Nuclear fusion underpins some of the most basic processes in our universe and holds the promise of virtually limitless, clean, carbon-free energy. Dr. Anne White, Professor of Nuclear Science and Engineering at The Massachusetts Institute of Technology, has been challenged to explain the nature of nuclear fusion to 5 different people; a child, a teen, a college student, a grad student, and an expert.
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00:00 My name is Anne White. I'm a professor of nuclear science and engineering at MIT,
00:04 and I've been challenged today to explain nuclear fusion in five levels of increasing difficulty.
00:09 Fusion is so exciting because it is extraordinarily beautiful physics,
00:14 which underpins some of the most basic processes in our universe. Nuclear processes has a tremendously
00:21 valuable application for humankind, a virtually limitless, clean, safe, carbon-free form of energy.
00:28 [Music]
00:31 What's your name? Tell me a little bit about yourself.
00:34 I'm Amelia. I'm nine years old. I'm in third grade,
00:37 and my favorite subject at school is definitely science.
00:40 So my son is five years old, and he asked me what kind of science I do, and I said fusion.
00:47 And I said, "I put a star in a jar." Does that make sense?
00:50 No.
00:52 That's a good answer, because it sounds a little ridiculous, right?
00:57 How can we put a star in a jar? Well, we're not actually going to put the sun, which is a star,
01:03 inside a jar, but instead we're going to take the same kind of material that the sun is made out of,
01:09 and we're going to hold it for a really long time in some kind of container.
01:15 So fusion is about bringing things together. That's what fusing means. When those fusion
01:22 reactions occur, a new particle is created, and energy is also released. Do you know what an atom
01:28 is? No. All right. So an atom is what everything in our world is made of, and at the very center
01:35 of the atom is what we call a nucleus, and inside that nucleus is a proton. We want to take those
01:43 protons and push them together to make them combine and release energy, fusion energy,
01:50 that we can use to make electricity, and there's a lot of different energies and forces that we
01:56 have to think about. Have you heard of gravity? Yes. Yes. Okay. So that is a big, important,
02:01 fundamental force. So another fun force to talk about that's important for fusion,
02:06 you're familiar with electricity? Yes. Right? And so there's also electric forces, electrostatic
02:13 forces, and you've heard of static electricity. So now let's see about static electricity lifting
02:18 my hair up. You can move this white strand that's like hanging down. The balloon took all the force
02:27 from like your hair and put it in here now. Just like move it. There you go. Yeah. And so if we
02:33 want to take those protons and push them together to make them combine and release energy, fusion
02:42 energy that we can use to make electricity, then we have to actually overcome that strong
02:48 electrostatic force that just want to make those balls bounce off of each other. There's another
02:52 force which you might be familiar with, which is like a magnetic force. We just learning about
02:58 that. Our teacher showed us putting one magnet on and then flipping the other one around and
03:03 made the top one kind of bounce. Yeah. And I was also thinking on how they do that.
03:09 So scientists are still studying exactly how magnetism works, right? It'll still be there for you to
03:14 tackle when you become a scientist. Have you ever seen one of these games? Yes. With the iron
03:21 filings. So if you take this and you take the magnetic end, maybe you can show us what's going
03:26 to happen with it while you move around those iron filings with the magnet. You're totally in control
03:32 of that material. You're pushing, you're pulling it, you're moving it around. And so you're using
03:38 this magnetic force to also do something useful for you. Have you learned about the states of
03:43 matter? Yes. Tell me about that. So we're in second grade and she put up a picture on the board,
03:50 three states of matter. She shows a picture of ice, a picture of water, and a picture of gas.
03:55 Did you learn that there's also a fourth state of matter? No. When you heat up a gas, you create a
04:02 plasma. A plasma is the fourth state of matter. The plasma I study is actually invisible. That's
04:09 really hard science. And the plasmas I work with are so hot that I can't see with my eyes,
04:15 but it's light that I can measure with very, very special instruments. What kind of instruments?
04:20 Because instruments we use to play music. That's a really great point. How do you keep the invisible
04:28 plasmas? Yes, they're invisible. You keep them in one spot so you can show where they are.
