Dennis Whyte is a nuclear scientist at MIT and the director of the MIT Plasma Science and Fusion Center. Please support this podcast by checking out our sponsors:
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EPISODE LINKS:
Dennis's Twitter: https://twitter.com/MIT_Fusion
Dennis's LinkedIn: https://linkedin.com/in/dennis-whyte-33474a54
Dennis's Website: https://www.psfc.mit.edu/whyte
SPARC: https://www.psfc.mit.edu/sparc
MIT Plasma Science and Fusion Center: https://www.psfc.mit.edu
MIT Plasma Science and Fusion Center's YouTube: https://youtube.com/@mitplasmascienceandfusionc6211
Commonwealth Fusion Systems: https://cfs.energy
Commonwealth Fusion Systems YouTube: https://www.youtube.com/@CommonwealthFusionSystems

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OUTLINE:
0:00 - Introduction
0:32 - Nuclear fusion
18:31 - e=mc^2
32:58 - Fission vs fusion
38:10 - Nuclear weapons
41:56 - Plasma
49:07 - Nuclear fusion reactor
1:04:27 - 2022 nuclear fusion breakthrough explained
1:25:04 - Magnetic confinement
1:44:14 - ITER
1:49:01 - SPARC
2:03:00 - Future of fusion power
2:11:33 - Engineering challenges
2:30:14 - Nuclear disasters
2:34:58 - Cold fusion
2:49:14 - Kardashev scale
2:58:38 - Advice for young people

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That it's overcoming the rate at which it's leaking heat to the outside world, and at some point it just becomes like a star.

Like a sun, our own sun and a star doesn't have anything plugged into it, it's just keeping itself hot through its own fusion reactions.

In the end, that's really close to what a fusion power plant would look like.

Does it look like, like you said, like purple plasma?

Yeah, actually it's invisible to the eye because it's so hot that it's basically emitting light in frequencies that we can't detect.

In fact, light goes through it, visible light goes through it so easy that if you were to look at it, what you would see in our own particular configuration, what we make is in the end is a donut-shaped, it's a vacuum vessel to keep the air out of it.

And when you turn on the plasma, it gets so hot that most of it just disappears in the visible spectrum.

And there's very, very cold plasma, which is between 10 and 100,000 degrees, which is out in the very periphery of it, which is kind of, so the very cold plasma is allowed to interact with the, kind of has to interact with something eventually at the boundary of the vacuum vessel.

And this kind of makes a little halo around it, and it glows this beautiful purple light basically.

And these are, that's the, that's what we can sense in the human spectrum.

I remember reading on a subreddit called Shower Thoughts, which people should check out.

It's just fascinating philosophical ideas that strike you while you're in the shower.

And 1 of them was, it's lucky that fire, when it burns, communicates that it's hot using visible light.

I don't know if there's a deep or profound truth to that, but Nevertheless, I did find it on Shower Thoughts subreddit.

Actually, I do have, this goes off on a bit of, you're right, this is actually, it's interesting, because as a scientist, you also think about evolutionary functions and how we got, like why do we have the senses that we do?

Like why can bees see in the ultraviolet and we can't?

Then you go, well, it's natural selection.

For some reason, this wasn't really particularly important to us, right?

Why can't we see in the infrared and other things can?

It's like, It's a fascinating question, right?

Obviously, there's some advantage that you have there that isn't there, and even colour distinguishing, right, of something safety, ease, non-safety, whatever it would be.

I'll actually go back to this because it's something that I tell all of my students when I'm teaching ionising radiation and radiological safety, Whatever you say, there's a cultural concern or that when people hear the word radiation, like what does this mean?

It literally just means light is what it means, right?

But it's light in different parts of the spectrum, right?

And so it turns out besides the visible light that we can see here, we are immersed in almost the totality of the electromagnetic spectrum.

There is visible light, there's infrared light, there is microwaves going around us, that's how our cell phone works, you can't, it's way past our detection capability.

But also higher energy ones, which have to do with ultraviolet light, how you get a sunburn, and even x-rays and things like this at small levels are continually being, like from the concrete in the walls of this hotel, there's x-rays hitting our body continuously.

