Inertial Confinement Fusion

(Transcript of the video commentary.)

Thermonuclear fusion is a process that takes place all over the universe in the cores of stars. The light nuclei of atoms fuse together, releasing large amounts of energy. To fuse, the atoms need to overcome the repulsive electrostatic forces that act between their positively charged nuclei. This can be achieved, for example, by high temperatures or by external forces pushing them together.

In thermonuclear fusion research, there are two main approaches to force particles to fuse. In magnetic confinement, a cage of magnetic fields is created in which particles are trapped and heated to hundreds of millions of degrees. In contrast, inertial confinement tries to compress the fusion fuel into the smallest possible volume in a brief moment. Inside, the temperature and pressures become so high that fusion occurs. It happens so fast that the fusing particles are held in place by mere inertia. That’s why this approach is called inertial.

It does not require any magnetic container or special device to hold the fusing atoms in place. All that is needed is a suitably designed fuel sphere called a target and a mechanism by which it is rapidly and symmetrically compressed called a driver.

The target contains the fuel for thermonuclear fusion. Usually, a mixture of deuterium and tritium in a 50/50 ratio, or pure deuterium. Both deuterium and tritium are isotopes of hydrogen. Deuterium contains one proton and one neutron in its nucleus, while radioactive tritium contains two neutrons. The fusion reaction between deuterium and tritium has an ignition temperature of 160 million degrees.

Since we need to compress the fuel target symmetrically into the smallest possible volume, logically the target has the shape of a sphere. Its surface is a solid membrane called an ablator. Most often this is a thin plastic membrane, but glass, beryllium, or even synthetic diamond can be used. Underneath the ablator layer is hydrogen fuel. To achieve the best symmetry and homogeneity, it takes the form of a frozen layer with the gas underneath in the center of the target.

The manufacturing of the target is extremely demanding in terms of precision. The surface of the ablator must be perfectly smooth. Even irregularities as small as bacteria could cause uneven compression of the target and prevent fusion from being achieved.

The same demands on smoothness apply also to the layer of frozen hydrogen under the ablator. After freezing, this layer is irradiated with a low-power infrared laser to partially melt and smooth out during refreezing. The interior of the target is then inspected by X-ray microscopy or even X-ray tomography. Any inaccuracy must be corrected. The finished target is then stored and handled at temperatures of approximately 18 kelvin.

The mechanism that can compress the target from all sides is technically called a driver. There are a variety of drivers — heavy ion beams, X-rays, hypervelocity pellet guns, or the pressure caused by a fission explosion. A number of these have been tried in practice, but the most effective has been found to be the laser. This word is an abbreviation of Light Amplification by Stimulated Emission of Radiation. It was invented in 1960 as a source of coherent light. This means that, unlike a conventional light source, a laser produces light of only one wavelength, and moreover, these waves are synchronised. As a result, the laser beam remains narrow over long distances and can be focused to a small point.

The core of the laser is the optical medium. This can be a solid, e.g. a ruby crystal or neodymium-doped glass, but also a gas such as carbon dioxide. The medium is illuminated by a flash lamp. This causes excitation of the electrons in the medium, which then release photons of the same wavelength when de-excited. The wave of photons bounces off mirrors at the ends of the optical medium and stimulates other electrons to de-excite as they pass through. A beam of monochromatic, coherent radiation is produced.

We can produce even very small laser diodes, so lasers can be found in a wide range of industries. But lasers for igniting inertial fusion are really big.

The National Ignition Facility laser spreads over an area equivalent to three football fields on the campus of Lawrence Livermore National Laboratory, USA. It is the most powerful laser for inertial fusion research, capable of delivering a peak energy of 500 trillion watts, more than the combined power of all reactors in the United States. Its 192 laser beams deliver about 2 MJ of energy to the surface of the target.

In order to achieve such a high energy, the original relatively weak laser signal must be amplified by passing it through a whole array of amplifiers. At their core are neodymium-doped glass and powerful lamps. The passage of the laser beam causes electrons to de-excite, creating more photons and increasing the beam energy. The original beam is split into 192 beams, all of which travel through sets of amplifiers and other optical elements until they reach a staggering energy of several million Joules. All this happens in just five microseconds, during which time the light travels a distance of 1.5 kilometres.

Finally, the originally infrared light is transformed into ultraviolet light. The laser beams pass into the reaction chamber and strike the surface of the target from all sides at once.

The reaction chamber is a large sphere 10 metres in diameter. Its walls are made of several centimetres thick aluminium panels covered with concrete. To act as a biological shield, the concrete is infused with boron to absorb neutrons. Dozens of openings pass through the chamber for laser beams, diagnostics and a target holder. The target is placed in the perfect center of the evacuated chamber.