04:33 Yes, we absolutely do. We hold it inside the container with the magnetic fields. So you
04:38 didn't have to actually touch the iron filings in the toy to move them around. You could pass
04:45 the magnetic field through the plastic and control them with it. So it's the same thing. We don't have
04:50 to touch this very, very hot plasma to control it and hold it in place because we use magnetic
04:56 fields. You are so smart. I'm so glad that science is your favorite subject.
05:01 What is fusion energy? The way our sun generates energy is by fusion reactions. It fuses hydrogen,
05:12 the lightest element we know about, into helium, and that gets fused into heavier and heavier
05:16 elements. So here on Earth, we're going to take some special kinds of hydrogen, a special flavor
05:22 of it, if you will, which we call an isotope, and we're going to combine them to create new particles.
05:28 And we can only get that combination of particles to happen if they are in a plasma. What's your
05:35 favorite exhibit at the Science Museum? I love the lightning show. I think it's so cool. You probably
05:40 have learned in school about three states of matter. Solid, liquid, and gas. Absolutely. We
05:44 take the gas and we add heat and we get a plasma. And a plasma is a state of matter where you have
05:50 an ionized gas. If we break down that gas, if we add enough energy to ionize it, we can take the
05:56 electrons and the ions and the atom and separate them. And now there's this soup of charged particles
06:02 that are moving around. That's the plasma. And it is what creates the beautiful light in lightning.
06:08 So you've already seen a plasma, in fact. So I'm going to show you this
06:12 fun demonstration. You've probably seen one of these before, right? Oh, that's so cool.
06:19 Yeah. So the way this is happening is this glass ball here is a container for our plasma.
06:25 And we've taken most of the air out of the container. So there's not a lot of particles
06:30 inside the glass ball and a very, very low temperature plasma. So it's continuously ionizing
06:36 and then recombining and becoming neutral again. And we see those energy transitions as the visible
06:42 light. So if we're going to put this plasma to use and do something helpful with it, like maybe make
06:47 some clean electricity, we would have to control it. And another word for controlling it is
06:53 confining it. So let me turn this off and set it back down. You're probably wondering, what is this
06:59 thing on this table? It's a model of a tokamak. And that's the name of the device that I work on
07:06 with the goal of creating clean energy. Have you played with magnets in school?
07:12 Okay. We've learned about how it has to be a positive and negative charge. And we've done
07:18 those things, you can like put them with something in between them and just move one and the other
07:22 will always follow. This is all very important to sort of understand how we would create a
07:28 container that would let us hold a plasma in place and control it. Have you ever played around with
07:33 an electromagnet in class? It's a coil of wire, much like this big red coil of wire right here.
07:38 And when we push an electrical current through this wire, it creates a magnetic field that goes
07:46 around the wire perpendicular. So if you want to know the direction of the magnetic field that's
07:51 being created by pushing the current through the wire, put your thumb in the direction of the
07:55 current and then curl your fingers like this. Yeah. And that's the right hand rule. So if we
08:00 push the current this way, we're creating a magnetic field in this perpendicular direction.
08:04 So if I drive a current in this red wire like this, which direction will the magnetic field go?
08:10 Yeah, exactly. Perpendicular. And if I drive the current in this green wire,
08:15 which direction will it go? Exactly. Yeah. The long way perpendicular. Now this is a bit of
08:22 a trickier one. The blue wire is going to act like a transformer action. And so by changing
08:27 the current in the blue coil, we are going to be able to run a current in this direction
08:32 around the tokamak. And now think back to how the wires worked. If I have a current going like this,
08:38 where's the magnetic field? Back this way. Exactly. Back this way, the short way around
08:43 the tokamak. We can now put together the pieces and understand the three magnetic fields that we
08:48 need to confine a plasma in our tokamak. So our plasma will be inside this vessel
08:53 in the shape of a donut. What could the tokamak be used for in like real life?
08:58 Oh, I'm so glad you asked. So what we want to use the tokamak for in real life is to confine
09:03 this super hot plasma. And we're talking 100 million, 150 million degrees. Because the plasma
09:09 is so very hot, the particles have enough energy to interact with one another and fuse. When those
09:16 fusion reactions occur, we are releasing energy that's inside the nucleus. And we can harness
09:23 that energy to make clean electricity. So what have you heard about fusion already before today?
09:33 The impeding joke is that, you know, we've looked forward to fusion for a long time,
09:37 but you're not exactly gotten there yet. But if we do ever get there, it would solve a lot of
09:42 our energy problems in a dramatic way. Do you have any idea about any of the
09:46 challenges? Like why has it taken us so long to get to fusion?
