In Our Time - Nuclear Fusion

Episode Date: October 30, 2014

Melvyn Bragg and his guests discuss nuclear fusion, the process that powers stars. In the 1920s physicists predicted that it might be possible to generate huge amounts of energy by fusing atomic nuclei together, a reaction requiring enormous temperatures and pressures. Today we know that this complex reaction is what keeps the Sun shining. Scientists have achieved fusion in the laboratory and in nuclear weapons; today it is seen as a likely future source of limitless and clean energy.Guests:Philippa Browning, Professor of Astrophysics at the University of ManchesterSteve Cowley, Chief Executive of the United Kingdom Atomic Energy AuthorityJustin Wark, Professor of Physics and fellow of Trinity College at the University of OxfordProducer: Thomas Morris.

This BBC podcast is supported by ads outside the UK. Every Sunday, we talk about the week's tech news on this week in tech. Hi, this is Leo Leport. Inviting you to join me this week with Lisa Schmeiser, Dan Patterson, and Yanko Rekkers. We're going to talk about the new 49 megabyte web page. It's the standard, you know. We'll also talk about Elon Musk. You've got some spleenin to do and the Yassify filter, new from Nvidia.

That's this week on this week in tech. You'll find it at Twitter. or wherever you get your podcasts. Thank you for downloading this episode of In Our Time. For more details about In Our Time, and for our terms of use, please go to BBC.co.com.uk slash Radio 4. I hope you enjoy the program. Hello, we'll be talking about nuclear fusion.

If you crash together two nuclei of hydrogen, with enough force, you create a helium atom, a spare neutron, and a great deal of energy. This is nuclear fusion. And you get an idea of just how much energy it produces when you consider that it's the physical process that powers the sun. Nuclear fusion is a meeting point between research into atomic physics, astronomy, magnetic fields and material science. Scientists understand the physics of fusion well enough to make it happen in the laboratory,

but doing so involves heating gases to temperatures of more than 200 million degrees centigrade. Enthusiasts for fusion say that if we can make it work efficiently, it'll provide as much clean, cheap energy as we will ever need. But putting the theory into practice turns out to be extremely difficult. With me to discuss nuclear fusion are Justin Walk, Professor of Physics and Fellow of Trinity College at the University of Oxford. Philippa Browning, Professor of Astrophysics at the University of Manchester, and Steve Cowley, Professor of Plasma Physics at Imperial College London

and Chief Executive of the UK Atomic Energy Authority. Justin Walk, can you give us an idea of the structure in an atom that's relevant to nuclear fusion? Well, with the subject of nuclear fusion, obviously we're talking. talking about the nuclei of atoms, and so let me try and set the scene and set the scale. And to do that, let's use the microscope of our imaginations. And to start off with, to get a scale, let's envisage that our whole field of view has something that we would be familiar with, say something the size of the width of human hair.

We'll now magnify that by a factor of a million, and then you would get something that looks like the size of an atom. And if we could see an atom, it would look a little bit like a miniature solar system with the nucleus, a bit like the sun at the centre, which is positively charged, and the very light electrons which are negatively charged orbiting it. But this miniature solar system is held together by the forces of electromagnetism and not gravity. But it's the nucleus in the middle that is the thing that we're interested in here. Although that's got most of the mass, it is very, very tiny.

In fact, so tiny that although we've done a magnification of a million on a human hair already, we have to go another factor of 100,000 for that nucleus to fill our view. And if we could see that nucleus, it would look like a clump of particles, some of which would be positively charged, which we call the protons, and others which don't have any charge at all, which are the neutrons. And the positive charge, the number of positive charges in the new ones, nucleus determine the element. So if there's just one positive charge, one proton there, that's ordinary hydrogen. When we get two, that's helium, three, lithium, four, beryllum, so forth,

all the way up to, well, if you get to 92, that would be uranium. So these positive charges, that's the periodic table. Exactly the periodic table, which was put in the proper order exactly 100 years ago by Henry Mosley, I would know of my college. And the positive charges want to repel due to the electromagnetic force. So we know there must be something else that's holding them together. And that's something else is the nuclear strong force that acts between both the protons and the neutrons. It acts at very short distances, the sort of distance when they touch together and binds that nucleus together. Now there's a lot in there. So just to simplify for your next question, can you tell us how those forces at work, how they give themselves

as well, an opportunity to generate power. Well, the power can come about via both splitting of heavy nuclei and the coming together of smaller nuclei. Now, like many things in nature, what's going on here is a competition. It's a competition

between the force that's trying to push the positive particles apart, the electromagnetic force, a bit like if you try and push two north poles of a magnet together, they want to repel. That's what the protons are doing. but the protons and neutrons together by this other force of nature, the strong force, are getting pulled together.

