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Hello.
Glad you could make it.
Welcome to the PSFC.
Today, we're going to be
taking a tour of the Alcator
C-Mod tokamak laboratory.
We learned about tokamak in 3.2.
Our tour guide today is
a graduate student here
at MIT, Alex.
I'll catch up with
you after the tour.
Have fun.
Welcome, everyone.
Let's head on in and
see Alcator C-Mod.
Welcome, everyone, to the
power room, which is the room
we're standing in right now.
So as you learned about
earlier in this series, when
we make a fusion
plasma, we confine it
with really, really
powerful magnets.
And in order to turn
these magnets on,
we use almost 100
megawatts of electricity
every time we run Alcator C-Mod.
And so this room,
in the power room,
is where we take all
of that electricity
and divide it up to power
different parts of the tokamak.
So some of the power goes
to the heating system.
Some goes to the magnets.
Some goes to other
parts of the tokamak.
And once we've divided
it up in the power room,
we then send it through the
wall here into Alcator C-Mod,
into the cell, and use
it to run the machine.
So after this, we'll walk into
the cell and see Alcator C-Mod.
So we're now in the cell.
And the machine behind
me is Alcator C-Mod.
So before I talk too much
about the machine itself,
I'll point a little bit
to this diagram here,
and I'll explain a few things
about the different pieces
inside the machine.
So the plasma is this
red or orange donut kind
of moving around
here in the middle
the machine that we confine
with these magnets which go
right around the machine here.
And all of that is inside
this big concrete dome
on the outside.
And that dome is what
you can see behind me.
All that blue stuff is concrete.
So the machine itself
is inside there.
But all we have a bunch of
other stuff around the machine.
So when we make this plasma--
maybe 100 million
degrees or however hot
it is-- we need to measure it.
We want to learn from it.
We're trying to do science here.
So we measure temperature.
We measure density.
We basically, any
way that we can
think of measuring something
and anything we can think
of measuring, we
build the diagnostic,
we install that on
the machine, and we
use that to measure the plasma.
So I can point out a few
of them while I'm here.
So one of the things
I'll point out
is down here there's actually
a probe that what it does
is it taps very quickly
the edge of the plasma.
So you can't really put a
thermometer in the plasma,
or you'd melt your thermometer.
But if you're very quick
and very careful about it,
you can actually
use a metal probe
and tap the edge
of the plasma just
to see what's going
on in the edge.
That's one way we
measure the plasma.
Other things I'll point out--
up here we have some diagnostics
that measure temperature.
So one of the things
I like to explain
is if you have a piece of metal,
and it gets a little bit hot--
it turns red.
And you heat it up even
more, it turns white.
So based on the
color of the metal,
you can tell how hot it is.
We also do something
similar in the plasma--
it's not quite the
same, but it's similar--
that based on the frequency
of the light that comes out
and the intensity,
how much light,
you can tell how hot the
plasma is at a given location.
So some of that data is
taken out here in the top
in these little copper pipes.
We call that electron
cyclotron emission.
And that's one of the things
that I work on personally.
Some other things
I can point out
is we want to make a hot plasma.
And so you can't just
put it in the oven.
It's not going to get quite
to 100 million degrees.
But we use basically
giant microwaves.
So if you think of a
microwave in your house
that's maybe a kilowatt, well,
we use megawatts of electricity
to heat up the plasma.
And those are actually
sent through pipes up here.
So we generate them on the
other side of the room.
And then think about like
a giant fiber-optic cable.
So it's a light pipe.
The light kind of comes
through these pipes
and is fired into
the machine down here
on these copper pipes that
are now covered up with foil.
And these are actually
car radio frequencies.
So think of your car radio.
The frequency of these waves
is exactly the same as that,
but it operates almost like
a microwave in your house.
So I can show you a lot
of other diagnostics,
but let's step down and
look at some of those now.
So now we're standing by one
of the ports for Alcator C-Mod.
So these are right in here.
If you want to get inside the
machine to do maintenance,
to do some upgrades,
to change something,
you have to crawl through
this port right here.
It's a pretty tight fit.
So I've been in the machine.
Some other people have been
in the machine as well.
But there's maybe
only 10 or 15 people
in the lab that will physically
fit inside the machine.
And why did they make
the port so small?
It's not to be mean.
It's because the magnets
for Alcator C-Mod
are so powerful that they
take up a lot of space.
