MATLAB for Control of Cryogenic DT Fuel for Nuclear Fusion Ignition Experiments

[AUDIO LOGO]

Good day to everyone. My name is Suhas Bhandarkar. And I work as a part of a team that does experiments on nuclear fusion using a very powerful laser.

So that's what I'm going to talk about today. This is a big science project, many interesting things that we can talk about. But, specifically, I'm going to talk about MATLAB, MATLAB for controlling the fueling for these experiments that we do.

What's the big deal about this? Here it is. In December of last year, we demonstrated what we call as ignition fusion. And this is a case where the fusion nuclear energy out is greater than the optical energy, the laser energy that went in.

And this is considered a breakthrough achievement, which has been in the making for 50 years. This has been going on since the lasers were first invented. And it's now that we have been able to do this very complex experiment so as to be able to demonstrate this.

And, throughout the talk, I will mention to you the highlights of the work that led to this and why this is an important achievement for mankind as a whole. So, as I go through, I'll have some introductory slides. And, very quickly, I will talk about the role of MATLAB in these experiments.

So where was this done? It was done at the US Department of Energy's National Lab, specifically the Lawrence Livermore National Lab where I'm located. And it's about 45 miles or so east of San Francisco. So you can sort of locate us from San Francisco.

We study high-energy density science, among other things. And this is one of the pinnacle of plasma research. And the National Ignition Facility, which you see here, is a large facility. That's the flagship facility for doing these experiments.

What is the National Ignition Facility? It is the world's most powerful laser, which can deliver over 2 megajoules of UV light with extremely high precision of pointing. So here it is. This is the facility, all of the NIF as we call it.

It's about the size of three football fields, which is very large for a single laser. It has approximately 200 laser beams. So the energy is split into 200 laser beams, which combine to give us 2 megajoules of energy. And that amounts to 50 terawatts, or 50 trillion watts of UV light.

So put it in context-- a laser pointer for instance is 1 milliwatt of laser light or a laser-cutting system that most of you may have heard of or have seen is about 10 watts. So, in that context, this is several hundred million times the energy of a laser pointer, all focused into a single spot, which can then create extremely high temperatures and pressures, temperatures in the order of 100 million kelvin and pressures that exceed 100 billion atmospheres.

And it is these conditions that we would like to utilize to study nuclear fusion. And you can see the complex architecture of NIF. It is a very, very carefully designed high-energy science tool that people can use world over.

Nuclear fusion is sort of the pinnacle of research in plasma physics. It is the reaction that powers the sun and the stars. It takes two atoms, specifically the atom of deuterium and the atom of tritium, which are both small atoms.

It combines their nuclei to form a single nucleus. That's hard to do because both of these have positive protons. That leads to coulomb repulsion.

So, in order to do that, you need extremely high compressions. And that's what I'm going to talk about. So two atoms are deuterium and a tritium atom being combined to form a single nucleus, which is a helium nucleus, which comes out of the reaction, as well as a neutron.

What is the big deal about this? A little bit of the mass between the starting materials and the ending materials is lost. So the mass of deuterium, tritium compared to the mass of the helium atom and the neutron is slightly different. That mass gets converted to energy through e equal gram c squared, which is an enormous amount of energy, which is shown here, 3.3 times 10 to the 11 joules per gram.

For comparison, the combustion of a gasoline gives us energy that's, as you can see, approximately, 100 million times lower than the energy that you can get from fusion. So it's a very, very concentrated form of energy. One of the important things of this reaction is that there are no radioactive byproducts. Helium is basically a noble gas. And the neutron is essentially used up in the reaction, post the nuclear fusion reaction.

So how do we do this? We basically use NIF's laser energy to compress the fuel, the deuterium-tritium fuel to extremely high conditions that I talked about earlier. In order to do that, we will first put the fuel inside a sphere. The sphere acts as a rocket.

I'll show how that rocket works. And, in order to make that sphere into a rocket, we will put that sphere into an X-ray generator called the hohlraum. So each one of the 192, almost 200, beams that come out of the laser are focused on different points inside the hohlraum.

