Title: “Laser Fusion Research at the U.S. Naval Research Laboratory: The Challenges and Progress in Meeting the Challenges of Achieving Inertial Fusion with Lasers” Speaker: Dr. Steve Obenschain
Affiliation: Head of Laser Plasma Branch, Plasma Physics Division, Naval Research Laboratory Abstract: Inertial confinement fusion (ICF) would involve the compression and ignition of frozen deuterium-tritium “fuel“ contained within few mm diameter pellets. The most developed approach involves use of high-energy high-power lasers to drive the implosion. This is being attempted either by directly illuminating and thereby heating the pellet surface, or the indirect-drive approach where the laser beams heat the inner wall of a gold hohlraum containing the pellet and the x-rays from the heated wall drive the pellet implosion. In both cases the pellet surface is ablated and the thrust from it drives the pellet implosion to speeds of several hundred km/sec. Most inertial fusion research is being done using frequency-tripled Nd:glass lasers operating at λ=351 nm. Short laser wavelength improves the coupling efficiency to the target and allows operating at higher laser intensity before undesired laser-plasma instability appears. The laser fusion program at the Naval Research Laboratory (NRL) has developed and utilized a different krypton fluoride (KrF) gas laser technology.* The KrF laser has substantial target physics and technological advantages towards achieving robust direct-drive implosions that ignite and provide high energy gain. The physics advantages arise from its shorter wavelength (λ=248nm), capability for more uniform target illumination, and broader bandwidth than existing frequency tripled glass lasers. In addition the focal diameter of a KrF laser can easily be zoomed down to follow an imploding target which further increases the coupling efficiency and also helps suppress unwanted Cross Beam Energy Transfer (CBET) in the coronal plasma. Recently we have begun research on the potential of the still shorter wavelength ArF (λ=193nm) laser for ICF. In addition, NRL ICF researchers are developing the means to achieve extreme bandwidths with existing ICF laser beams using Stimulated Rotational Raman Scattering (SRRS) in diatomic gases. Large laser bandwidth (>> 1 THz) is an additional means to suppress laser plasma instability. This presentation will provide an overview of the scientific and technological challenges to achieving high-performance inertial fusion with lasers and recent ICF research at NRL.
Bio: Dr. Obenschain is Head of the Laser Plasma Branch and leads the laser fusion program at the Naval Research Laboratory. The laser fusion program includes research efforts in laser-matter-interaction experiments, in large scale simulations of pellet implosions, and in development of high-energy krypton-fluoride (KrF) laser technology. This research has been primarily funded by the Department of Energy, National Nuclear Security Administration. Dr. Obenschain was project manager for the design and construction of Nike, the world’s largest KrF laser facility. He was co-inventor with Dr. Robert Lehmberg, of the induced spatial incoherence (ISI) technique that provides uniform illumination of targets by high-energy lasers. He led the first experimental efforts that showed such laser beam-smoothing schemes can help suppress deleterious laser-plasma instability. For this work he was a recipient of the 1993 APS-DPP award for Excellence in Plasma Physics Research. He received the Fusion Power Associates Leadership award in 2012. In selecting Dr. Obenschain, the FPA Board recognized his many scientific and technical contributions to fusion development and the leadership he has been providing to the U.S. and world inertial fusion efforts, including the leadership and vision he has been providing to planning for a next-step inertial fusion test facility. He is a fellow of the American Physical Society. He received a B.S. degree in physics from the University of Virginia and a Ph.D. in plasma physics from UCLA.