09:50 Making a star out of it is not easy.
09:53 So we are trying to bring a star to Earth. We are not going to be using hydrogen the way our star
10:00 in our solar system, our sun, uses hydrogen to make helium and generates fusion energy that way.
10:05 Instead, on Earth, we're going to be using isotopes of hydrogen, deuterium and tritium.
10:10 What do you know about charged particles? If I want to try and push two positively charged
10:16 particles together, two protons together, what do you think is going to happen?
10:19 They repulse each other and they don't like being close together. So they
10:22 push back on that force.
10:25 Or we're going to call the pushback is a Coulomb interaction or Coulomb collision.
10:29 So you can sort of imagine if I were to take a deuteron and a triton. And so those are the
10:34 positively charged ions of deuterium and tritium. And I try and combine them together. Those two
10:40 positively charged particles just sort of bounce off of each other. So we have to give them
10:45 enormous amounts of energy. And it has to do with getting up to very high temperatures. So
10:50 we're talking about over 100 million degrees Celsius. And we typically put that into an
10:57 energy unit that we use a lot in plasma physics called an electron volt. And so we describe being
11:02 up at 100 million degrees that we're at sort of 15 kilo electron volts. That's very, very hot
11:08 temperature. But the other thing we need is a lot of particles. That's the density. We are able to
11:12 combine a deuteron and a triton in a fusion reaction at lower temperatures at lower energies
11:19 than other fuel. And this has to do with some very nice properties of the deuteron and the triton
11:24 that when we get them close enough to one another to fuse, there's actually a resonance, which is
11:29 predicted by quantum mechanics. And that really helps have a little bump up in the cross section
11:35 for the deuterium tritium fusion reaction.
11:38 - Compared to just hydrogen.
11:40 - Yes, exactly. Exactly. That little bump up is good for us because it means that we have a
11:46 higher probability of getting the deuterium and the tritium to fuse than otherwise at those
11:51 manageable temperatures. And when we say manageable for fusion scientists, yeah,
11:55 50 million, 100 million, 150 million Celsius.
11:58 - So the problem you described is that we get to those high temperatures, we have dense plasma,
12:04 but the problem is the hotter the plasma is, more likely is the heat to get sucked out of it by-
12:10 - Absolutely. Yeah, absolutely.
12:12 - So the plasma itself is not staying hot enough for the time we need it to stay.
12:17 - We've come so far in the study of magnetically confined plasmas, which is what I work on,
12:23 that we sort of tamed all the other types of major instabilities that would cause loss of the plasma.
12:29 So you might be asking yourself, what is the energy that's coming out of the fusion reaction?
12:34 So we've got the deuteron and we've got the triton, and so they combine in a fusion reaction,
12:39 and that produces a neutron and a helium nucleus, but the neutron doesn't have any charge.
12:47 - Yeah, it comes out.
12:48 - Exactly. So it comes right out and it's the kinetic energy of the neutron, and we want it
12:54 to interact with our overall energy system. And as it interacts with that material, it heats the
12:59 material up. It transfers its kinetic energy to this material. Take that thermal energy and run
13:05 a turbine, run a generator, and convert it into electricity. So once you get to that stage,
13:11 it starts to look a lot like any other thermal power plant, whether it's fission or natural gas.
13:16 So a fusion plant could basically be the plasma core coming in, setting it in place, and driving
13:22 your thermal system to electricity. We often call it an alpha particle. And that is a charged
13:28 particle, right? So it's actually going to stay in the plasma. It's an energetic particle compared
13:34 to the fuel. So it actually is going to give its kinetic energy back to the fuel via Coulomb
13:42 collisions. So now they're good. Now we like them. - So you get this kind of self-sustaining cycle.
13:47 - Yes. You said exactly the right word, self-sustaining.
13:53 I am in soft condensed matter physics, and my research kind of dips into material science,
13:59 but I feel like people are always asking me about fusion. - What are they asking you about fusion?
14:04 - So usually people ask me, like, do you think that we'll ever really replace all of our other
14:09 energy sources with fusion? - I think that it actually has a lot of mystery around it,
14:15 because the fuel for fusion is a plasma. And we don't experience plasmas on Earth in our
14:22 everyday life. They exist in space at the event horizon of a black hole, in the solar wind,
14:27 in our sun, or very rapid events like lightning is also sort of a very weakly ionized plasma.
