A review of laser-plasma interaction physics of indirect-drive fusion

The National Ignition Facility (NIF) has been designed, constructed and has recently begun operation to investigate the ignition of nuclear fusion with a laser with up to 1.8 MJ of energy per pulse. The concept for fusion ignition on the NIF, as first proposed in 1990, was based on an indirectly driven spherical capsule of fuel in a high-Z hohlraum cavity filled with low-Z gas (Lindl et al 2004 Phys. Plasmas 11 339). The incident laser energy is converted to x-rays with keV energy on the hohlraums interior wall. The x-rays then impinge on the surface of the capsule, imploding it and producing the fuel conditions needed for ignition. It was recognized at the inception that this approach would potentially be susceptible to scattering of the incident light by the plasma created in the gas and the ablated material in the hohlraum interior. Prior to initial NIF operations, expectations for laser-plasma interaction (LPI) in ignition-scale experiments were based on experimentally benchmarked simulations and models of the plasma effects that had been carried out as part of the original proposal for NIF and expanded during the 13-year design and construction period. The studies developed the understanding of the stimulated Brillouin scatter, stimulated Raman scatter and filamentation that can be driven by the intense beams. These processes produce scatter primarily in both the forward and backward direction, and by both individual beams and collective interaction of multiple beams. Processes such as hot electron production and plasma formation and transport were also studied. The understanding of the processes so developed was the basis for the design and planning of the recent experiments in the ignition campaign at NIF, and not only indicated that the plasma instabilities could be controlled to maximize coupling, but predicted that, for the first time, they would be beneficial in controlling drive symmetry. The understanding is also now a critical component in the worldwide effort to produce a fusion energy source with a laser (Lindl et al 2011 Nucl. Fusion 51 094024, Collins et al 2012 Phys. Plasmas 19 056308) and has recently received its most critical test yet with the inception of the NIF experiments with ignition-scale indirect-drive targets (Landen et al 2010 Phys. Plasmas 17 056301, Edwards et al 2011 Phys. Plasmas 18 051003, Glenzer et al 2011 Phys. Rev. Lett. 106 085004, Haan et al 2011 Phys. Plasmas 18 051001, Landen et al 2011 Phys. Plasmas 18 051001, Lindl et al 2011 Nucl. Fusion 51 094024). In this paper, the data obtained in the first complete series of coupling experiments in ignition-scale hohlraums is reviewed and compared with the preceding work on the physics of LPIs with the goal of recognizing aspects of our understanding that are confirmed by these experiments and recognizing and motivating areas that need further modeling. Understanding these hohlraum coupling experiments is critical as they are only the first step in a campaign to study indirectly driven implosions under the conditions of ignition by inertial confinement at NIF, and in the near future they are likely to further influence ignition plans and experimental designs.