I can bring out, we can go down to the lab at MIT and bring out a detector and show you.

By our body you mean the 10 to the 28 atoms?

Yeah, the 10 to the 28 atoms, and they're coming in and they're interacting with those things.

And those, particularly the ones where the light is at higher average energy per light particle, those are the ones that can possibly have an effect on human health.

So we, it's interesting, humans and all animals have evolved on Earth where we're immersed in that all the time.

There's natural source of radiation all the time, yet we have 0 ability to detect it, like 0.

Yeah, and our ability, cognitive ability to filter it all out and not give it down.

It would probably overwhelm us, actually, if we could see all of it.

But my main point goes back to your thing about fire and self-protection.

If ionizing radiation was such a critical aspect of the health of organisms on Earth, we would almost certainly have evolved methods to detect it, and we have none.

The physical world that's all around us, it's just incredible.

Okay, so you have experience with magnetic confinement, you have experience with inertial confinement, most of your work has been in magnetic confinement.

But let's sort of talk about the sexy recent thing for a bit of a time, there's been a breakthrough in the news that laser-based inertial confinement was used by DOE's National Ignition Facility at the Lawrence Livermore National Laboratory.

Can you explain this breakthrough that happened in December?

Yeah, so it goes to the set of criteria that I talked about before about getting high energy gain.

So in the end, what are we after in fusion is that we basically assemble this plasma fuel in some way and we provide it a starting amount of energy, think of lighting the fire, and what you want to do is get back significant excess gain from the fact that the fusion is making more, is releasing the energy.

So it's like the equivalent of We want to have a match, a small match, light a fire, and then the fire keeps us hot.

So as I said, we've made many of the, and what do I mean by we?

It's like the fusion community has pursued aspects of this through a variety of different confinement methodologies, is that the key part about what happens, what was the threshold we had never gotten over before was that if you only consider the plasma fuel, not the total engineering system, but just the plasma fuel itself.

We had not gotten to the point yet where basically the size of the match was smaller than the amount of energy that we got from the fusion.

Is there a good term for when the output is greater than the input?

Well, there's several special definitions of this.

So 1 of them is that if you, like in a fire, if you light a match and you have it there and it's an infinitesimal amount of energy compared to what you're getting out of the fire, we call this ignition, which makes sense, right?

This is like what our own sun is as well too.

So that was not ignition in that sense as well too.

So what we call this is scientific, what the 1 that I just talked about, which is for some instance, when I get enough fusion energy released compared to the size of the match, we call this scientific break-even.

Break-even, and it's because you've gotten past the fact that this is unity now at this point.

What is fusion gain or as using the notation Q from the paper review of the Spark talk before, using just the same kind of term?

Yeah, actually, so the technical term is Q, capital Q.

We actually use capital Q, or sometimes it's called Q...

What it means is that it's in the plasma.

So all we're considering is the energy balance or gain that comes from the plasma itself.

We're not considering the technologies which are around it, which are providing the containment and so forth.

Well, because for 1 reason, it's a rather simple threshold to get over to understand that you're getting more energy out from the fusion, even in a theoretical sense, than you were from the starting match.

It's conceptually simple that you get past 1, that everyone, like when you're less than 1, that's much less interesting than getting past 1,

because obviously you start to get energy gain.

But it really is a scientific threshold because what Qp actually denotes is the relative amount of self-heating that's happening in the plasma.

So what I mean by this is that in the end, in these systems, what you want is something that where the relative amount of heating, which is keeping the fuel hot, is dominated by, from the fusion reactions themselves.

And so it becomes, it's sort of like thinking like a bonfire is a lot more interesting physically than just holding a blowtorch to a wet log, right?

There's a lot more dynamics, it's a lot more self-evolved and so forth.

And what we're excited as scientists is that it's clear that in that experiment that they actually got to a point where the fusion reactions themselves were actually altering the state of the plasma.