14:33 Even among plasmas, there are so many different kinds of plasmas. There are low temperature,
14:40 higher density plasmas. There are, of course, the astrophysical plasmas and space plasmas. And then
14:46 there are fusion plasmas. They are predominantly fully ionized plasmas. They are also plasmas where
14:57 we have a certain ability to basically kick up micro-instabilities. So they're plasmas which
15:05 are held in a stable enough state by strong external magnetic fields, confining the plasma
15:11 into a donut shape. And this has a lot of advantages for us because charged particles
15:18 want to follow the magnetic field lines. But things start to get really interesting when we're
15:23 no longer thinking about individual particle motions in the plasma. And instead, we start to
15:28 think about collective effects. - It's never occupied any space in my mind to think about
15:32 what happens when you have something so high temperature and like precisely confined. And
15:39 now you have to deal with presumably turbulence. - Plus magnetic fields. When we start thinking
15:45 about turbulence in the plasma, we can no longer even think about the plasma as a single fluid.
15:51 Instead, we have to consider electron fluid and ion fluid separately. We have to use a full-blown
15:58 kinetic equation to explain how this state of matter is behaving because we have collisions.
16:04 So we have to add collisions back in to understand and track how all the particles are moving and how
16:10 these collective motions, this turbulence can get kicked up. So that's pretty intractable, right?
16:17 I mean, if people talk about simulating that system and following those particles,
16:21 it's probably going to take millions and millions of years on even the fastest supercomputer.
16:26 So one really big advance in plasma theory over the last, I'd say three or four decades,
16:32 has been the development of a gyrokinetic theory that we use to model the microturbulence
16:40 in the plasma and get that under control. And the reason it's so important to get the turbulence
16:45 under control and understand it is because turbulence is the primary heat loss mechanism,
16:51 the primary way that heat is transported from hot to cold across confining field lines in a magnetic
16:58 confinement system. Being able to study it, measure it, and predict how it's going to behave is
17:02 really one of the big hurdles to overcome.
17:06 Could you say the name of the model again?
17:08 Absolutely. So it's a gyrokinetic model. And we talked about how challenging it would be to follow
17:13 every particle in space and know its position and know its velocity at all times. So what
17:19 gyrokinetics actually does as a theory is it takes advantage of the fact that when we drop a charged
17:24 particle into a strong external magnetic field, the Lorentz force bends that particle's trajectory
17:30 into a helix. And so now if we know that wherever the field line is going, that particle is following
17:37 it in this helical, in this corkscrew trajectory, we can say, "Aha, I no longer have to worry about
17:43 following that particle's velocity around in a circle." Because at every point in time,
17:49 I know it's going in a circle. So we average that out. We do a gyro average
17:54 because the motion is typically called a gyrofrequency. That's how fast it goes around
17:59 the field line. And it has a particular radius of that helix called the gyroradius because it's just
18:05 gyrating. So what we know from studying the plasma and making direct measurements of the turbulence
18:10 and also what comes from the simulations is the scale size of the turbulence is about 5 to 10
18:16 gyroradii. You said that density and temperature fluctuations are what drive these turbulent
18:22 flows that end up reducing your heat transport. Is there anything that can be done to minimize
18:28 those density and heat fluctuations or is that just down to the statistics of things?
18:33 I love the way you framed it because originally, in the '60s and '70s, people did not think that
18:38 microturbulence would even be a problem. But as we started to make more and more measurements and
18:42 build higher and higher, basically performing devices, we started to see nothing is matching
18:48 the expected performance. And that's because people thought that Coulomb collisions between
18:52 the particles, just interactions of charged particles, would dominate cross-field transport.
18:57 What happens with turbulence is it enhances the transport of particles because now we're not just
19:02 talking about this random walk of collisions. We're talking about conduction, convection,
19:07 eddies, structures, microstructures, flow generation, very complex soup of activity.
19:14 Turbulence for me really hits on one of the most beautiful parts about physics. It's so
19:20 complex and that's what makes it visually beautiful. That's what makes it mathematically
19:25 interesting. And it's also what keeps us so puzzled about it.
19:30 Yeah, turbulence is beautiful and so fun to study.
