Time-resolved spectroscopic studies of laser-produced plasmas

Abstract

High speed optical measurements are important for laser fusion studies. At present, streak cameras provide the most versatile direct optical recording techniques with picosecond time resolution, but they possess limited dynamic range. Their poor performance is believed to be partly due to space charge effects at the electron optical crossover point. This was experimentally investigated in this thesis, but found to be relatively unimportant. Effects inside the photocathode are suggested as an alternative explanation. The dynamic ranges, sweep speeds and linearity of two commercial streak camera systems were measured using a modelocked dye laser and an etalon. The spatial resolution was measured with a test chart. These two streak cameras were then used to study the temporal evolution of the backscattered fundamental and harmonic emission spectra from micro balloon targets when they were irradiated by a neodymium laser. The spectra at the fundamental frequency displayed the characteristic red shift of Brillouin scattering. The spectral features of the 2wq harmonic are explained in terms of ion acoustic turbulence driven by the electron return current. The harmonic spectra were observed to consist of two wings, symmetrically shifted to the blue and red, with a separation depending on the electron temperature. Both the 2w and 3w/2 harmonic emission were found to consist of picosecond pulsations which are attributed to density perturbations driven by the unstable ablation flow across a steepened density profile. Finally, proposals are made for the development of diagnostic techniques in laser produced plasmas.