I mean, we'd seen it in glimpses before in magnetic confinement at relatively small levels, but apparently it seems like in this experiment it's likely to be a dominant, dominated by self-heating.

That's a very important, that's a very interesting thing.

It's more self-sustaining, it's more self-referential system in a sense.

Again, it's not that it's gonna evolve to a dangerous state, it's just that we want to see what happens when the fusion is the dominant heating source.

And we'll talk about that, but there's also another element which is the inertial confinement, laser-based inertial confinement is kind of a little bit of an underdog.

So a lot of the broad nuclear fission community has been focused on magnetic confinement.

Can you explain just how laser-based inertial confinement works?

So it says 192 laser beams were aligned on a deuterium tritium DT target smaller than a pea.

But this is like throwing a perfect strike in baseball from a pitch.

This is like a journalist wrote this I think.

This is like, oh no it's not a journalist, it's DOE

This is like throwing a perfect strike in baseball from a pitcher's mount 350 miles away from the plate.

The United States Department of Energy wrote this.

Actually there's usually mass confusion about this.

So what's going on in this form of energy?

So the fuel is delivered in a discrete, the fusion fuel, the deuterium and tritium is in a discrete spherical, it's more like a BB, let's call it a BB, so it's a small 1.

And all the fuel that you're going to try to burn is basically there, okay, and it's about that size.

So how are you going to get, and it's literally, it's like at 20 degrees above absolute 0 because the deuterium and tritium are kept in a liquid and solid state.

Oh wow, so the fuel is injected not as a gas, but as a solid.

It's actually, and these particular experiments they can introduce 1 of these targets once per day approximately, something like that.

Cause it's very, it's very, it's a kind of amazing technology actually that I know some of the people that worked on this back in the days, they actually make these things at a BB size of this frozen fuel, it's actually at cryogenic temperatures and they're almost like smooth to the atom level.

I mean, they're amazing pieces of technology.

So what you do in the end is think, what you have is a spherical assembly of this fuel, like a ball, and what is the purpose of the lasers?

The purpose of the lasers is to provide optical energy to the very outside of this, and what happens is that energy is absorbed because it's in the solid phase of matter, so it's absorbed really in the surface.

And then what happens is that when it's absorbed in something called the ablator, What does that mean?

It means it goes instantly from the solid phase to the gas phase.

So it becomes like a rocket engine, but you hit it like very uniformly.

So all, there's like rocket engines coming off the surface.

Think of like an asteroid almost where there's like rockets coming off and all this thing.

It actually pushes, by Newton's laws, right, it pushes the other thing on the other side of it, equal and opposite reaction, it pushes it in.

So what it does is that the lasers actually don't heat.

People think the lasers, oh, we're gonna get it to 100 million degrees.

In fact, you want the exact opposite of this.

What you want to do is get essentially a rocket going out like this, and then what happens is that the sphere, and this is happening in a billionth of a second or less, actually, this rapidly, that force so rapidly compresses the fuel that what happens is that you're squeezing down on it and it's like, what was the...

Goes from a basketball down to something like this, and a billionth of a second.

And when that happens, I mean, scale that in your mind.

So when that happens, and this comes from almost from classical physics, so there's some quantum in it as well too, but basically if you can do this like very uniformly and so-called adiabatically, like you're not actually heating the fuel, what happens is you get adiabatic compression such that the very centre of this thing all of a sudden just spikes up in temperature because it's actually done so fast.

It's because you're doing this on such fast timescales that the inertia of the hot fuel basically is still finite, so it can't push itself apart before the fusion happens.

This is why you use lasers, because you're applying this energy in very, very short periods of time, like under a fraction of a billionth of a second.

And so basically that, and then the force which is coming from this comes from the energy of the lasers, which is basically the rocket action which does the compression.

Well, So is the force, is the inward facing force, is that increasing the temperature exponentially?

wanna keep the fuel cold and then just literally just ideally compress it.

And then in something which is at the very center of that compressed sphere, because you've compressed it so rapidly, the laws of physics basically require for it to increase in temperature.

