Indeed, the achievement of fusion firing and energy recovery in the world's most intense laser late last year was a major scientific achievement. Yet fusion as a practical energy source would be a long way off, if not a dead end. The majority of scientists agree that the strategy used at the National Ignition Facility (NIF), where the fusion milestone took place, is unlikely to build a laser-guided fusion power plant, even if it eventually becomes a reality.
Deuterium-Tritium Fuel
The shot, which took place in December, silenced skeptics who said that small deuterium-tritium capsules could never be ignited by laser striking them, and produced 2 times the energy of 1,5 MJ fired at fusion fuel.
Stephen Bodner, retired chief of the US Naval Research Laboratory's laser fusion branch and a former staunch opponent of NIF's strategy, believes they have demonstrated ignition and combustion and this is a very important achievement.
Despite the exaggeration surrounding the announcement, the fusion energy yield from the successful shot was less than 192% of the 300 MJ used from the mains to power the 1 beam of NIF. And the energy released was enough to heat ten kettles. According to many scientists, for fusion to be economically viable, the energy gain from fusion reactions must be at least 100 times greater than the energy stored in the fuel capsule (or twice as much as the NIF shot).
According to Bedros Afeyan, a consultant who has worked in fusion R&D at three national laboratories, as a result of NIF's success, IFE is now 10% of the way to commercialization.
Unlike the few hours between NIF shots, an IFE power plant will need to fire a laser every few seconds at least. The machine will also need to produce its own fuel; this fuel will then be pumped into an extremely hot reactor chamber and then loaded into extremely small capsules that must somehow be maintained at cryogenic temperatures for a short time. Moreover, the plant must produce hydrogen, electricity or industrial process heat in a cost competitive manner.
As IFE developers move away from NIF, which was created to model nuclear weapons operations rather than power generation, at least three fundamental questions need to be answered. First, should the indirect propulsion method used by NIF be imitated, in which the light is first converted into x-rays and crushed the pellets, or should the light of the laser directly detonate the fuel capsules? Second, what kind of laser can do this best? What is the most cost-effective way to design and mass-produce targets carrying DT fuel? Responses to these concerns will be crucial if laser fusion can be made affordable.
Direct drive is maintained by two US startups, Focused Energy and LaserFusionX, which use various types of lasers. Longview Fusion Energy Systems, based in Orinda, California, creates a NIF-style, all-indirect propulsion strategy. An indirect-direct hybrid strategy has been developed by Xcimer Energy, based in Redwood City, California.
According to many laser experts, indirect drive cannot be made efficient enough to achieve the degree of gain needed to generate power at a reasonable cost. According to Michael Campbell, emeritus director of the University of Rochester Laser Energetic Laboratory, UV rays are absorbed and x-rays are emitted within the hollow cylinder, or hohlraum, surrounding the fusion fuel capsule, but a lot of laser energy is lost.
Indirect propulsion requires more sophisticated targets, and they are probably more expensive than the basic spherical fuel capsules that direct drive offers. NIF targets cost at least $10.000 each and are not mass produced. Each of the hundreds of thousands of unique targets to be detonated each day would have to cost less than $1 to be economically viable. Afeyan recommends keeping goal design as simple as possible. “Skip the indirect driver. This is not possible,” he says.
Capsules carrying cryogenic DT fuel can have some protection from the hohlraums of indirect propulsion while being rapidly injected into the target chamber. However, debris left behind by hohlraum eruptions can quickly build up and pose a clean-up problem.
Much of the direct drive research has been done in the Department of Energy-funded Laser Energy Laboratory, which is also home to the Omega laser. Working with direct drivers, researchers have so far been unable to create explosions with the exact symmetry required for ignition. According to Campbell, the more powerful the laser, the less precision will be required for direct-actuated explosions that will result in ignition and gain.
With a light output of just 25 KJ, Campbell claims the Omega is insufficient in any case. Another research facility with sufficient energy must be built to allow the plasmas to be fired in direct drive mode.
Source and Further Reading: Physics Today – physicstoday.scitation.org/doi/10.1063/PT.3.5195
Günceleme: 13/03/2023 16:29