Sat. Feb 4th, 2023
Fusion efficiency increased by exploding a small gold shell from the inside

Inertial confinement fusion has mostly been the playground of the US. Because the basic idea is that you need a big laserTM that heats and compresses a nuclear fuel, you have to be rich enough to afford the laser system. Not long ago, China joined that club with a laser designed to deliver 100 kilojoules per pulse. While this is still a generation behind the 1.8 MJ at the National Ignition Facility in the US, the researchers in China are focusing on innovative ways to achieve high compression.

The key to inertial confinement fusion is that the laser crushes a pellet of nuclear fuel, increasing the pressure and temperature to the point where fusion can occur. This works if you can get a set of laser beams that illuminate the pellet from all sides at once, delivering an even and clean crushing force.

But what would happen if, instead of applying the force from the outside, the force was applied from the outside? inside and outward?

Everyone hates turbulence

At first glance, the idea of ​​harnessing the power from within seems both bizarre and counterproductive. But the goal is to correct some of the shortcomings that have slowed the progress of inertial confinement fusion.

Let’s follow the train wreck in slow motion. The laser pulses approach the pellet from many different angles. They must all hit at the same time and the intensity profile of each beam must be smooth and identical. If this is not achieved, much of the fuel will spray out the side and escape from the combustion area. Think of it like squeezing an inflated balloon: Although you can squeeze it between your hands, it will bulge through the gaps between your fingers.

Even if you have the timing and intensity profile of the beams near perfect, you will still run into problems. The implosion is indirect: a heavy metal casing around the fuel absorbs the radiation and the outside burns down. As the material flies away from the casing, the back reaction forces the casing inside, compressing the fuel. This compression is so powerful that, in the center of the pellet, the fuel begins to melt.

The fusion forces the inner material to expand into the material that is still collapsing, and this heats and compresses that material so that the fusion can continue. In theory, all the fuel is consumed and a lot of energy is released in the form of high-energy neutrons.

But what actually happens is that all the imperfections in the outer casing, the shape of the fuel pellet and the laser beams get out of hand. Due to the imperfections, the pressure shock wave that initiates fusion is not perfectly spherical, so fusion may start slightly off center and with a non-spherical distribution. The resulting outgoing shock wave is even more aspherical than the incoming shock wave. The material reacts to this by forming a turbulent flow that absorbs energy, causing the pressure and temperature to drop.

Ultimately, the amount of neutrons produced is disappointingly low.

Excuse me, you’ve got your fusion inside out

The problem is that the way the shock wave is created is very finicky, so instabilities turning into turbulence are almost inevitable. Even if turbulence could be avoided, the instabilities lead to large variations in the efficiency of fusion.

Researchers believe that by changing the way the shock wave is applied, these instabilities can be significantly reduced. The basic idea is the same: a spherical heavy metal case has a shell of deuterium (hydrogen with a neutron) coated on its inner surface. (It’s actually a polymer with all hydrogen atoms replaced by deuterium, but the idea is the same.) Two holes are then drilled into the shell. The laser beams are aimed at the holes so that they hit the shell from the inside.

The main point here is that the outer shell does not provide the force for compression. Instead, the direct and rapid vaporization and ionization of the fuel creates a rapidly expanding plasma that collides and stops at the center of the sphere. But stopping means that a lot of energy has to be given up by the ions. As a result, the temperature of the plasma shoots up, allowing fusion to take place. Essentially, directed motion with few collisions is converted into random motion with many collisions.

This is very similar to standard inertial confinement fusion, but the important difference is that the outward shock wave is almost certainly spherical because of the way the plasma stops at the center. This causes the plasma to naturally assume a spherical shape.

In a series of experiments, performed at fairly low energy (~6 kJ per pulse), researchers showed that the number of neutrons per shot scaled with input power as expected. They also showed that the neutron yield was quite stable from shot to shot, which is rather important.

The team also showed that the total neutron yield still depended on how evenly the laser pulses illuminated the inside of the shell. The scientists believe their results would be improved by switching to an octahedral target with a hole in the center of each face. That would be almost spherically symmetrical and give even better results than the current design could do.

They also calculated what they could achieve with more energy per shot. If they did the same thing at the National Ignition Facility, they could get 1017 neutrons per shot, assuming deuterium-tritium reactions (tritium is a hydrogen atom with two neutrons). By comparison, the National Ignition Facility folk reports about 1014 neutrons per shot. However, the number of neutrons also depends on the amount of fuel available, and that is not so easy to estimate from the papers.

Is it really that much better?

Those estimates should be taken with your annual salt intake. The problem is not in the estimates themselves, but in the idea that the scaling continues as the laser energy is increased by about four orders of magnitude. That may be true, but my first thought is that the outer gold casing will burn through, reducing the energy going into the plasma.

I also think that as the energy increases, the number of gold ions in the plasma will increase, and those ions will suck energy out of the plasma. By comparison, hydrogen has one electron – once it’s gone, all the energy has to be expressed by the nucleus zipping around and bumping into things. That’s perfect for fusion. But gold has a lot of electrons and not all of them will be stripped. So when a gold ion collides with a hydrogen ion, it can absorb some energy from the hydrogen ion by putting an electron in an excited state. The electron then relaxes by emitting some light. The light escapes the plasma and removes energy from the plasma. And the gold ion is ready to repeat the process, transferring more energy.

Whether this is important or not depends on how fast fusion happens. When the laser hits from the outside, everything happens so fast that the ions from the gold shell have little chance to cool the plasma. However, since the process is much slower here, I wonder if there is any chance of the gold mixing with the plasma before the outgoing shock wave arrives to complete the fusion burn.

Physical assessment letters2017, DOI: 10.1103/PhysRevLett.118.165001

By akfire1

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