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

Abstract

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.

(cont.) Comparisons to scaling laws show that the electron spectrum is colder than the laser ponderomotive potential derived from the peak intensity. For intensities above 2 x 1019 W/cm2, the spectrum is slightly hotter than widely used empirical scalings. More accurate comparisons to ponderomotive scaling using a synthetic energy spectra generated from the intensity distribution of the focal spot imply slope temperatures less than the ponderomotive potential, but is within the range of a correction due to the neglect of resistive transport effects. The impact of resistive transport effects were estimated using an analytic transport model and may lead to higher total conversion efficiencies but lower conversion efficiencies into 1-3 MeV electrons.