Now, the analogy that we like to give, and in fact it's the name of a model that we first teach our students, is the liquid drop model. So although we're used to the inclement weather in the UK, when we go outside, we don't find raindrops the size of a football. They're all raindrop-sized. And the reason for that is there are two things working in competition. one when droplets of rain fall from the sky. They like to agglomerate because of the stickiness of water, but also when passing through the air, that will break up the droplets as well if they get too big, and you end up with a droplet, all raindrops come in raindrop sizes.

And the similar thing happens for the nucleus. Because we've got things in these forces in competition, the protons want to repel, and the neutrons and protons together want to attract, there's a specific size of nucleus that all nuclei in the universe would like to be, if you like the raindrop size for a nucleus, and that's something with 26 protons, 30 neutrons, and it's iron.

So anything heavier than iron can split apart, like a very large raindrop coming down from the sky, to go towards iron and release energy, usually by throwing off fragments. Things smaller than that can condense together if we can push them together against this electromagnetic force and also release energy, again, by shooting off fragments. Peter Browning, can you tell us what happens in a fusion reaction? How is specifically this power generated in this new universe that Justin just explained to it?

Yeah, well fusion as the name suggests, is bringing together. So it's the bringing together of small nuclei into larger ones. And we have to obviously contrast that with fission, which is perhaps a term which is more familiar to people, which is splitting up of large nuclei and smaller ones. Let's just nail fission. We're not going to do fission. Just to get the contrast. The other side of it, yeah. Yeah. So fusion, so the most basic fusion, well, the most obvious fusion reaction,

which will come to, which happens in the sun, is bringing together four protons into a helium, or the way, it doesn't happen in quite such a simple way. But you start with four protons, you end up with helium, which is a larger nucleus. Now, you would think that if you take the mass of a nucleus, it would be, as it were, equal to the sum of its parts.

So if you take, for example, a helium nucleus, it's two protons, two neutrons, and you would think that would weigh the same as two protons and two neutrons separately, or equivalently the same as four protons. But that's actually not quite the case. There's a very small change in mass when you bring the small particles together into bigger ones.

So you actually lose a little bit of mass, and because of basically, Einstein's famous equation that equals mc squared you lose a little bit of mass that's equivalent to energy so when you bring the small nuclei together into larger ones the very small change in mass actually gives you a very large release in energy and that's that's the energy we're talking about in in nuclear fusion so it's energy from converting mass into energy and that is a process that powers the sun how does it work there right well in the sun I say the basic, and actually in most stars, at least for most of their lives,

the main process is converting hydrogen into helium. Now, in principle, the overall effect is you take four protons and you convert those into a helium, helium, say two protons, two neutrons. But, of course, you can't do that all in one go. You can't bring four protons together. That will sort of never happen. So it proceeds by a series of steps, which we call the proton proton chain. And the reactions for that were, well, more or less discovered in the late 30s by Gamov and others.

So what happens is first of all, two protons come together, and they form a deuterium nucleus, which is an isotope of hydrogen. So that's just a proton and a neutron. How did that initial condition come about? Was it because of the sun, these gases, and they're pressed so tightly on it? What made that force, the force for energy it now is? Right. Well, I think we'll come to the conditions for fusion,

but certainly you need sufficiently high temperatures, as we shall see. So that's why it only happens in the centre of stars when sufficiently high temperatures and sufficiently high densities have been reached. So when a star is forming, In the early days, you start with a cloud of gas, and that collapses and becomes sort of smaller and smaller. And the centre of the star becomes hotter and hotter. And I say in formation stage, it's just losing energy through radiation, it's heating up. But eventually it gets hot enough, and then the fusion process starts.

Steve Carly, scientists as I understand the sun, discover nuclear fusion by thinking about the age of the sun. How did they get to that? It's a beautiful story really. Back in around about 1860, Lord Kelvin had said he'd sat down to try and work out what the age of the sun was. And what he did was he said, okay, the sun is a ball of gas that's collapsing and radiating away its energy. And from the rate of which it's radiating its energy, which he measured, you were able to determine the age of the sun. And he said it's 20 million years old. And this became a problem. towards the end of the 19th century because if the Sun is only 20 million years old, how come things on Earth seem to have lasted several billion years? So geologists and most of all Darwin, Darwinism and Darwin were very disturbed by this. Exactly. They were very disturbed by it. And it's one of those issues where, you know, the paradigm didn't shift. It took a long time before people thinking about the physics of the Sun

