High-energy bremsstrahlung diagnostics to characterise hot electron production in short-pulse laser-plasma experiments

Abstract

Summary form only given. Short-pulse, high-intensity (10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sup> Wcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ) laser-target interactions produce high-density plasmas, in which hot electrons are generated, via a number of energy absorption mechanisms. A key requirement in many applications of these interactions is the achievement of the maximum conversion efficiency of laser energy into hot electrons of a specific energy. Measurement of the hot electron temperature and the associated hot electron fraction will assist in the understanding of absorption mechanisms present in the 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sup> Wcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> intensity regime. One highly useful signature of hot electron generation is the bremsstrahlung radiation emission due to the hot electrons interacting electromagnetically with cold target atoms. We present the design, full characterisation and modelling of two target diagnostics to measure the high-energy (100 keV to 2 MeV) bremsstrahlung emission from hot electrons; a thermoluminescent dosimeter (TLD) array and a hard X-ray spectrometer (HXRS). These diagnostics will be used to determine the hot electron distribution generated with the short-pulse beamlines of the upcoming high-power Orion laser system at AWE. Past bremsstrahlung dose measurements obtained with a TLD array (of similar design to the Orion one) are used to demonstrate how the hot electron production efficiency can change with laser parameters, target type and experimental geometry. In addition, we use results from pre-installation testing of the HXRS to establish a charge-collection method enabling the diagnosis of hot electron temperatures in laser-plasma experiments.