The milestone achieved at the NIF (National Ignition Facility) on December 5 involved the third of three fusion pulses, or shots, that together comprise the most important episode in the seven-decade history of controlled fusion research.1 For the first time, researchers have demonstrated definitively the scientific feasibility of at least one approach to controlled fusion energy. This new result also serves to reinforce many of the points I made in an essay for Inference published earlier this year.2 Especially the observation that inertial confinement fusion (ICF) has established a clear lead over magnetic confinement fusion (MCF) in attaining reactor-relevant fusion conditions.
When my essay appeared at the end of the May, the NIF at Livermore had already produced the first of these so-called supershots. Achieved on August 8, 2021, that initial result was heralded as reaching the “threshold of fusion ignition,”3 where heating from the fusion reaction itself exceeded energy losses from the hot deuterium-tritium (D-T) plasma. After a year of less spectacular results, the first supershot was essentially replicated on September 19, 2022.4 By contrast, the December 5 record shot generated more than twice the fusion output of the earlier supershots and indicated both definitive achievement of thermonuclear ignition in the D-T core and substantial propagation of a thermonuclear burn into the surrounding D-T fuel.
The NIF’s latest success has only accentuated the disparity described in my essay between the recent advances of ICF and the quarter-century stagnation of MCF. Other approaches lag even further behind. The promoters of different fusion concepts have conflated the NIF results with their own systems, claiming that the NIF success shows that their approach will work as well. But the NIF results say nothing about the feasibility of any other proposed method for controlled fusion. All other current fusion concepts have performance parameters that fall drastically short of any similar demonstration, and most—perhaps all—will never make the grade.
With these latest results in mind, an obvious question is whether an ICF facility similar to NIF could form the basis for a power-producing reactor in the near future. There is no chance whatsoever that such a power plant will appear any time soon because a host of barely existent technologies must first be developed—or in some cases invented—before a fusion reactor that produces net electric power can be realized. In the following comments, the acronym IFE denotes Inertial Fusion Energy, which commonly refers to the application of ICF plasmas for power production.
Consider the following characteristics of the current NIF system:
- The neodymium-glass laser system has an electrical efficiency of about 0.5% and can only fire a full-energy pulse once or twice each day.
- The tiny and intricate target assemblies comprising the hohlraum and fuel capsule cost more than US$10,000 each, require several weeks to fabricate, and hours of careful adjustments to position precisely in the target chamber.
- No attempt is made to capture the fusion output or to replenish the burned tritium fuel.
By comparison, the basic requirements for a fully-realized power-producing IFE system are as follows:
- The implosion driver must deliver 5MJ with an overall electrical efficiency of at least 10% and be capable of firing several times per second.
- A half-million identical fuel targets must be fabricated daily at a cost of no more than 20 cents each.
- After each shot, all debris must be removed and the next target inserted and precisely positioned in a fraction of a second—a process that will be repeated a half-million times each day.
- The survival of exposed optical and target-tracking components must be ensured.
- The target must be surrounded by a liquid metal falls—analogous to a waterfall—to absorb the explosive fusion output of radiation and neutrons.
- The metal falls must breed adequate tritium from those neutrons, and the molten metal must be circulated to drive a turbine.
While there are nascent technologies that have the potential to satisfy the above requirements, many decades of work, at the very least, will be required to develop and perfect all of them. Candidates for driving the fuel implosion process include krypton fluoride and argon fluoride gaseous lasers, diode-pumped solid-state lasers, heavy-ion beams, light-ion beams, and relativistic electron beams—although some of these devices presently have markedly insufficient pulse energy or a pulse length that is orders of magnitude too long.
Methods for the mass production of inexpensive fusion targets, including so-called additive manufacturing techniques, have been proposed, but the components remain largely untested. While the rapid and precise loading and illumination of the targets can benefit from high repetition rate techniques perfected for semiconductor EUV lithography, the technology necessary for the generation and sustainment of molten metal falls remains speculative.
In the realm of physics, the principal challenge is that the fusion energy gain, Q, of each pulse must exceed a value of 100 in order to realize significant electric power output after satisfying all the power drains in an IFE system.5 Achieving Q > 100 once seemed fanciful, but is now plausible. For the foreseeable future, major advances in physics performance must come from the NIF after its planned increase in delivered laser energy to 2.6MJ. Even then, the achievable Q will be limited to the range 10 to 15, which represents 40% burnup of the tritium fuel—about 10 times the amount burned in the December 5 record shot.
Attaining still higher Q requires more massive targets and higher laser energy. A new facility with a 5MJ implosion driver must be implemented to demonstrate Q > 100. Ideally, such a facility will feature an implosion driver of a type listed above that is suitable for deployment in an IFE system. At the time of writing, no new substantial installation is under construction, nor has one even been seriously proposed. For this reason alone, achieving Q > 100 is more than 10 years away.
During the last year, there have been only slow advancements, if any, in the key reactor technologies mentioned herein. Nothing at all has changed concerning the potential time scales for developing a fusion power plant. As I concluded in my earlier essay, “fusion-based electric power remains a distant prospect that is likely unachievable in the next half-century.”6