said, oh come on, this must be wrong. And it was Sir Arthur Stanley Eddington in 1920 at the British Association who stood up to give the presidential address and he said, look, it's got to be wrong. It cannot be that the sun is only 20 million years old. Why did he say it's got to be? Was this a hunch that he'd done some work? By then we knew from geology. By then we knew... What did we know from geology? We knew from the radioactive dating of rocks that the earth was several billion years old. Right. it's almost 5 billion years old

and we also he also knew from astronomy that there were stars out there that seemed to be from an age much much earlier because they were so far away it took like a long time to get to us so he was making all these deductions and he was saying

if the sun is older than 20 million years old it's got to have a power source inside it because how did it shine for all those years longer and he was It was very lucky because at the same time, actually just the year before, Francis Aston had measured the mass of hydrogen and helium and noticed that in fact the mass of four hydrogens was slightly more than the mass of helium. And Eddington made a conjecture. He said, perhaps the sun is turning hydrogen into helium.

And if it is, there's a little mass left over. and if there's a little mass left over, that mass can be used as energy through E equals MC squared to power the sun. And then he made a simple back of the envelope calculation. He said, supposing the sun has converted about a third of its mass from hydrogen to helium, because he knew both hydrogen and helium were in the sun. He said, supposing about a third of the mass has been converted from hydrogen to helium, we can work out how much energy that is.

and then we can work out how long it would take the sun to radiate that energy and he came up with 10 billion years which is much, much closer to the real age of the sun and so he'd discover nuclear fusion the conversion of hydrogen into helium without any detailed understanding of what was going on in the nucleus did it help him that Rutherford a few years before 1911 had discovered the nucleus Oh absolutely yeah

So how important was Rutherford's discovery to Eddington's discovery Well, Rutherford's discovery was that there was a nucleus in the middle of the atom, but what it was made of, we didn't even know there was such a thing as a neutron in 1920. We didn't know what the structure of that little thing in the middle. All we knew was it contained the positive charge. And when did scientists begin to think about nuclear fusion? Well, actually, Eddington. As a possible, Eddington again.

Eddington himself says in rather Victorian English that, you know, man will someday, be able to exploit this energy source. He said man, of course, because these were very sexist age. And Justin Mann did, and man made a bomb. Yes, they did. The atom bomb was fission-driven, nuclear fission, but the hydrogen bomb is our subject, isn't it? That's a fusion bomb.

Yes, and we can't in this case separate them. But the idea of a fusion bomb was, in fact, formed at a very similar time as the idea of a fission bomb. So after that famous Einstein-Rosevelt letter in 39 that ultimately led to the Manhattan Project and trying to produce a fission bomb, very early on in that process, scientists realized that they could perhaps also use fusion.

In fact, Enrique Fermi talked about that in 1941 in a conversation with Edward Teller. But producing the fission bomb was comparatively easier things, to do than create a fusion bomb. So really the ideas then only came after the war, after the Second World War, Edward Teller continued to work on this amongst others. And the idea that they had is that if you have the fission bomb, the splitting of the atom bomb, that produces, as we know, an enormous amount of energy. We know that for fusion we have to heat something, this heavy hydrogen to enormous temperatures, you've already mentioned 200 million degrees, and to try and do that

at enormous densities. So the idea came about to use the radiation from a fission bomb to heat and compress next to it a device which is containing heavy hydrogen, which can produce far more energy for a given number of protons and neutrons than the original fission bond. So already they worked for a little while after the war.

Edward Teller came up with a design in 1951. the first big test in 1952 of the Ivy Mike test, which produced the first fusion bomb of a 10 megaton yield, just a few years after the war. And to put that in perspective, that's nearly 500 times as powerful as the bombs that were dropped on Hiroshima and Nagasaki. And they're in existence as we sit and speak this morning. Oh, thank you. Philippa Browning. Can you tell us what conditions are necessary to achieve fusion? to try to catch up with the sun,

replicate the sun here on... Yes. Well, one thing, as has been mentioned, fusion is a sort of competition between the electromagnetic forces and nuclei have positive charges so they'll tend to repel. And the only way to overcome that and let them to get close enough together

that these strong nuclear forces can kick in is basically for them to be moving very fast. Now, this is an important point, so can we spare it out completely? These things repel each other. Yes. And there's no way of getting together except one way.

and that is to make it unimaginably hot and because it's unimaginably hot, what then happens that's different and they stop repelling each other? Well, it's not quite unimaginably hot. It's a little bit... It's a little bit... It depends what you're imagining.

