fusion ignition

On 8 August, 2021, 192 laser beams pumped vastly more power than the entire US electric grid into a small gold capsule and ignited, for a faction of a second, the same thermonuclear fire that powers the Sun.

The experiment in fusion power, conducted by the National Ignition Facility at Lawrence Livermore National Laboratory in California, is explored in detail in three new papers — one published in Physical Review Letters and two papers published in Physical Review E — that argue the researchers achieved “ignition,” a crucial step proving that controlled nuclear fusion is achievable. But definitions of what constitutes “ignition” vary, and however defined, the results of 2021 are still very far away from a practical fusion reactor, despite producing a very large amount of energy.

Nuclear fusion involves the fusion of two elements, typically isotopes of hydrogen, into the heavier element helium, releasing tremendous amounts of energy in the process, which is the process that powers stars like the Sun.

A fusion power plant would produce abundant energy using only hydrogen from water as fuel, and producing helium as waste, without the risk of meltdowns or radiation. This is in contrast with nuclear fission, the type of reaction in contemporary nuclear power plants, which splits the nuclei of heavy elements like uranium to produce energy.

But while fusion reactions take place in the Sun, and uncontrolled fusion takes place in thermonuclear weapon explosions, controlling a sustained fusion reaction for generating power has eluded nuclear engineers for decades. Experiments of varied design have managed to produce fusion reactions for very small amounts of time, but never have they reached “ignition,” the point where the energy released from a fusion reaction is greater than the amount of energy required to generate and maintain that reaction.

But the team at the National Ignition Facility and authors of one of the three new papers, the one published in the journal Physical Review Letters, argue that “ignition is a state where the fusion plasma can begin ‘burn propagation’ into surrounding cold fuel, enabling the possibility of high energy gain.” That is, fusion began in cold hydrogen fuel and the reaction expanded to generate far more power than in previous experiments.

The 8 August 2021 experiment required 1.9 megajoules of energy in the form of ultraviolet lasers to instigate a fusion reaction in a small, frozen pellet of hydrogen isotopes, — an inertial confinement fusion reaction design — and released 1.3 megajoules of energy, or about 70% of the energy put into the experiment. The output, in other words, was more than a quadrillion watts of power, even if released for only a small fraction of a second.

“The record shot was a major scientific advance in fusion research, which establishes that fusion ignition in the lab is possible at NIF,” Omar Hurricane, chief scientist for Lawrence Livermore National Laboratory’s inertial confinement fusion program, said in a statement. “Achieving the conditions needed for ignition has been a long-standing goal for all inertial confinement fusion research and opens access to a new experimental regime where alpha-particle self-heating outstrips all the cooling mechanisms in the fusion plasma.”

But subsequent attempts to replicate the experiment have produced far less output energy, most in the 400 to 700 kilojoules range, leading some researchers to suggest that the experimental design of the National Ignition Facility is a technical dead-end, according to reporting by the news department at the journal Nature.

“I think they should call it a success and stop,” physicist and former US Naval Research Laboratory laser fusion researcher Stephen Bodner told Nature.

The National Ignition Facility cost $3.5 billion, more than $2 billion more than expected, and is behind schedule, with researchers initially targeting 2012 as the deadline to prove ignition was possible using the design.

But the new studies suggest that researchers are willing to keep exploring what the National Ignition Facility is capable of, especially because unlike other fusion researchers, the researchers at the facility are not primarily focused on developing fusion power plants, but better understanding thermonuclear weapons.

“We’re operating in a regime that no researchers have accessed since the end of nuclear testing,” Dr Hurricane said. “It’s an incredible opportunity to expand our knowledge as we continue to make progress.”