Advantages of KrF Lasers for Inertial Confinement Fusion Energy

Advanced concepts for direct drive inertial confinement fusion (ICF) have emerged that may lead to sufficient gain for the energy application (g>140) at laser driver energies as low as 1 megajoule. For example, recent “shock ignition” designs compress low aspect ratio pellets then apply a final high intensity spike pulse (10 16 W/cm 2 ) to ignite the fuel via a converging shock wave. These analyses were based on an excimer laser with a Krypton-Fluoride lasing (KrF) medium. KrF systems are particularly well suited to these new ideas as they operate in the deep ultraviolet ( =248 nm), provide highly uniform illumination, possess large bandwidth (1-3 THz), and can easily exploit beam zooming to improve laser-target coupling for the final spike pulse. While this driver option is strongly supported by a multitude of attractive technological features, decisive factors for any candidate technology must assess target physics for high gain operation. This presentation will examine advantages of KrF lasers in relation to the new implosion designs. Supporting experimental and theoretical studies of hydrodynamic instabilities and laser plasma instabilities (LPI) conducted by the Nike laser group at the U. S. Naval Research Laboratory will be discussed. Recent studies of the two plasmon decay instability have made the first determination of the threshold intensity for this instability in an ICF-relevant plasma driven by a KrF laser. This instability is a major concern due to its ability to generate hot electrons that may preheat the compressed fuel prior to ignition. The observed threshold intensities are higher than that reported for longer wavelength lasers. An expanded range of allowed intensities would help KrF lasers meet the requirements of advanced ICF designs. Recent experimental work has also shown that the high ablation pressures and smooth profiles obtained with the Nike laser can be used to accelerate planar targets to velocities consistent with the requirements of impact ignition.