200 million degrees centigrade, that will do. The centre of the sun is about 15 million degrees. Equally unimaginable to me, and fusion there actually happens rather inefficiently. The sun actually generates power at a rather low rate. But to do it on Earth, we actually need, to do it efficiently, we need somewhat higher temperatures. So why does it happen under high temperatures?

Right. Basically, because the particles have got enough energy to come together. But it's actually, we are helped by a few things. Quantum mechanics helps us. So we don't need quite as much energy as we think we might do. Because the particles can actually tunnel through barriers using quantum mechanics. And so we need temperatures to do fusion effectively in the laboratory of around 100 million. degrees. I think the first thing to understand about high temperature is temperature means how fast the particles are moving. So the higher the temperature, the faster your particles are moving.

And if they bump into each other, they'll repel due to their two positive charges repel. And if they bump into each each other really hard, they get very, very close. And they get close enough that this strong force, which only acts at short distance, can grab them, pull them, them together. What's this strong force you're talking about? That's the thing that holds the nucleus together. But it only acts when you get really close. Justin wants to pop in. Now I'll come back to your... Yes, I mean

Steve's absolutely right, but it's fascinating to think about the speeds that we're talking about. When you're heating something up to a couple of hundred million degrees, you're throwing together these particles at about a thousand kilometres per second. And of course,

everything's happening by chance as well. They have to have a head-on collision slowing down due to this repulsion only just touch and then bam the strong force takes over and from that spewing out these neutral particles. Now, to help... Do you want to come in for a moment? Yeah, and I think it's just worth saying

the very early fusion experiments where fusion was demonstrated were actually done with accelerators where particles were literally slammed together at very high speeds. But in a gas you have just many particles moving around with high velocities essentially at random

and every so often they will happen to come sufficiently close and then they will be able to do the fusion reactions and hopefully get the energy out. This turns into a sort of ungovernable plasma unless you find a way to govern it and the Russians found a way as I understand it, the Tokomak. Now what's that and what does it do? Well, so if you want to heat something to 200 million degrees,

you don't want it to touch the wall. So you've got to suspend it somehow in mid-air. So what both the Russians and the Americans and the British decided to do in programs that were secret at the time in the 1950s is let's take our fusion fuel, which is two kinds of hydrogen, deuterium and tritium, heavy hydrogen and super heavy hydrogen. Let's turn it into a ionized gas like in a fluorescent light bulb and let's hold it with a magnetic field in a cage so that it doesn't touch. the wall anymore. Why doesn't it touch the wall? Because the magnetic field lines provide a force that squeezes the plasma, the hot gas, off and holds it in the middle. And these charged particles that are, you know, the charge nuclei, which are positive charges, are held by this magnetic field in this cage-like structure that squeezes it down towards the center. And the

Tocomac, which is a Russian acrhic in fact, for a donut-shaped configuration of magnetic field is a way in which you can make this cage and the plasma just basically goes around in a circle, never touching the walls. And what's the advantage of that just in Walker? Are these the conditions that you need to develop the notion of fusion, the fact of fusion? Well, I think fusion we need two things. We've already been talking about the high temperature because then by chance the particles, the positively charged particles will be moving fast enough that before they stop when they're repelling, they'll just touch and the nuclear force will take over.

But we also need to confine, as Steve has said, the plasma, but we need to confine it for a certain time. In the Tokomac that we've been discussing, the pressure in the gas, in fact, is not very much different from the pressure in this room. Because even though something is at a couple of hundred million degrees, the number of particles there, it's very, very sparse, roughly about a million of thirds a C, than you have of particles in a normal gas. And so that means that the collisions, the head-on collisions that you need,

don't take place very often. And what we end up with is a criterion that was brought together by John Lawson, called the Lawson criteria, which is quite simple. It basically says that if your fusion, reaction is dense, it takes less time for the reaction to take place, and you don't have to wait as long to get more energy out than you put in to heat the system up. So in the Tocomac, it's about as I say, 100,000th or a millionth of the density of normal air, and it takes, I think, the criteria is about a few seconds to get more energy out than you put in once you've heated it

up to 100 or 200 million degrees. Philip Browning, that was obviously, I would guess, very difficult to construct. What were the difficulties in the way of constructing it? Right, well, in terms of, the first thing is just to design a magnetic field configuration that works. It turns out out, of course, we've learnt that the charged particles generally move

along magnetic field lines. So the first thing is you can't just have a straight magnetic field because they would sort of fall out the ends. You'd have to have ends somewhere. So as Steve's end, the particles would just go off the ends of the field lines. So it's no good having a straight magnetic field lines.