So the magnets are spaced
right next to each other.
And it turns out
that there's only
about that much space
in between the magnets.
So if you want to put a port, it
has to fit between the magnets.
And that's the space you
have to crawl through.
So once you get inside, you
can kind of crouch down.
You can crawl
around a little bit
But you can't stand up if you
want to do any maintenance.
On the other hand, the powerful
magnets of Alcator C-Mod
are actually one of
its big advantages.
So the very strong
magnets allow you
to have a very hot, dense
plasma in a very small space.
And that's actually what
allowed Alcator C-Mod to have
the volume average pressure
record of any tokamak
in the world at the
end of 2016, where
the very high magnetic
field approach,
the strong magnets
of Alcator C-Mod.
So most of the structure
actually of Alcator C-Mod
is steel keeping the
magnets together.
And the space inside is
where the actual plasma is.
And that's all just
surrounded by the magnets
and the superstructure
used to keep them together.
So now we'll head up to
the top of Alcator C-Mod
and see a few extra
things up there.
So we're now standing
on top of Alcator C-Mod.
And I'll show you a few
things while we're up here.
So as I said before,
we want to measure
every aspect of the plasma
in Alcator C-Mod that we can.
And one of those is the
density of the plasma--
how much plasma is there in
any given volume of space?
And one way we can do
that is by shooting
a laser through the plasma
and basically seeing
how long it takes to
get through the plasma.
So is it slowed
down a little bit
as it goes through the plasma?
And so up here, we
have a laser system
that will shoot that
laser through the plasma.
Here we just have a big
tank of liquid helium
that cools various aspects
of the system and makes sure
that the system is
operating properly.
So also, if you look over
here to my right, your left,
you'll see a bunch
of other systems
that we use to heat the plasma.
So this is another
type of heating system,
in addition to the
one you saw earlier,
that generates
waves and pipes them
through these little copper
tubes here, the square ones.
And that goes into the
machine in the side
and also heats up
the plasma inside.
So now that we've seen
the top of Alcator C-Mod,
we'll head back out into the
control room where we analyze
the data, where it's a
little bit easier to talk--
it's not quite as
loud in there--
and continue our conversation.
We're standing now
in the control room.
And Alex is going to
say a little bit more
about how we actually operated
Alcator C-Mod from this room.
So as Anne just mentioned,
this is the control room.
So when the machine itself
is running in the cell,
all of the scientists and
grad students and professors
and everyone are in
this room in order
to run the machine itself.
So most of these
desks are for people
taking their own
individual data--
so temperature, density
measurements, all
this kind of stuff.
And some of the stuff up
here on the projectors
is information that maybe
everyone wants to know.
So there's two very
important people
playing a role when we run
an experiment at any tokamak
actually in the world.
And here, in the
control room, they
would sit at these
machines right
behind me, at these
two computers.
Maybe you can say a
little bit about that?
Yeah, exactly.
So the person in charge of the
experiment-- to some extent,
for the day--
is the session leader.
And so they're the one
deciding what kind of science
you want to do, what kind of
physics you want to measure.
And the other person is
the physics operator.
And they're kind of in charge
of taking that scientific goal
and translating it into--
how do I want to
run the machine?
So what is a
typical plasma going
to look like if we were to,
say, visualize it with a camera?
Great.
Yeah, so actually, the
picture on the left
there-- on the far
left-- is a video
of the inside of the
machine during operation.
So you see the bright
area around the edges
and the see-through
area in the middle.
And so that's actually showing
a two-second discharge,
slowed down a little bit, in
order to make it easier to see.
And that's actually our
record-breaking discharge
from the end of 2016, where we
set the volume average pressure
in a tokamak.
So there's a few
counterintuitive things
about that video behind us now.
Can you say a few words,
Alex, about the parts
that look really
bright and the parts
that aren't so bright actually,
that seem a little invisible?
We can see through them.
Sure.
So the middle of the
plasma is actually so hot
that it emits X-rays
rather than visible light.
So if you look at the video,
the section in the middle you
can see through, that's actually
the hottest part of the plasma.
And it's the sections around the
edge that are glowing brightly
that are the, quote, unquote,
"cold" parts of the plasma that
are only, quote, unquote,
"10,000 degrees rather than
100 million degrees."
And those are cool
enough that they'll
emit infrared light
and visible light,
and you can pick them up
on a camera like that.
