Absorption and Fluorescence Spectroscopy as a Diagnostic of Laser Driven Experiments

Abstract

Point projection absorption spectroscopy has been used for many years in the measurement of radiative opacity [1]. This powerful diagnostic technique allows both spectral and two-dimensional spatial information to be obtained by radiography of targets using a near point source of x-rays. Recently the authors have applied this technique to measure absorption spectra, not to study opacity, but as a diagnostic of a range of radiation-hydrodynamics phenomena relevant to inertial confinement fusion. Characterization of targets by absorption or emission spectroscopy at frequencies close to the peak of thermal emission is possible but can have problems with self-emission, high background and non-thermal emission and may reduce the size of targets which can be probed. Therefore K-shell absorption spectroscopy was chosen because the line spectra sampled are at much higher frequencies than the peak thermal emission of the targets thus avoiding the problems mentioned above. K-shell spectroscopy also allows the use of crystal spectrometers which tend to be simpler to implement than gratings of similar resolution. Examples of the use of the technique are given. For instance detailed characterization of conditions in shock and radiation waves have enabled models of transonic flows to be tested against experiment. Also the preheating effect of forming a plasma buffer to smooth laser beam imprint has been studied under conditions relevant to direct drive inertial confinement fusion. To infer the target conditions the measured spectra must be compared with synthetic spectra generated by modelling the opacity in the region of the diagnostic lines. The opacity model used in this work is briefly described.