So the first trick was, as Steve has described, to bend your magnetic field lines into a ring. And of course, a ring has no ends. So in principle, the particles can just sort of move around and around the ring. But it turns out then the particles, Steve has described magnetic field lines as a cage. Unfortunately, they're not a perfect cage. Particles can gradually move across the magnetic field lines. And worse than that, magnetic field lines themselves can change. they're influenced by the motion of the charged particles themselves

and so the magnetic field lines form a rather slippery sort of cage in which you say the particles can sometimes slip through the field itself can change and not trap the particles as well so it's a very complex problem but nevertheless it gets results can I just Steve can ask you would you say what you want to say then I'll ask you my question Oh, magnetic field is like lines in space. We call them field lines. And those magnetic field lines act like elastic bands on the fuel. And they wrap round the fuel and it's like wrapping wool on a ball of wool. And if you wrap wool on a ball of wool and you just wind in one direction, the ball will fall apart. And what we had to do is we had to construct a magnetic field, which is very like, you actually do wrap your wool on the ball is you rotate the ball and you wrap it in a slightly

different direction and one piece of wool holds the other piece of wool down and then you rotate again and you do this and this is how your grandmother taught you to wind up the wall because otherwise it just falls apart and in the early machines that we made we did it wrong and what happened is that the magnetic field would fall apart and with it this hot plasma and it would touch the walls and immediately get cold can we the great thing we would put this into action wouldn't it but it doesn't seem possible yet you tell me if I'm wrong please

that you can from a fusion reaction you can generate electricity are you generating electricity yet well we're not generating electricity yet in principle you could generate a very tiny amount if you wanted to but it would be minuscule compared to the amount of energy that you put in to start the fusion reaction

in the first place so that doesn't help very much So fusion, as Steve's described it in the method with magnets, and there are other methods of doing it as well, does work. The problem is that at the moment, we can't sustain it for long enough or can't do it sufficiently efficiently, that we get more energy out than we put in. But that leads obviously to the question about how would you generate electricity if you could get more power out than you put in. And to describe that, we have to describe how this energy manifests itself. So we've said that we're slamming together two nuclei of heavy hydrogen to form helium. But when that happens, and I've said they're coming together at about 1,000 kilometers per second, when that happens, it spits off one little neutron and recalls.

That neutron, when you fire it off, because that's where the energy manifests itself. That neutron comes off at 50,000 kilometres per second, like 100 million miles an hour. If it could, it would get from here to the moon in less than eight seconds. So where the electricity would come from is you simply would have, outside your reactor, that neutron would be slowed down in some blanket of liquid metal or something. In the end, it will, by atomic friction, heat something up. You use that to heat up water or steam and dry the turbine. In the end, it all comes down to the good old steam turbine.

Philip, do you want to develop that? To take on from what I thought you were going to come in from Rochester was saying. No, I think it is probably just worth emphasising again, though, that we have achieved. Fusion power has already been generated in existing in Tochamax, have generated substantial amounts of power using the reactions between the isotope's heavy hydrogen, but not quite yet as much power as is put in to heat the plasma. but it's close to you know a sort of an equal amount

power in equal amount power out but that's obviously no use for a power station so the next step is to move on to increasing the amount of power out and then we'll be you know are you using power instead of electricity deliberately well I say existing devices don't

as it were bother to try and generate electricity because that's the last step so that won't be done until in the future the first time substantial fusion was done was in 1997 at Cullum in Oxfordshire on the jet device and we made 16 megawatts

of fusion power typical person is using about a couple of kilowatts so much less than a thousandths of that so it's a substantial amount of power we made 16 megawatts but at the same time in order to keep the fuel hot we had to inject 24 megawatts

at that time. So what was coming out was the 24 megawatts we put in plus the 16 megawatts of fusion power that we made in that reactor. The temperature in the middle was 230 million degrees. So we've actually achieved the conditions for fusion, but we've not yet harnessed it to make commercial power. Other ways that people are seeing too harness it, Justin, Justin Moore? Well, there's other ways. There's two approaches to trying to achieve fusion in the land. and Steve's outlined one or been talking about the Tokomac, where you hold something in a magnetic cage. And as we were discussing there, the density is very, very low,

and the time for the reaction to get more power out than you initially started with is several seconds, according to the Lawson criteria. The other way, which is being attempted in a laboratory in California, is to compress a small pellet, a pinhead-sized pellet, of Deuterium and Tritium, and to compress it to extremely high densities, comparable to those as you go towards the centre of the sun. And in that reaction, the reaction then takes place extremely quickly in less than a tenth of a billionth of a second,

because you're at such a high density. And because it takes place so quickly, you don't need a magnetic cage. It takes time when you compress something to a very high density like that, it can't fly apart because of its own inertia. And we call this inertial confinement fusion. And the way this works is you take a spherical pellet of your heavy hydrogen and you shine the most powerful laser on the planet onto the outside surface. It's actually a little more complicated than that.