When you have this much energy hit the hohlraum, you get strong X-rays coming out, which is shown here. These X-rays can impinge onto the sphere called the capsule. You can see that the sphere is just about the size of a peppercorn.

And these X-rays impinging on the capsule can cause the capsule to implode, as you can see in the middle picture, to velocities in the order of 500 kilometers per second, extremely high velocities. These velocities then cause compression on the inside of the fuel due to Newton's second law. And that's how you can make these things go to extremely high densities, which basically lead to the fusion of the two nuclei, deuterium, tritium nuclei.

Temperatures can reach about 100 million centigrade. This fuel then spreads through the rest of-- the burn then spreads to the rest of the fuel. And you get the complete reaction going through, which is what happened in December of last year for us to get the nuclear fusion.

So here is a video that I'm going to show. You will see the various steps of how we carry out this. I will try to point out some of the highlights. A lot of things are happening all at once. This is happening over a very short period of time.

The video is obviously a schematic of what's happening. And I'll point out some of the main highlights. So here we go.

[VIDEO PLAYBACK]

So we're getting ready for a shot. This is the NIF control room. They're counting down. So they're counting down. Everything is ready. Here we go.

So we have large capacitor banks. The capacitor banks are first charged. Here is a fiber-optic laser. The fiber-optic laser generates a small pulse that starts getting split into multiple pulses.

These multiple pulses go through amplifiers, optical amplifiers. They're going back and forth. And the power of the laser slide is getting amplified by a large amount, by over a billion times.

These beams go through various beam conditioning optics. They go back to get amplified even further. And, now, they're getting ready to go into the NIF chamber where we have the experiment at hand.

They're going through the various chambers, getting precisely aligned. And this is still red light. In fact, it's infrared light.

As it approaches the NIF chamber, these beams are split so that they can point exactly where we would like them to be pointed. And they will very soon convert to UV light through a frequency conversion process. So the red light became UV light. The UV light impinges on what we call a target.

There is the target. All that light is going in. The light hits the hohlraum.

The hohlraum gets very, very hot. Note the time. It's a billionth of a second.

The hohlraum emits very strong X-rays. There is the capsule in the middle. The capsule gets very hot, starts blowing. As it blows off, the interior of the capsule gets compressed. And we get fusion.

[END PLAYBACK]

The fuel for the nuclear fusion experiment is a mixture of deuterium and tritium, as I mentioned. This fuel needs to be in the form of a solid layer, extremely thin. The thickness of the fuel is in the order of a hair's width.

And this layer of fuel, solid fuel, needs to be near perfect. It needs to have very uniform thickness around the whole sphere in the order of half a micron. It needs to be very homogeneous and smooth.

And we need that because any defects in this fuel layer can cause hydrodynamic instability. So if you're trying to compress it and you have instabilities, you cannot compress it uniformly. And it disrupts the whole process. So that's why we have to have a perfect layer.

One of the challenges here is the temperature. The temperature at which this needs to form is about 19 kelvin or minus 254 centigrade. So this is why it becomes very challenging to make this near perfect layer of DT fuel in the form of a solid layer.

So, in order for us to make that, we make a very, very specific assembly of parts. For the physics of this, we only need the capsule and the hohlraum. But there are over 100 parts that go into making this whole thing such that we can make millikelvin control over the temperatures around the capsule so that we can make a perfect layer of the fuel.

The capsule itself is made out of a material called high-density carbon, or diamond, which is essentially opaque. And so we have to use X-rays to be able to image it through the diamond layer. And we can do that through the top where there's a large hole for the lasers to come in.

We can use that to image the capsule from the top. And we have cutouts on the side. So we can image it from the side as well.

So here is a picture of the actual system where this is done. There are X-rays that can image it from three sides, as I pointed out. And here is an image of the capsule as seen from the top.

The capsule needs to have the fuel put into there. We do that using a small fill tube. The fill tube is a very small glass tube that enables the fuel to go in there in the order of a 2-micron diameter.