Compared to its few millimetres size, the 10 metre diameter chamber seems huge. But the fusion of even such a tiny target releases a lot of energy that could damage the smaller chamber. If 1 mg of deuterium — tritium fuel completely undergoes fusion, the released energy will be 340 MJ, which corresponds to the explosion of 75 kg of TNT. The best experiments so far release energy in units of MJ.

During compression, laser beams impact the surface of the target from all sides. Their immense energy instantly vaporizes the upper layer of the ablator. It blows away from the target, while due to the law of conservation of momentum, the rest of the ablator is hurtling into the centre of the target, compressing it. A shock wave propagates into the core of the target from all sides. The density at the target’s centre reaches up to 1,000 g/cm3, or 1,000 times the density of water, and the temperature exceeds 100 million degrees. Thermonuclear fusion is ignited.

The alpha particles and neutrons produced by the fusion strike the surrounding particles in the target, heating them up so that the fusion can continue. In an incredibly short moment, just a few tens of nanoseconds, part of the target undergoes fusion. Much later, the rest of the target is blown apart. Megajoules of energy are released.

In December 2022, the NIF facility achieved a fusion that released 3.15 MJ of energy, while the lasers that ignited the fusion delivered only 2.05 MJ. This marks the first time in the history of fusion research that the scientific breakeven threshold, where the fusion released more energy than was put into igniting it, has been passed. Ignition was achieved. NIF has since repeated the successful ignition several times.

There are two basic approaches to target compression, direct drive and indirect drive. In direct drive, the lasers strike directly on the surface of the sphere. However, the spots where the center of the beam hits are always compressed more than the spots between the two beam impacts. The resulting instabilities mix the denser and thinner parts of the target. It is therefore very difficult to achieve perfect compression.

A method called indirect drive attempts to disperse the energy of the radiation hitting the surface of the target more evenly. The entire target is suspended on plastic fibres in the middle of a hollow cylinder called a hohlraum. It’s made of lead, gold, or gold-coated uranium. The laser beams are directed through holes inside the cylinder so that they do not hit the target but fall on the inner walls of the chamber. Upon impact, intense X-rays are produced which homogeneously fill the entire interior of the hohlraum. This radiation then begins to symmetrically compress the pellet until fusion occurs.

The indirect drive method leads to better compression of the target, higher temperatures and pressures, and therefore better fusion ignition. However, in addition to the production of the target, it is also demanding for the precise production of the hohlraum, whose inner surface must be extremely smooth with irregularities not exceeding 100 nanometres. The entire hohlraum and the target are single-use devices because they are destroyed during the experiment.

Other methods, such as fast ignition, have also been tried in inertial fusion research. In this method, a cone is introduced into the target and directed towards its centre. First, the fuel is compressed to a certain density and temperature by lasers hitting the surface of the target, and then an ultrashort pulse sent through the cone to the centre of the target ignites the precompressed fuel.

The use of inertial fusion as an energy source is complicated by the fact that most devices are only capable of compressing a target once a day at most. However, a power plant would need the fusion reaction to occur ideally several times per second. To do this, the production of the target would need to be made cheaper and simpler and the target would need to be placed periodically in the centre of the chamber. In addition, heat dissipation and the generation of tritium, which is almost non-existent on Earth and is an essential component of the fuel, would need to be resolved.

There are several concepts that address the possibility of using inertial fusion for power generation.

LIFE, or Laser Inertial Fusion Energy, was a project developed at Lawrence Livermore National Laboratory. It was abandoned in 2013, as ignition was still not achieved at that time. The project analyzed two possible approaches.

The first option addressed the question of how to increase the repetition rate of the laser, which normally has to be cooled and adjusted between shots. Inventing a way to simplify and cheapen the production of the fuel target was also essential. Rapid freezing of liquid hydrogen droplets was considered. These would then be thrown into the chamber and ignited when passing through its centre.

The second version featured a hybrid fussion-fission power plant. The reaction chamber would be surrounded by an envelope of uranium fuel in the form of TRISO particles. The neutrons produced from the fusion would be used to fission natural uranium. The envelope would be cooled by liquid salts containing lithium, which would produce tritium, an important component of fusion fuel.

The HYLIFE II project opted to use a heavy ion beam to compress the target. The surface of the chamber is covered with liquid Flibe salts, which serve as radiation shielding, to wash the chamber of target debris, and to breed tritium. This project also faced the question of how to produce cheaply enough targets.

An alternative is a combination of different approaches such as magneto-inertial fusion. In this, the plasma is first pre-compressed using magnetic fields, eliminating the need to produce a costly target. Subsequently, it is compressed by laser or ion beams to temperatures and densities at which fusion is ignited. Development of magneto-inertial fusion is being pursued by companies such as General fusion or Helion energy.

Thanks to the success of the NIF, inertial fusion is the first type of fusion that has broken the magic threshold of scientific breakeven and achieved ignition. But it still has a long way to go to be used as a power plant.