But that vaporizes the outside surface of the pellet, and that vapor rushes off into the vacuum, a little bit like the exhaust gas of a rocket. So this is a bit like a spherical rocket that gets imploded to very high densities and high temperatures. And that mechanism is also showing some interesting results. It's at the level where just this year they're getting about 1% of the energy out compared to that put in via the light. But it's already starting to fizz.

We know the reaction is starting to self-generate some heat. So there's good progress being made. Philip Browning, how has all of this research into the physics of nuclear fusion on Earth helped us to understand the way that nuclear fusion works in the sun and other stars? Well, I think the studies on Earth tell us a lot about what's going on in the sun and stars, but not so much about the actual fusion processes, because for one thing, the sun is doing fusion by different reactions than we're studying on the Earth.

I say the Sun's doing it actually using these very slow reactions, working directly from hydrogen, simple, hydrogen, which is all it's got, whereas on Earth, we're working with heavy hydrogen, deuterium and tritium and so on. So we don't learn so much about the actual nuclear reactions, but we can learn a lot. One thing we can learn about for magnetically confined fusion, strangely enough, is not about the interior of the sun, the core of the sun where the fusion happens, but actually the atmosphere. So if we look at the atmosphere of the sun, the corona, which we only see usually at a total eclipse, that has got the hot gas, the plasma, confined by magnesium.

magnetic fields in a very similar way to what happens in a tocomac. So we can learn a lot about the atmosphere of the sun and actually the space environment of the earth by studying the magnetic fields and the plasma in a tocomac and the processes are really rather similar. To put it extremely crudely, have you got what you want out of the sun at this stage in terms of information? Well, I think we understand the actual nuclear processes

by which the energy is generated in the centre rather well. and it seems like that's been tested in various ways by using seismology and looking at the neutrinos, the particles which come out. And I say it seems that the fusion in the centre is working very much as we expect it to. But as we go into the outer layers of the sun, there's an awful lot we don't understand. For example, how does the sun generate its magnetic field? What does that magnetic field do? How does that affect the earth? And those processes, I say, have a lot in common.

with what's going on in a tocomac. Steve Carly. So one of the most dramatic and beautiful things you can see on the sun is the eruptions from the surface, the solar flares and what are called coronal mash injections when the sun suddenly spews out a big slug of plasma. People have seen movies of these. You can go on YouTube and you can see them.

And that's when the magnetic field that's holding plasma on the surface of the sun suddenly breaks apart and erupts. and unfortunately we get that in our experiments sometimes sometimes the magnetic field in the experiment sort of parts and out comes a slug of plasma we call it a disruption and the mechanism on the sun and the mechanism of our experiment seem to be enormously similar and in fact the calculations seem to show a very very similar mechanism

Justin Wolk So Philippa and Stephen have been talking about what could have been learnt from Tokomax in nuclear fusion reactions which you are asking about There has been quite a bit learnt in the last couple of years in these very dense implosions I was discussing earlier produced by lasers. And the reason for that is although we're only getting 1% of the energy out that we put in, that is still 10,000 trillion neutrons.

And that number being produced in a less than a tenth of a billionth of a second in something the size of a pinhead is a neutron flux greater than that which exists when a supernova goes off. And so what's been found in just ride-along experiments, actually, is that nuclear scientists are finding a whole set of nuclear reactions that they hadn't been able to study before, some of those which will go on within stars. So, yes, we are learning about solar physics, astrophysics,

by making these, what I like to think of as miniature stars in the lab. Of course, supernovae are the exploding stars, the star of Bethlehem, in fact, is thought to be. have been a supernova, and it's the end of a star's life. And at that point, there are a bunch of nuclear reactions that we really don't understand in those exploding stars, and these experiments are pointing them out. Can we talk about cold fusion,

which came in as an idea in 1989 from the University of Utah, and why was it so striking? it seems such a very plausible idea? Steve, you first. Oh, did it seem a plausible idea? There was a phenomenon before that that had been discovered. One of the difficulties of making fusion happen

is this repulsion that happens between positive charges. If you could get a negative charge that could cancel out that repulsion so that you could stick a negative charge between the two positive charges and cancel it out, then you could get them, much closer. But you can't get an electron in there because it's rushing around too much. It never stays in one place long enough. But you can get a heavier particle in there. And there was a phenomena

noticed in the in 1946 actually whereby fusion took place because a heavy negatively charged particle called a muon got bound to deuterium, heavy hydrogen. And that allowed the deuterium to get close to another deuterium, close enough that this strong force and actually cause them to fuse. So here was another way of making fusion happen by having a heavy negatively charged particle. And fusion could then happen without having these immense energies and speeds of the particles. And so in 1989 there was this announcement from the University of Utah that you could actually do this in an electrochemical cell, you know, by passing electricity through a liquid. And it wasn't really plausible because

