New target designs are projected to have much higher gains using KrF lasers and direct drive. KrF lasers have several inherent advantages for direct drive targets including: capability for ultra-uniform target illumination, which reduces perturbations imprinted by the laser; and deeper ultraviolet light (wavelength=248 nm) which provides more efficient coupling to the target, thereby allowing higher drive pressures and implosion velocities. The combination of KrF's short wavelength and very broad bandwidth capability (3 THz) suppress deleterious laser plasma instabilities (LPI) that limit the maximum usable laser intensity. In addition, KrF's optical architecture allows easy implementation of focal zooming where the laser spot size follows the imploding pellet, thereby further increasing the coupling efficiency. A new family of high gain direct-drive designs capitalizes on these advantages. Simulations predict a conventional KrF-driven implosion can produce power-plant-class gains (> 120) with laser energy of only 1 MJ. Still higher performance "Shock Ignition" designs have predicted power plant gains at 500 kJ and gains greater than 200 at 1 MJ. In Shock Ignition, which was invented at the University of Rochester, the laser pulse first drives a conventional but lower velocity implosion; then near peak compression a high intensity spike generates a shock to heat the center of the target to ignition temperatures. Work by Rochester and Livermore National Laboratory scientists corroborate these promising results. High resolution 2-D simulations at NRL indicate that the high gains of these shock ignition designs are robust against hydrodynamic instabilities. Experiments on the Nike KrF laser have explored the 10¹⁵-10¹⁶ W/cm² intensities used in these designs, and support the expectation that KrF laser light suppresses laser plasma instability. Shock Ignition designs have predicted gains similar to Fast Ignition, but do not need a multi-petawatt laser or complex targets.

A breakthrough increases the durability and reliability of KrF lasers. The 30-cm aperture Electra KrF laser amplifier recently demonstrated 10 hours of continuous laser operation at 2.5 Hz (approximately 90,000 shots) utilizing two electron beams to pump the laser gas. This is a significant increase in the durability of large KrF laser systems, which was previously limited to runs of 10,000-20,000 shots. This breakthrough in continuous operation was achieved by eliminating voltage reversals in the high voltage pulse that drives the electron beam pump. This eliminated a failure mode in the pressure foil that separates the electron beam source, which is in vacuum, from the laser gas. The foil must be thin enough to provide high electron transmission efficiency while surviving a hostile environment that includes shock waves, thermal cycling, high electron currents, and exposure to heated laser gas that contains fluorine. Blocking the reverse voltage prevents the foil from emitting electrons, which can carry enough current in isolated spots to drill small holes in the foil. Since this change was made, the Electra facility has fired well over 180,000 electron beams shots on one of its two diodes without a foil failure. Work is now underway to develop an all solid state pulse power system for Electra, which would allow similar testing to the >10⁷ shot regime.

For further information contact Steve Obenschain: steve.obenschain@nrl.navy.mil