So here is the highlight of the process that we use to do that. I won't go into the details, but I will basically point out the main steps. We start out by fueling the capsule through the fill tube to a very, very precise amount.

That's because the amount that goes in will result in the exact thickness that we need. Once that's done, we quickly solidify the fuel. It forms many crystals. We then melt back all these crystals, except for a small crystal, a single crystal, and use that single crystal to grow the final layer inside the capsule.

So this is the process. It looks simple. It's actually very complicated to get it to this precision. And we use MATLAB to automate the whole process.

Essentially, we have come up with a MATLAB script whereby we can press start, and a layer comes out at the end. What does the MATLAB do? It does these following things-- the system control, image acquisition, image analysis, and then make decisions along the way to advance to the next step.

So NIF, as a system, is very complex. It has many, many-- over 100 diagnostics, many subsystems. And, in order for all of those to run in synchronization at a nanosecond timescale, it requires a specialized script.

And that's done using our internal Integrated Computer Control System script, ICCS. And we use MATLAB through a Java interface to communicate with the ICCS system. By doing that, we can turn the cryostat, which is the system that cools the whole thing down to 19 kelvin.

We can turn the cryostat on and off at selected times. We have temperature controllers. We have to know exactly what the temperature is. That's done through MATLAB control as well.

And then these X-ray cameras. We take large format images and these images have to be processed. So image analysis is a big deal.

We get these X-ray images. There are multiple circles that we see, et cetera, all these other features. And we've written scripts so that the MATLAB can analyze these images.

And then because there's vibration from the cryostat, each subsequent image can be shifted a little bit. And so we use MATLAB to register these images. For instance, the immediate first thing we do is a quality control check.

We have to have the capsule perfectly centered in what we call the hohlraum. We have a reference point for the hohlraum, as you see, the red circle. And then the green circle is the capsule fit. And any offset is reported by MATLAB and becomes a go/no-go process for proceeding.

Likewise, in other images, we can see any defects particles or any other defects that exist in this whole thing. And we can use that to, again, do a quality control on an experiment. Image analysis is basically the key for layering. This is the process where we form the DT solid layer.

We control the fill using MATLAB, basically, down to 1 micron because that then dictates what the thickness of the layer is. We unwrap the final ice layer. You can see here, we have unwrapped it into a linear scale, which then allows us to analyze for what we call low modes, which is basically out of round deviations from [INAUDIBLE] down to submicron levels.

We can also look at micro roughness. And we use the method of a power spectrum to measure that. And we can do this at different scale. So out around something like a golf ball and something like a rough sphere, all of that is captured in this power spectrum. But you may also have isolated defects, PPM level defects, Parts Per Million level defects, or what we call wafer edge grooves. All of that is captured by MATLAB.

So these are the things that we use to demonstrate the ignition fusion reaction. This is basically the reactant that gave us the very high yield, yield approaching a gain of 1.5. So the laser energy going in is 2.5 megajoules. The fusion energy coming out was 3.15 megajoules, thereby a gain of over 1.5. And what we're working on next is seeking even higher gains.

What does this allow us to do? So we basically work to support our nation's nuclear stockpile in a mission called Stockpile Stewardship Mission. Beyond that, NIF, in this unique way, can do astrophysics of the kind that cannot be done elsewhere.

Essentially, you are creating a star in a laboratory. So you can study what happens inside stars. You can study what happens inside planet. And, equally importantly, nuclear fusion is sort of the holy grail of fusion clean energy. So we would like to basically pursue this further.

And the ignition demonstration that we had basically provides a fresh impetus for pursuing nuclear fusion energy. Fusion energy would be the most unique form of energy in that it's very, very high density. And you can also have no byproducts that are harmful.

This is the next grand challenge for us because, even after the demonstration of ignition, there is a long road ahead of us to make this-- to be a viable commercial process. And it'll require development of a whole new generation of technologies and talent. And our goal at Lawrence Livermore National Lab is to accelerate this so-called Inertial Fusion Energy, IFE, in support of the Department of Energy's decade-old vision of enabling the commercialization of fusion energy. Thank you.

[AUDIO LOGO]