Spectrum and conversion efficiency measurements of suprathermal electrons from relativistic laser plasma interactions

摘要

Fast Ignition is an alternative scheme for Inertial Confinement Fusion (ICF) that uses a petawatt laser to ignite a hot spot in precompressed fuel. The laser delivers its energy into relativistic electrons at the critical surface of the blowoff plasma. These electrons must propagate to the fuel core and deliver their energy to a hot spot. Electrons of energies between 1 and 3 MeV have the appropriate range for efficient energy deposition. This thesis experimentally explores the coupling efficiency and spectrum of the laser produced electrons. The experiments make use of Bremsstrahlung and K-shell emission from planar foil targets to infer the electron distribution produced in the laser-plasma interaction. This thesis describes the development of a filter stack Bremsstrahlung spectrometer with differential sensitivity up to 500 keV. The spectrometer is used with a single photon counting camera for measuring K[alpha] emission in experiments on the Titan laser (1.06 [mu]m, 150 J, 0.7 ps) at Lawrence Livermore National Laboratories. The Bremsstrahlung and K-shell emission from 1 mm3 planar targets irradiated with intensities from 3x1018-8x1019 W/cm2 were measured. The target emission is modeled using the Monte Carlo code Integrated Tiger Series 3.0 in order to unfold the electron spectrum from the x-ray measurements. Conversion efficiencies into 1-3 MeV electrons of 12-28% were inferred, representing 35-60% total conversion efficiencies. Laser diagnostics were used to characterize the laser focal spot and pulselength in order to provide proper comparisons to intensity scaling laws.