So it's actually
really interesting
because when we talk about
heating up a piece of metal,
we talk about it getting
red hot and then white hot.
But if we think about
heating up the plasma,
it has to get invisible hot.
By the time we reach the 10
kiloelectron volt temperatures
that we described
in the lectures
that we need to achieve fusion,
you can't see a plasma anymore
because it's coming out in
the non-visible wavelengths.
Yeah.
So in addition to the
visible video of the plasma,
we also show a few other things.
So we show a cross-section
of the plasma.
So basically, if you
have your plasma donut,
and you take a little
slice out of it,
we call that a cross-section.
And so that's what you
see up there on the right.
So the red is the
edge of the plasma.
The blue is inside.
The white is the outside.
And on the far right, we
also show some information
about what's going
on in the plasma--
so current magnetic
field density, heating
power, other things that you'd
like to know about the plasma.
So for this particular
plasma, this was a record shot
that Alcator C-Mod
produced, right?
And what was the record?
What was so special about it?
Right.
So we set the volume
average pressure,
so the most pressure inside of
a tokamak ever in the world.
It was about two atmospheres.
And the reason you
want pressure is
because fusion power is really
just based on the pressure.
So the more pressure you have
in the tokamak, the more fusion
power you can get out.
And it turns out
that Alcator C-Mod
is really great at making
this high pressure.
So I was going to ask, why
is it so great at making
this high pressure?
What was the magnetic field?
What was the plasma current
that we ran this tokamak at?
We're going to be learning in
the lectures-- in 3.2 and 3.3--
we're going to be learning
about those two parameters
and how important they are
for confinement and stability.
So maybe you can say
a bit about them.
Yeah.
So Alcator C-Mod, one of the
reasons that the machine is
has had such high
performance is that it
is very high magnetic field.
So this particular
discharge was run
a little bit over 5 teslas, so
something like 5.6, 5.7 tesla--
compared to most
machines in the world,
which run maybe up to
2 or 3 tesla at tops.
We also ran more than
1 and 1/2 mega amps
of current inside the machine.
So that current also
helps confine the plasma
and keep it inside.
So because Alcator C-Mod can
run at such high magnetic field
and high current, you can have
a very small compact machine
which still has performance
that is equal to
or better than much larger
machines in other areas
around the world.
That's a fantastic segue
to a lot of the discussion
that we're going to hear in
3.3 of the lectures online
and the lecture notes.
We're going to be talking about
the high field path to fusion,
which MIT has really been
pioneering for many, many years
with the Alcator
C-Mod tokamak being
one of the major
experiments in that history.
And we're looking
ahead to the future
now, where we'd like to
build a compact device which
can allow us to achieve a
fusion power gain of capital
Q greater than 1.
Remember, capital Q
was one of the metrics
that we were
talking about in 3.2
that help us describe the
performance of a plasma, how
good it's doing on
the path to fusion.
And so later on in
the series, we're
going to be discussing
the SPARC tokamak, which
is something Alex might want
to say a little bit more
about before we leave, and
the field path to Fusion
that MIT is pushing ahead with.
So yes, as Anne
mentioned, MIT would
like to build this
machine called SPARC.
And the idea of SPARC is
basically take the path
of Alcator C-Mod-- take this
high-field compact path--
and scale it up just
a little bit bigger,
and use some technology that has
only recently been developed.
So you may at some point hear
about yttrium barium copper
oxide, this high
temperature superconductor.
They'll hear about it.
They'll hear about it.
Yes, we're going to
talk about that in 3.3.
Excellent.
And we'll use this new material,
this new superconducting
material, to build a tokamak,
this SPARC tokamak, which
will have fields that are much
higher than even the 5, or even
8, tesla in Alcator C-Mod--
maybe up to 11 or 12 tesla.
And with this really
powerful magnetic field,
you can have a
relatively small device
which confines the
plasma well enough
to get this capital Q, this
break even point of greater
than 1, and generate net
energy from your tokamak
for the first time in the world.
So we really have a
blend of going forward
with the basic plasma physics
and the basic tokamak physics
that we and other
labs around the world
have been establishing
for decades.
Combining that with new
technologies recently
available, we can accelerate
the path to fusion--
fusion in time to make a
difference for humanity.
So with that, Alex, I think
we should tell everyone,
thanks for joining us.
Thanks for joining us.
And goodbye, everybody.
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