Prospects for inertial fusion energy with a KrF laser

Previous reactor studies indicate that a practical laser fusion power plant will require target gains of about 100. This level of energy gain appears possible with direct-drive targets now being designed and optimized at the Naval Research Laboratory (NRL). With direct-drive, the light is absorbed directly on the pellet shell, thereby maximizing the coupling efficiency. The current status of NRL's high gain target designs will be presented. To obtain sufficiently high target gains for a fusion reactor, NRL has had to take advantage of three optimizations. First, the laser beam illumination on the pellet has to be extremely uniform. High-mode beam nonuniformities in the range of 0.2% rms are required, along with low-mode nonuniformities of about 1%. The equivalent nonuniformity levels have already been achieved, in planar geometry, with NRL's KrF laser. Second, the rocket efficiency has to be maximized by depositing the laser energy deeply into the pellet. KrF, with 1/4 micron wavelength light, deposits at a high plasma density. Third, the target gain is optimized by "zooming" the laser beam inward during the implosion, thereby matching the laser spot size to the decreasing pellet diameter. This optical zooming is easily implemented on KrF lasers. Although the laser-target physics leads us to KrF, there are several engineering challenges in developing a laser of this type with sufficient energy, rep-rate, reliability, and economy for a practical reactor. Some of these challenges are the lifetime of the emitter and pressure foil in the electron-beam pumped amplifiers, the ability to clear the laser gas between pulses without sacrificing beam quality, and the overall efficiency of the system. Technologies and techniques which might meet these challenges have been partially developed elsewhere, but they are not necessarily in a parameter range appropriate for laser fusion, and they have yet to be integrated into a single system. We have a conceptual design for a 400-Joule, 5-Hz KrF laser which would serve as a test bed for these technologies. There are also engineering challenges in the design of a target chamber for a laser fusion reactor, including the protection of the first wall from the transient x-ray flux, and the final grazing incidence metal mirror which will be in direct line of sight of high energy neutrons from the burning pellet.