The approach being taken by the National Ignition Facility to achieve thermonuclear ignition and burn is called the “central hot spot” scenario. This technique relies on simultaneous compression and ignition of a spherical fuel capsule in an implosion, roughly like in a diesel engine (see How to Make a Star). Although the hot-spot approach has a high probability for success, there also is considerable interest in a modified approach called fast ignition (FI), in which compression is separated from the ignition phase. Fast ignition uses the same hardware as the hot-spot approach but adds a high-intensity, ultrashort-pulse laser to provide the “spark” that initiates ignition. A deuterium-tritium target is first compressed to high density by lasers, and then the short-pulse laser beam delivers energy to ignite the compressed core—analogous to a sparkplug in an internal combustion engine.
An advantage of the FI approach is that the density and pressure requirements are less than in central hot-spot ignition, so in principle fast ignition will allow some relaxation of the need to maintain precise, spherical symmetry of the imploding fuel capsule. In addition, FI uses a much smaller mass ignition region, resulting in reduced energy input, yet provides an improved energy gain estimated to be as much as a factor of 10 to 20 over the central hot-spot approach. With reduced laser-driver energy, substantially increased fusion energy gain—as much as 300 times the energy input—and lower capsule symmetry requirements, the fast-ignition approach could provide an easier development pathway toward an eventual inertial fusion energy power plant.
In the compression stage, x rays generated by laser irradiation of the hohlraum wall deposit their energy directly on the outside of a spherical shell, the ablator shell, that rapidly heats and expands outward. This action drives the remaining shell inward, compressing the fuel to form a uniform dense assembly.
To ignite the fuel assembly, about 20 kilojoules of energy must be deposited in a 35-micrometer spot in a few picoseconds (trillionths of a second), heating the fuel to the ignition temperature and initiating thermonuclear burn. The leading approach to FI uses a hollow cone of high-density material inserted into the fuel capsule to allow clean entry of the second laser beam to the compressed fuel assembly (see more on Fast Ignition).
The physics basis of FI, however, is not currently as mature as that of the central hot-spot approach. The coupling efficiency from a short-pulse laser to the FI hot spot is a critical parameter dependent on very challenging and novel physics. Fast ignition researchers must resolve these physics problems to justify advancement to the next stage. Success in demonstrating efficient transport of a high-energy pulse into dense plasma, development of a target design for the compression phase, and definition of a power plant concept could lead to a new energy source for the nation and the world.
“Titan Leads the Way in Laser-Matter Science,” Science & Technology Review, January/February 2007.
“New Targets for Inertial Fusion,” Science & Technology Review, November 2001.
“Taking Lasers Beyond the National Ignition Facility,” Science & Technology Review, September 1996.