They didn't have any murens in there, no heavy negatively charged particles, but they were claiming that there was a lot of energy coming out of their cell. Philippa running, what would have been the excitement of cold fusion? Well, I mean, had it worked, I suppose, which I think we probably all agreed, it effectively didn't. Obviously, it would have saved, you know, all the difficulty of having to, you know, have these plasmas at very high temperatures and so on. But I think it was, you know, experiments were done.

you know, very soon after really, which showed that this process wasn't really a way of generating energy. So it would have been attractive, but... Sorry, is there now thought there be any possibility that coal fusion could sometimes be proved to work? I mean, I don't think there's much... No. A shaking of heads is going. Yes. Yes.

Yes. Yes. Yes. It generates a lot of excitement. I mean, nobody knew what this announcement meant. All we knew is that there was hydrogen, heavy hydrogen, had been somehow, pumped into this metal. I mean, in 1988, I remember the announcement myself.

I mean, the very next day, a colleague across the corridor from me, I asked him if he knew about it, he said, well, no, but I do have some deuterated pladium. And I took it, I went down to the Rutherford Laboratory. I thought to myself, I don't know what's going on, but if I can squash this stuff, which I knew

how to do with a laser, maybe the fusion reaction would, if there is one, would increase a bit in rates. So I put it into the beam, I fired a laser at it, I had a counter. The neutron counter went off scale. I was very excited, but I didn't call the press straight away. I went a cup of tea and I put more shielding between the experiment and the neutron counter and the signal went away. So I think the problem with the Fleischman and Pons scheme is,

well, perhaps it's a Fleischman-Ponsi scheme. It really, I don't think anybody now looks upon it seriously. People keep talking about it in the same way as they keep talking about perpetual motion machines. Philip O'Barney, we mentioned en bassohn nuclear fission. What are the advantages that you see of nuclear fusion over nuclear fission? Well, I think there's many advantages. Perhaps the fundamental ones occur with the reactions. The end product of fusion is helium, and helium is a totally harmless gas. You know, we have it in party blooms and so on, whereas the end products of fission are radioactive materials. We have long-lived radioactive waste, which has to be dealt with in rather difficult ways.

So that's one advantage. Another major advantage is that fusion has no possibility of a sort of catastrophic, like any kind of runaway reaction. Fission, as we know, has the possibility of fairly catastrophic accidents. But in fusion, if things fail, even like the disruptions that Steve, you've mentioned. Basically, the reactions stop, the plasma cools down and everything just kind of goes away. I mean, fusion, sorry. In that sense, it's safe. And the fuel is also one of the major advantages of fusion is the fuels are virtually limitless. I mean, the core fuel is simply hydrogen,

which comes from water. We also need to create tritium, which is another form of heavy hydrogen, which we'll need lithium for, but in rather small quantities, and it's believed there are large reserves of lithium. So we have sort of limitless, almost limitless fuel, safe waste product, no possibility of a catastrophic accident, and very large amounts of energy, as we said at the beginning, from very small quantities of fuel. And actually fusion is even, in some sense, more effective than fission

in the amount of energy that you get for the sort of massive, mass of fuel. It's a paradise of energy, and the answer to everything, of it, isn't it? Were you going to say that just in on something different? Well, I was going to say there is another, if you could get it to work, another advantage from a political, if you like, point of view, which is there's no chance of producing a weapon with just the fusion reaction. I described earlier that you need to have a fission bomb to get a fusion bond to work. And of course, we know the political ramifications of states using nuclear power can possibly, they can then possibly develop their nuclear reactors to produce nuclear weapons. And so it would take, if it it can be achieved, it could potentially take away those sorts of very complicated issues. Steve? I always say it's the perfect way to make energy except one thing. It's really hard to do. That, of course, is why it's so much fun to work on, because as a scientist, there's all

