Particle accelerators are among the most amazing
scientific instruments ever built, and they
allow us to explore the behavior of matter
under unbelievable conditions.
The world’s highest energy particle accelerator
is called the Large Hadron Collider, or LHC,
and it allows physicists collide together
beams of protons to generate temperatures
100,000 times hotter than the center of the
sun.
The last time those temperatures were common
in the universe was a tenth of a trillionth
of a second after the Big Bang.
While the LHC is expected to be the world’s
flagship accelerator for the next two decades,
physicists are already thinking of a replacement
accelerator.
We’re thinking of it now because it will
take at least twenty or thirty years to design
and build it.
To give you some historical context, people
started thinking about the LHC in the early
1980s and the first official meeting was in
1984.
Given that the LHC first saw circulating beam
in 2008 and collisions in 2010, this gives
you some sense of how long the process will
take.
So in designing a particle accelerator, there
are lots of things you need to take into account.
For one thing, you need to decide the energy
you’re interested in exploring.
The LHC design collision energy is 14 trillion
electron volts.
The next accelerator should be an appreciable
increase in capability, so physicists are
proposing the round number of 100 trillion
electron volts, or about a factor of 7 increase.
I’ve made a couple of videos on accelerators
and the possible shapes they can have and
the possible beams they can have.
If you’re interested in some of the reasons
for the design that is under consideration,
you might want to take a look at them.
After considerable thought, calculation and
simulation, physicists have decided to pursue
a circular accelerator with two counter rotating
beams of protons.
The circular design is to limit costs and
the proton beams both limit costs and allows
for the most diverse research program.
The tentative name for this accelerator is
the FCC for Future Circular Collider.
Now, where the FCC is located is actually
a global geopolitical question and depends
on which country wants to host it.
That decision is actually decades in the future,
with China being one option.
The decision of where it will be located or
even if it will ever be built at all will
be made at a pay grade well above mine.
However, there is a community that thinks
that the most sensible place to locate an
FCC would be at the CERN laboratory in Switzerland.
The reason is pretty compelling, at least
to me.
For one thing, there is a talented and professional
technical staff there with advanced accelerator
knowledge about accelerators already assembled.
Plus, there is a considerable amount of infrastructure
and the existing LHC infrastructure can be
used to raise the energy of the beam before
the FCC brings it to the highest energy.
Once you’ve decided to make a circular accelerator
with about seven times the energy of existing
accelerators, you’re ready to start the
design process.
The first question is how big of a ring to
make.
This is actually pretty straightforward and
uses only introductory physics.
The thing that makes a charged particle follow
a circular path is a magnetic field and the
circumference of any particle accelerator
is just full of magnets.
The magnets push the trajectory of the charged
particles towards the center.
But each magnet can only push so hard and
if you have a higher energy beam, you need
a bigger circle.
This idea is demonstrated in this simple equation
here.
The strength of the magnetic field times the
radius of the circle equals the energy of
the beam.
A careful physics student will remind me that
I really should say momentum and not energy,
but at these energies and for my purposes,
we can use the words interchangeably.
So it’s easy to see that if we increase
the energy by a factor of seven, we need to
increase the radius of the accelerator by
seven as well.
Given that the LHC has a circumference of
27 kilometers or about 16 miles, that means
that the FCC would have to be about 189 kilometers
around or about 116 miles.
And yes, if you do the calculation yourself,
there are some rounding issues.
So that’s huge.
You have to dig a very long tunnel, build
lots of magnets, et cetera.
So what can you do to make things easier?
Well, you could also increase the strength
of the magnets.
If you could double the strength of the magnets,
then you only need to increase the accelerator’s
circumference by a factor of three and a half
or so.
The current LHC magnets are made by making
wires of an alloy of niobium and titanium.
If you cool them to near absolute zero, you
can run an electrical current through them
and they’ll make a magnetic field of eight
Tesla, which is a magnetic field about 160,000
times stronger than the Earth’s magnetic
field.
Using niobium/titanium technology you can
make a magnetic field of 9 or 10 Tesla, but
no higher.
So scientists have been working with an alloy
of titanium and tin, which should be able
to make stronger magnets.
Magnetic fields of 13.5 Tesla have already
been achieved and it is thought that 16 Tesla
is possible.
So that might be the way to go.
There is a problem though.
While niobium/titanium alloys are malleable
and can bend, niobium/tin alloys are brittle.
So that means it’s hard to make long wires
and that means that there are some real engineering
challenges ahead.
But the technology seems very promising.
Beyond the magnets, we need to figure out
better ways to make stronger electric fields
to accelerate particles.
That’s also a big deal, but probably the
magnets are the biggest challenge.
Make stronger magnets and you can make a smaller
accelerator.
So if we build such a powerful accelerator,
what will we find?
That’s always a tough question to answer.
First, we’re in the business of exploring
and discovering.
If we knew the answer ahead of time, we just
wouldn’t do it.
But we do know what we’d be looking for.
We’ll be searching for dark matter and to
see if there are even smaller building blocks
than the familiar quarks and leptons.
We’ll be looking for something unexpected.
And it’s probably worth noting that the
LHC accelerator program is just getting started.
If LHC scientists find something, we’ll
have to significantly change our plans.
That’s the cool thing about working on the
edge of knowledge.
We don’t know what we’ll find.
So will the FCC ever be built?
I don’t know.
As I said, that decision will be made at a
much higher pay grade than mine.
But there’s no technical reason why we can’t.
And, in my opinion, an FCC is a natural step
in humanity’s millennium long effort to
understand the world around us.
And it would be a glorious effort for the
next generation of scientists to undertake.
