Real significance of the current fusion breakthrough at the NIF facility

This week, a number of sources wrote about a breakthrough on the path towards a fusion power plant achieved at the US NIF facility. Conditions for fusion thermonuclear reactions are achieved there through the extreme compression of fuel using laser beams. In the latest success, more energy was obtained from fusion reactions than was carried by the laser beams used to create extremely hot and dense plasma. Let us look at the real significance of this achievement.

A whole range of articles in the press and online celebrated the crossing of an imaginary threshold at which more energy was generated in fusion reactions than was contained in the laser beams at the US NIF (National Ignition Facility) at the Livermore laboratory. LLNL (Lawrence Livermore National Laboratory) focuses intensively on laser physics. This is also why this key facility for studying inertial plasma confinement on the path to a controlled thermonuclear reactor was built there.

Let us recall that there are two distinct methods of achieving the conditions for thermonuclear fusion. The required temperature and a sufficient product of plasma density and confinement time must be achieved. For the deuterium-tritium reaction, 150 million degrees must be reached. The required value of the product of plasma density and confinement time can be achieved in two ways. The first is magnetic confinement, in which low-density plasma is maintained for a long period, on the order of tens or even hundreds of seconds. The second is inertial confinement, in which an extremely high plasma density is achieved for a very short period, on the order of nanoseconds. A detailed analysis of these options and the conditions needed to achieve thermonuclear fusion can be found in an earlier article.

The NIF facility achieves inertial plasma confinement using a laser whose initial beam is split into 192 beams, which irradiate as symmetrically as possible a peppercorn-sized sphere containing a frozen mixture of deuterium and tritium. It should be stressed that the very powerful lasers must accumulate the energy needed for a given shot, which lasts on the order of nanoseconds, over a longer period. The energy of the resulting laser beam is then two orders of magnitude lower than the energy initially stored. The NIF laser has a power of 0.5 PW. As mentioned, the beam must be split into the 192 beams in question for symmetrical irradiation of the fuel capsule, and it is also spread out over a longer period.

At present, to improve the symmetry of irradiation, the deuterium-tritium sphere is placed inside a gold enclosure with a cavity known by the German term hohlraum (cavity). Its dimensions are around one centimetre. The laser beams heat the inner walls of the cavity to extreme temperatures, causing them to emit X-rays. This creates a very homogeneous bath of this radiation inside the cavity. It is only this radiation that then compresses and heats the fuel sphere. This indirect approach makes the heating process as isotropic as possible. The rapid heating causes the outer layers of the sphere to expand and explode and, under the law of action and reaction, the inner layers to implode. This achieves an extremely high plasma density and temperature in the inner regions. It is essentially a microscopic thermonuclear explosion.

Not only the gold hohlraum but also the fuel-containing sphere have a complex structure. In particular, the quality and composition of the ablative surface layer are crucial to the quality of a given shot. Intensive work is therefore under way to improve both the hohlraum and the sphere containing tritium and deuterium. The first reports that the parameters of the generated plasma had been dramatically improved emerged in the summer of 2021, but detailed information on this achievement was published at the end of 2021. It gradually became possible to overcome some of the key milestones on the path to controlled thermonuclear fusion.

Let us recall the limits most frequently discussed in specialist articles. Scientific breakeven denotes the level at which fusion reactions produce the same power as is required to heat the plasma. Ignition breakeven is reached when the fusion power absorbed in the plasma balances the plasma's loss power. Engineering breakeven refers to the balance between the gross output of a fusion power plant and the plant's own consumption.

A more detailed description of the path to the record shot on 8 August 2021, together with a detailed analysis of this shot, was published later. It shows that increasingly uniform distributions of the high temperatures and densities needed to carry out fusion reactions are gradually being achieved in an expanding internal volume involved in the implosion. These shots also exceeded the first of the aforementioned breakeven points several times over, and even the second one.

That this was not a random success but the result of long-term, continuous intensive research is also demonstrated by the fact that another record shot was achieved on 5 December 2022. In this case, 3.15 MJ of energy was released in fusion reactions, while the energy of the ultraviolet laser beam was 2.05 MJ. More energy was thus produced in fusion reactions than was contained in the laser beam entering the hohlraum. It should also be recalled that the conversion of ultraviolet radiation into X-rays and losses caused by part of the radiation missing the “peppercorn” sphere result in total losses of up to 99% of the laser beam's energy. Only roughly one percent of the original laser beam energy was therefore used to heat and compress the fuel.

In the previous record shot of 8 August 2021, the laser beam had an energy of 1.9 MJ and 1.37 MJ of energy was released in fusion reactions, or around 70%. The 100% threshold has now finally been exceeded. This is certainly enormous progress, especially as it appears that shots of this quality can be achieved routinely and that their parameters continue to improve. I very much look forward to a detailed publication on the record shot and the progress that has been achieved.

However, it must be remembered that we are still a long way from engineering breakeven, which must be surpassed on the path to a fusion power plant. Just to balance the energy needed to carry out a laser shot with the parameters described, we need, as already mentioned, energy production two orders of magnitude higher. The other energy requirements of any power plant and the losses in converting thermal energy into electricity must also be covered.

Ensuring a high shot frequency and their continuous long-term operation is an equally major challenge. A real power plant will require a shot frequency of units to tens per second. The current NIF facility can carry out one shot per day. This shows that a number of challenges and a very long road still lie ahead in this area.

Magnetic confinement and tokamaks remain closer to a fusion power plant. Here, the ratio between the energy needed to perform a shot and heat the plasma and the energy actually deposited in the plasma is far lower. At the same time, we can achieve much more stable and continuous operation. This is also why such expectations are placed on ITER. We wrote about earlier and current results from fusion experiments at the European JET tokamak in a recent article, while an earlier article also describes some records achieved by current tokamaks.