The effect is like, if you know the thing, so adiabatic cooling we're actually fairly familiar with, if you take a spray can, right, and you push the button, when it rapidly expands, it cools.

This is the nature of a lot of cooling technology we use actually.

Well, the opposite is true that if you would take all of those particles and jam them together very fast back in, they want to heat up and that's what happens.

And then what happens is you basically have this very cold compressed set of fusion fuel and at the centre of this, it goes to this 100 million degrees Celsius.

And so if it gets to that 100 million degrees Celsius, the fusion fuel starts to burn.

And when that fusion fuel starts to burn, it wants to heat up the other coal fuel around it.

And it just basically propagates out so fast that what you would do ideally, you would actually burn in a fusion sense, most of the fuel that's in the pellet.

So this was very exciting because what they had done was it's clear that they propagated this, they got this what they call a hot spot, and in fact that this heating had propagated out into the fuel, And that's the science behind inertial fusion.

So the idea behind a reactor that's based on this kind of inertial confinement is that you would have a new BB every like.

10 times a second or something like that.

And there's some kind of, so there's an incredible device that you kind of implied that kind of has to create 1 of those BBs that.

There's reports on this, but about what does it mean, you know, the starting point is can you make this game?

So this was a scientific achievement primarily because – Right,

The rest is incredibly complicated engineering.

Well, in fact, there's still physics hurdles to overcome.

And it's actually because If you want to make an energy source out of this, this had a gain of around 1.5, that namely the fusion energy was approximately 1.5 times the laser input energy.

Okay, this is a fairly significant threshold.

However, from the science of what I just told you is that there's 2 fundamental efficiencies which come into it, which really come from physics really.

What I mean by this is that it's a rocket.

So it just has a fundamental efficiency built into it, which comes out to orders of like 10%.

So this means is that your ability to do work on the system is just limited by that.

And then the other 1 is the efficiency of laser systems themselves, which if their wall plug efficiency is 10%, you've done spectacularly well.

In fact, the wall plug efficiency of the ones using that experiment are more like 1%, right?

So when you go through all of this, the approximate place that you're ordering this is for a fusion power plant would be a gain of 100, not 1.5.

So you still, you know, and hopefully we see experiments that keep climbing up towards higher and higher gain, but then the whole fusion power plant is a totally different thing.

So it's not 1, it's not 1 BB and 1 laser pulse per day, it's like 5 or 10 times per second.

Like, da-da-da-da-da-da-da-da-da-da, like that, right?

And then comes the other aspect, so it's making the targets, delivering them, being able to repeatedly get them to burn.

And then we haven't even talked about like how do you then get the fusion energy out, which is mainly because these things are basically micro implosions which are occurring, so this energy is coming out to some medium on the outside that you've got to figure out how to extract the energy out of this thing.

How do you convert that energy to electricity?

So in the end, you have to basically convert it into heat in some way.

So most of the, in the end, what fusion makes mostly is like very energetic particles from the fusion reaction.

So you have to slow those down in some way and then make heat out of it.

So basically the conversion of the kinetic energy of the particles into heating some engineered material that's on the outside of this.

And that's, from a physics perspective, is a somewhat solved problem, but from an engineering is still...

Yeah, it's physics, I can show you all the equations that tell you about how it slows down and converts kinetic energy into heat, and then what that heat means, you know, you can write out like an ideal thermal cycle, like a Carnot cycle.

The integrated engineering of this is a whole other thing.

I'll ask you to maybe talk about the difference between inertial and magnetic, but first we'll talk about magnetic.

But let me just linger on this breakthrough.

You know, it's nice to have exciting things, but in a deep human sense, there's no competition in science and engineering.

First of all, we are humanity altogether, and you talk about this, it's a bunch of countries collaborating, it's really exciting.

There's a nuclear fusion community broadly, but then there's also MIT, there's colors and logos and it's exciting.

And you have friends and colleagues here that work extremely hard and done some incredible stuff.

Is there some sort of, how do you feel seeing somebody else get a breakthrough using a different technology?

I have, so to just to wave the flag a little bit, so MIT was a central player in this accomplishment.

