Guest essay by Eric Worrall
Inertial confinement fusion researchers have claimed a near break even experimental nuclear fusion burn, in which energy produced by the fusion reaction was comparable to the energy injected to initiate the burn.
Laser fusion experiment yields record energy at LLNL’s National Ignition Facility
LIVERMORE, Calif. — In the early morning hours of Aug.13, Lawrence Livermore’s National Ignition Facility (NIF) focused all 192 of its ultra-powerful laser beams on a tiny deuterium-tritium filled capsule. In the nanoseconds that followed, the capsule imploded and released a neutron yield of nearly 3×10 15 , or approximately 8,000 joules of neutron energy — approximately three times NIF’s previous neutron yield record for cryogenic implosions.
The primary mission of NIF is to provide experimental insight and data for the National Nuclear Security Administration’s science-based stockpile stewardship program. The experiment attained conditions not observed since the days of underground nuclear weapons testing and represents an important milestone in the continuing demonstration that the stockpile can be kept safe, secure and reliable without a return to testing.
This newest accomplishment provides an important benchmark for the program’s computer simulation tools, and represents a step along the ” path forward ” for ignition delivered by the NNSA to Congress in December 2012.
Early calculations show that fusion reactions in the hot plasma started to self-heat the burning core and enhanced the yield by nearly 50 percent, pushing close to the margins of alpha burn, where the fusion reactions dominate the process.
“The yield was significantly greater than the energy deposited in the hot spot by the implosion,” said Ed Moses, principle associate director for NIF and Photon Science. “This represents an important advance in establishing a self-sustaining burning target, the next critical step on the path to fusion ignition on NIF.”
The experiment was designed to resist breakup of the high velocity imploding ablator (shell of the target capsule) that has degraded the performance of previous experiments by lowering compression of the target. To create this resistance, the laser power is turned up during the picket that occurs at the beginning of the laser pulse. This raises the radiation temperature in the foot or trough period of the pulse (hence the name “high-foot” pulse), increasing the stability of the ablator but reducing compression later in the implosion.
The high-foot campaign was born after systematically exploring possible causes for the shell breakup observed in a series of lower foot, more compressed experiments, and developing hypotheses for how to address the issue.
“In the spirit of what Livermore is good at, this work was born out of the fierce competition of ideas of how to fix the problem, but then coming together as a team to move the best ideas forward,” said Omar Hurricane, lead scientist on the campaign. “In this particular experiment, we intentionally lowered the goal in order to gain control and learn more about what Mother Nature is doing. The results were remarkably close to simulations and have provided an important tool for understanding and improving performance.”
These promising returns were the result of a laser experiment that delivered 1.7 megajoules (MJ or million joules) of ultraviolet light at 350 terawatts (TW or trillion watts) of peak power. NIF is the world’s largest and most energetic laser system, which has already pushed past its design specifications of 1.8 MJ and 500 TW, leaving headroom for more exploration of this idea. The campaign is the product of a strong collaboration between LLNL’s NIF and Photon Science and Weapons and Complex Integration directorates.
Moses expressed his gratitude to the team of designers and experimentalists. “Much thanks to the many who seamlessly integrated their capabilities in order to field this experimental campaign,” he said. “It’s hard not to feel encouraged by the progress we’ve made with great new and planned diagnostic capabilities, promising results with high-foot experiments, a team that is working extremely well together and a go forward plan that, by and large, is well supported by the community.”
I find inertial confinement fusion exciting, because in principle, unlike magnetic confinement fusion, it might be possible to scale inertial confinement down to an affordable size.
The gigantic international magnetic confinement ITER tokamak currently being constructed in France in a sense represents a brute force approach to viable nuclear fusion. The heat produced by a nuclear fusion reaction is related to the volume of the plasma, while the heat lost is related to the surface area. Simple geometry dictates that if you make the plasma volume really large, the heat generated by such a large volume of fusing plasma is more likely to overcome surface losses, leading to a self sustaining fusion reaction.
My concern with this magnetic confinement approach is that even if ITER succeeds, the sheer size and cost of the precision engineered reactor vessel will represent a formidable barrier to adoption. Nuclear fusion reactors which cost $50 billion each and take decades to construct are unlikely to contribute significantly to the global energy mix, so long as cheaper options are available.
There is also a real risk that after all these billions of dollars of expenditure and man millennia of effort, ITER’s most expensive components will simply disintegrate under the blast of radiation from a sustained fusion burn. Deuterium Tritium fusion produces a blizzard of hot neutrons, which are more than capable of causing physical structural damage to anything near the plasma. The search for structural materials which can survive such a hostile environment without collapsing into dust is ongoing.
The Lawrence Livermore facility which produced the near break even burn is large, but it is a lot less expensive than the ITER facility.
Lawrence Livermore still have a long way to go to prove that inertial confinement is a viable path to connecting an operational nuclear fusion reactor to the national grid. Although the energy produced was comparable to the energy deposited to initiate the burn, the lasers which deposited that energy are not 100% efficient. The total energy expended conducting the experiment likely vastly exceeded the fusion yield.
An inertial confinement fusion generator would have to economically perform thousands of burns per day, rather than a single exciting experimental burn. And of course we still don’t know how much a net energy producing inertial confinement fusion reactor would cost, even if such a thing is possible.