Notes on the NIF nuclear fusion breakthrough

My explainer/analysis of the US nuclear fusion breakthrough was published today. Some stuff didn’t make it to the final draft for space and tone constraints; I’m publishing that below.

1. While most US government officials present at the announcement of the NIF’s results, including the president’s science advisor Arati Prabhakar (and with the exception of energy secretary Jennifer Granholm), were clear that a power plant was a long way off, they weren’t sufficiently clear that the road from the achievement to such a power station was neither well-understood nor straightforward even as they repeatedly invoked the prospect of commercial power production. LLNL director Kim Budil even said she expects the technology to be ready for commercialisation within five decades. Apart from overstating the prospect as a result, their words also created a stark contrast with how the US government has responded to countries’ demand for more climate financing and emissions cuts. It’s okay with playing up a potential source of clean energy that can only be realised well after global warming has shot past the Paris Agreement threshold of 1.5º C (if at all) but dances all around its contributions to the $100 billion fund it promised it would contribute to and demands to cut emissions – both within the country and in the form of investments around the world – before 2050.

Also read: US fusion bhashan

2. A definitive prerequisite for a fusion setup to have achieved ignition [i.e. the fusion yield being higher than the input energy] is the Lawson criterion, named for nuclear engineer John D. Lawson, who derived it in 1955. It stipulates a minimum value for the product of the ion density and the confinement time for different fuels. For the deuterium-tritium reaction mixture at the NIF, for example, the product must be at least 1014 s/cm3. In words, this means the temperature must be high enough for long enough to allow the ions to get closer to each other given they are packed densely enough, achieved by compressing the capsule that contains them. The Lawson criterion in effect tells us why high temperature and high pressure are prerequisites for inertial confinement fusion and why we can’t easily compromise them on the road to higher gain.

3. Mentions of “gain” in the announcement on December 13 referred to the scientific gain of the fusion test: the ratio of fusion output to the lasers’ output. Its value is thus a reflection of the challenges of heating plasma, sources of heat loss during ignition and fusion, and increasing fusion yield. While government officials at the announcement were careful to note that the NIF result was a “scientific breakthrough”, other scientists told this correspondent that a scientific gain of 1 was a matter of time and that the real revolution would be a higher engineering gain. This is the ratio of the power supplied by an inertial confinement fusion power plant to the grid to the plant’s recirculating power – i.e. the power consumed to create, maintain and heat the fusion plasma and to operate other facilities. This metric is more brutal than the scientific gain because it includes the latter’s challenges as well as the challenges to reducing energy loss in electric engineering equipment.

4. One plasma physicist likened the NIF’s feat to “the Kitty Hawk moment for the Wright brothers” to The Washington Post. But in a January 2022 paper, scientists from the US Department of Energy wrote that their “Kitty Hawk moment” would be the wall-plug gain reaching 1, instead of the scientific gain, for fusion energy. The wall-plug gain is the ratio of the power from fusion to the power drawn from the wall-plug to run the power plant.

5. The mode of operation of the inertial confinement facility at NIF is indirect-drive and uses central hotspot ignition. Indirect-drive means the laser pulses don’t directly strike the capsule holding the ions but the hohlraum holding the capsule. When the lasers strike the capsule directly, they need to do so as symmetrically as possible to ensure uniform compression on all sides. Any asymmetry leads to a Rayleigh-Taylor instability that rapidly reduces the yield. Achieving such pinpoint accuracy is quite difficult: the capsule is only 2 mm wide, so even a sub-millimetre deviation in a single pulse can tamp the output to an enormous degree. Once the laser pulses have heated up the hohlraum’s inside surface, the latter emits X-rays, which then uniformly compress and heat the capsule from all sides.

A schematic of the laser, hohlraum and capsule setup for indirect-drive inertial confinement fusion at the National Ignition Facility. Source: S.H. Glenzer et al. Phys. Rev. Lett. 106, 085004

6. However, this doesn’t heat all of the fuel to the requisite high temperature. The fuel is arranged in concentric layers, and the heat and pressure cause the 20 µg of deueterium-tritium mix in the central portion to fuse first. This sharply increases the temperature and launches a thermonuclear “burn wave” into the rest of the fuel, which triggers additional reactions. The wisdom for this technique arises from the fact that fusing two hydrogen-2 nuclei requires a temperature corresponding to 5-10 keV of energy (a few million kelvin) whereas the yield is 17,600 keV. So supplying the energy for just one fusion reaction could yield enough energy for hundreds more. Its downside in the inertial confinement contest is that a not-insignificant fraction the energy needs to be diverted to compressing the nuclei instead of heating them, which reduces the gain.

7. As the NIF announcement turns the world’s attention to the prospect of nuclear fusion, ITER’s prospects are also under scrutiny. According to [Shishir Deshpande of IPR Gandhinagar], who is also former project director of ITER-India, the facility is 75% complete and “key components under manufacturing” will arrive in the “next three to five years”. It has already overrun several cost estimates and deadlines (India is one of its funding countries) – but [according to another scientist’s] estimate, it has “great progress” and will “deliver”. Extending the “current experiments” – referring to the NIF’s tests – “is not a direct path to a power station, unlike ITER, which is far more advanced in being an integrated power station. Many engineering issues which ITER is built to address are not even topics yet for laser fusion, such as survival of key components under high-intensity radiation environments.”