19:32 I'm a research scientist at MIT and I work on computational plasma physics,
19:40 basically doing simulations that can accurately describe what's going on inside these fusion
19:46 reactors like tokamaks and stellarators. They have plasmas that are magnetically confined,
19:51 so we're trying to predict how the plasma behaves so that we can build in the future better reactors.
19:56 What's one of the most exciting parts of your research right now?
20:00 Something that we were not able to do until very recently was actually using first principle
20:06 simulations to predict the performance and efficiency of reactors. The developments in
20:13 plasma theory and computation and simulation that has been thoroughly validated over the years in
20:19 many experiments that now we're using those simulations to inform how to best operate our
20:25 future reactors. Very exciting because so far we've been getting great results. It's very,
20:30 very promising. Where we're going with a lot of the experiments right now is trying to produce
20:36 some maybe outside the box datasets that we haven't seen before. And then of course, ultimately
20:41 compare them to the simulations and do a bit of this validation maybe where we're not just looking
20:46 into the lampposts, where we're going a little bit outside the comfort zone. That means going
20:50 from measurements really sort of more in the middle of the plasma at about mid radius, pushing
20:56 all the way out to the edge where the turbulence starts to become very different in its nature.
21:01 It becomes a lot more electromagnetic. It becomes sometimes larger in scale, just physical scale
21:07 size. And some of the things we're starting to find was that turbulence features and turbulence
21:13 characteristics in the edge of some of these high performance plasmas don't always behave the way
21:19 we think they do. So as we think about pushing our measurements and our study of the turbulence from
21:25 the core to the edge, how does that influence what you're working on now? The edge of the plasma
21:30 gives you the boundary condition really for the simulations that then we do in the core. You need
21:35 to start somewhere determining what is the temperature very close to the wall really of
21:41 the machine. And when you get that temperature, then you can actually integrate inwards with the
21:45 rest of core model. It's going to be very exciting in the next years when we can actually
21:49 make some measurements in those devices and compare them to simulations so that we can
21:54 have more trust in the predictions for the next step, for the reactors, the power plants.
22:00 Maybe both of us in our own way answer the question that we always get asked,
22:05 when is fusion going to happen? When are we going to have fusion electricity on the grids?
22:09 It's hard to say when it's going to arrive. I think that with the arrival of private companies
22:14 and venture capital, that is accelerating things a lot. So I don't think fusion is 30 years away,
22:23 and it will always be. I don't think that's true anymore.
22:26 So you're saying lots of private companies have entered, and that's injected a lot of
22:31 private funding, not just government funding. Oh, yeah. The nature of private ventures is you want
22:39 to get commercial as soon as possible. So I think they're accelerating things. They are actually
22:43 taking advantages of discoveries in other fields, like in the case of high-field fusion with common
22:52 wave fusion systems and Tokamak Energy, those companies, they are using high-temperature
22:56 superconductors. It's an advancement that has come recently from material science,
23:00 or machine learning, artificial intelligence. Those breakthroughs in other fields, I think,
23:05 can really accelerate fusion. So I think we're seeing the next decades are going to be very
23:10 exciting. We have to diversify the different research that we do so that at the end,
23:15 we come with the most optimum solution for a fusion power plant.
23:18 I agree. Yeah, I think having multiple stakeholders who are all driven by different
23:22 missions and different purposes working synergistically is exciting. When I'm asked,
23:27 like, okay, what's the timeline for fusion? And why is now any different than five years ago or
23:32 10 years ago? Why is now the moment for fusion? My answer is it's finally for the first time,
23:38 all the pieces of the puzzle are here. We've advanced really the basic physics understanding
23:43 so far that we have got the predictive capabilities. But we also have alignment
23:48 with policy and science drivers that we didn't really have before. That's, I think,
23:52 what can get us there. Maybe a demonstration of net electricity in a decade. Is that the thing
23:57 folks are pushing for? We're pushing for it. Yeah, there are challenges still to overcome,
24:02 as you know, and hopefully we find solutions to those when we have new experiments and when we
24:06 actually push forward. Yeah, the potential is huge. Fusion energy research is an extraordinarily
24:14 exciting field that is pushing the frontiers of what we can do experimentally as well as what we
24:19 can do computationally. Fusion might be closer than we think and tremendous advances are being
24:23 made every day.