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.”