these challenges. But the thing is, we've done it. You know, at Jet and regularly in the experiments at our laboratory, we are creating plasmas with 200 million degrees in them that are just the conditions. for fusion. We can do it, but now we have to do it at a scale where we produce more energy than we consume and also at a cost the consumer wants to pay. And we've got to finish the job. I mean, we have to have fusion energy. We're running out of good ways to make energy. And fusion can do that for us. And it's only a matter of what we know. It isn't a matter of what we dig up. It's a matter

of what do we know? And can we actually tame this in a way that's cost effective? to you to save us. That's the answer to that. Thank you very much. Thank you very much. Justin Walks, Steve Coley and Philippa Browning. Next week we'll be talking about Hachepsut, the woman who ruled Egypt as a pharaoh in the mid-15th century BC. Thank you for listening. And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvin and his guests. What we didn't mention was that we are now building the device in southern France. Yes, it would have been nice to have got us to eat her.

in a way because that really is the future and it's being built and it's a major is it what is also I think interesting that you know is is the international nature of fusion research because I think it's it's a it's a model in some ways I think for other forms of science but just for the fact that you've got they say the countries that are participating in ITER you know you've got you know the Europe Japan Russia so on countries that working together on one project I mean, it has its difficulties, but I think that's still a phenomenal thing. The plan for Eater is to produce at least 10 times as much energy as you put in. So at that point, you've shown that it's scientifically possible to make electricity this way.

It's vaguely having you bring it up at the very end and say that was the future, but I should have prodded it somehow. Because there's also a possibility that the laser way of doing this, the machine in Livermore is called the National Ignition Facility. It was meant to be producing more or producing more energy than was put in from the light by 2012, as I had funding from Congress to do that. It hasn't quite happened. But there's some tremendous progress being made there.

As I say, the system is starting to fizz, and people understand a lot more about the problems than we did just a few years ago. Is it an area that attracts a young PhD student? Oh, yes. Absolutely. It's the big one. It's one of the big one. Yes. It's a very, very good area to get students involved. They learn lots of different skills, but they know that they're doing something

potentially very useful as well. But it's also, as we've been talking about, link to the stars, linked to astrophysics, link to high technology. It's amazing. Again and again when we do science programs, the thing that always no longer surprising, but it's really,

I don't say delights, which sounds a bit ponsie, but it is, it is that things that start off in the most abstract possible way end up being the things we can't do without if we're going to continue to run the civilisation. I mean, Rutherford works out that there's a nucleus.

Eddington works out, oh, well, the sun is not that old is that because he does all these calculations. And what they're doing is, calculations, they're doing what the 18th century is. They're following curiosity, aren't there? Francis Bacon's that followed. And just a few years later, they're going to solve the world's energy. Some of those initial calculations

as you're talking about what Eddington, they were very, very simple back of the end of the calculations that any first year, well, any A-level student could have done. It was the insight that enabled him to come up with this. I was actually quoted to students as an example of the fact that science isn't all about covering the blackboard in equations. Sometimes it's just about thinking clearly.

And Eddington had a very clear set of thoughts at that time. And then just simple calculations on the back of an envelope convinced him he had some... This envelope keeps cropping up. Was there a real envelope? And these days you carry a snapshot of an idea. We're right out of envelopes these days.

Actually, quite a few scientists that I know carry cards in their pocket like index cards because the envelope isn't naturally available. It doesn't sound quite as good as in the back of an envelope. It doesn't. But we are getting a lot of young people

involved in this because everybody's worried about the future. Yeah. It is actually one of I mean, well, arguably the biggest. If you think of, you know, problems that society has to resolve, energy is, you know, arguably the biggest, yeah. I mean, all other things, if you don't have energy, everything else, even sort of health and everything,

you can't, you know, you don't have health care if you haven't got energy. It's an absolutely fun. And, you know, let's say without fusion, it's very hard to see in 50 years what we would be doing. So there's a moment in the Echer experiment that I want to be around for. And it's taking a long time to build the season. but that's a moment where the plasma starts to sustain itself. The fusion reactions provide enough heat to replace the heat that's lost so that it's a burning fire.

Unlike, you know, when you light a fire, you put a match to it, the heat of the match starts the fire. What we'd like to do is to have a fire that burns on its own and you just add fuel to it. And you can do that with fusion. And the Eater program is supposed to get to that point where you can just keep supplying the fuel and it will keep burning. it will keep burning and you don't have to supply any energy. Yes. And if it does that, it'll be a historic moment in science. It's a moment comparable to the moment that Fermi pulled out the control rods on the first nuclear reactor.

There are many more Radio 4 arts and discussion programs to download for free. Find these on the website at BBC.co.uk.