Interesting, I would say it showed some of our 2 best traits.

So 1 of them was that the, like, how do you know that this happened?

So 1 of the ways to do this is, if I told you is that in the DT fusion, what it actually, the product that comes out is helium, we call an alpha, but it's helium, and a free neutron, right.

So the neutron contains 80% of the energy released by the fusion reaction and it also, because it lacks a charge, It basically tends to just escape and go flying out.

So this is what we would use eventually for, that's mostly what fusion energy would be.

But so what my colleagues, my scientific colleagues at the Plasma Science and Fusion Centre built were extraordinary measurement tools of being able to see the exact details of not only the number of neutrons that were coming out, but actually what energy that they're at.

And by looking at that configuration, it reveals enormous, I'm not going to scoop them because they need to publish the paper, but it reveals enormous amounts of scientific information about what's happening in that process that I just described.

I mean, and I have colleagues there that have worked for 30 years on this, for that moment.

I mean, and it's 1 of those, like, there is nothing, it's hard to describe to people who aren't, it's like almost addicting to be a scientist when you get to be at the forefront of research of anything.

Like when you see like an actual discovery of some kind and you're looking at it, particularly when you're the person who did it, right?

And you go, no human being has ever seen this or understood this.

So even in proxy, It's incredibly thrilling to see this.

It's not, I don't wanna say, it's rivalry or jealousy.

It's like, I can tell you already, fusion is really hard.

So anything that keeps pushing the needle forward is a good thing, but we also have to be realistic about what it means to making a fusion energy system.

fun, I mean, these are still the early steps, or maybe you could say the early leaps.

So let's talk about the magnetic confinement.

What is, how does magnetic confinement work?

So why inertial confinement works on the same principle that a star works.

So like what is the confinement mechanism in the star is gravity because it's own inertia of something the size of the sun basically pushes literally a force by gravity against the centre.

So the centre is very, very hot, 20 million degrees, and literally outside the sun it's essentially 0 because it's a vacuum of space.

It does that by, and it's out of, like why doesn't it just leak all of its heat?

It doesn't leak its heat because it all is held together by the fact that it can't escape because of its own gravity.

So this is why the fusion happens in the center of the star.

Like we think of the surface of the sun as being hot, that's the coldest part of the star.

So of our own sun, this is about 5500 degrees, a beautiful symmetry by the way, it's like, so how do we know all this?

Because we can't of course see directly into the interior of the sun, by knowing the volume and the temperature of the surface of the sun, you know exactly how much power it's putting out.

And by this, you know that this is coming from fusion reactions occurring at exactly the same rate in the middle of the sun.

Is it possible as a small tangent to build an inertial confinement system like the sun?

It is, of course, possible to make a sun, although we do have stars, but it is not impossible on Earth because for the simple reason that it takes, the gravitational force is extremely weak and so it takes something like the size of a star to make fusion occur in the center.

I mean, if you had to build like a second sun, how would you do it?

You can't, there's not enough hydrogen around.

So the limiting factor is just the hydrogen.

Yeah, I mean, the forces and energy that it takes to assemble that is just mind-boggling.

So in the 1 that I just described, it's like you say, so you have to replace this with some force, which is better than that.

And so what I mean by that is it's stronger than that.

So what I talked about the laser fusion, this is coming from the force, which is enormous compared to gravity, like from the rocket action of pushing it together.

So in magnetic confinement, we use another force of nature, which is the electromagnetic force.

And that's very, it's orders and orders of magnitude stronger than the gravitational force.

And the key force that matters here is that if you have a charged particle, that namely it's a particle that has an electric, net electric charge, and it's in the proximity of a magnetic field, then there is a force which is exerted on that particle.

So it's called the Lorentz force for those that are keeping track.

So that is the force that we use to replace physical containment.

So this again, how do you hold something at a hundred million degrees?

This is not like, you know, it's not this plastic bottle holding in this liquid or a gas chamber.

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Dennis Whyte: Nuclear Fusion and the Future of Energy | Lex Fridman Podcast #353