possible at this phase of research
and development. The theoretical and computational
tools needed for hydrogen bomb design are
closely related to those needed for inertial
confinement fusion, but have very little in
common with the more scientifically developed
magnetic confinement fusion.
=== Energy source ===
Large-scale reactors using neutronic fuels
(e.g. ITER) and thermal power production (turbine
based) are most comparable to fission power
from an engineering and economics viewpoint.
Both fission and fusion power stations involve
a relatively compact heat source powering
a conventional steam turbine-based power station,
while producing enough neutron radiation to
make activation of the station materials problematic.
The main distinction is that fusion power
produces no high-level radioactive waste (though
activated station materials still need to
be disposed of). There are some power station
ideas that may significantly lower the cost
or size of such stations; however, research
in these areas is nowhere near as advanced
as in tokamaks.Fusion power commonly proposes
the use of deuterium, an isotope of hydrogen,
as fuel and in many current designs also use
lithium. Assuming a fusion energy output equal
to the 1995 global power output of about 100
EJ/yr (= 1 × 1020 J/yr) and that this does
not increase in the future, which is unlikely,
then the known current lithium reserves would
last 3000 years. Lithium from sea water would
last 60 million years, however, and a more
complicated fusion process using only deuterium
would have fuel for 150 billion years. To
put this in context, 150 billion years is
close to 30 times the remaining lifespan of
the sun, and more than 10 times the estimated
age of the universe.
== Economics ==
While fusion power is still in early stages
of development, substantial sums have been
and continue to be invested in research. In
the EU almost €10 billion was spent on fusion
research up to the end of the 1990s, and the
new ITER reactor alone is budgeted at €6.6
billion total for the timeframe between 2008
and 2020.It is estimated that up to the point
of possible implementation of electricity
generation by nuclear fusion, R&D will need
further promotion totalling around €60–80
billion over a period of 50 years or so (of
which €20–30 billion within the EU) based
on a report from 2002. Nuclear fusion research
receives €750 million (excluding ITER funding)
from the European Union, compared with €810
million for sustainable energy research, putting
research into fusion power well ahead of that
of any single rivaling technology. Indeed,
the size of the investments and time frame
of the expected results mean that fusion research
is almost exclusively publicly funded, while
research in other forms of energy can be done
by the private sector. In spite of that, a
number of start-up companies active in the
field of fusion power have managed to attract
private money.
== Advantages ==
Fusion power would provide more energy for
a given weight of fuel than any fuel-consuming
energy source currently in use, and the fuel
itself (primarily deuterium) exists abundantly
in the Earth's ocean: about 1 in 6500 hydrogen
atoms in seawater is deuterium. Although this
may seem a low proportion (about 0.015%),
because nuclear fusion reactions are so much
more energetic than chemical combustion and
seawater is easier to access and more plentiful
than fossil fuels, fusion could potentially
supply the world's energy needs for millions
of years.Despite being technically non-renewable,
fusion power (like fission power using breeder
reactors and reprocessing) has many of the
benefits of renewable energy sources (such
as being a long-term energy supply and emitting
no greenhouse gases or air pollution) as well
as some of the benefits of the resource-limited
energy sources as hydrocarbons and nuclear
fission (without reprocessing). Like these
currently dominant energy sources, fusion
could provide very high power-generation density
and uninterrupted power delivery (because
it is not dependent on the weather, unlike
wind and solar power).
Another aspect of fusion energy is that the
cost of production does not suffer from diseconomies
of scale. The cost of water and wind energy,
for example, goes up as the optimal locations
are developed first, while further generators
must be sited in less ideal conditions. With
fusion energy the production cost will not
increase much even if large numbers of stations
are built, because the raw resource (seawater)
is abundant and widespread.Some problems that
are expected to be an issue in this century,
such as fresh water shortages, can alternatively
be regarded as problems of energy supply.
For example, in desalination stations, seawater
can be purified through distillation or reverse
osmosis. Nonetheless, these processes are
energy intensive. Even if the first fusion
stations are not competitive with alternative
sources, fusion could still become competitive
if large-scale desalination requires more
power than the alternatives are able to provide.A
scenario has been presented of the effect
of the commercialization of fusion power on
the future of human civilization. ITER and
later DEMO are envisioned to bring online
the first commercial nuclear fusion energy
reactor by 2050. Using this as the starting
point and the history of the uptake of nuclear
fission reactors as a guide, the scenario
depicts a rapid take up of nuclear fusion
energy starting after the middle of this century.Fusion
power could be used in interstellar space,
where solar energy is not available.
== See also
