Spectrally smooth X-ray source produced by laser direct driven DT implosion target on SG-Ⅲ laser facility

Abstract

Spectrally smooth X-ray sources can be used in point projection radiography and absorption spectrometry diagnostics of dense plasmas. But conventionally they are end at about 3.5 keV, which can only be used to diagnose materials up to Z=18. Spectrally smooth X-ray sources above 3.5 keV are needed to study higher-Z materials. Bremsstrahlung radiation from a laser driven implosion target can produce a small size, short duration and spectrally smooth X-ray source in the range of 1-100 keV. They have been successfully applied in the investigations of middle-Z materials in the 3-7 keV X-ray range. Despite much interest for backlit X-ray studies of middle- and high-Z dense materials, research on implosion X-ray sources are scarce. Characterization of the implosion X-ray source is needed to understand and improve its performance.To provide a physical basis for optimization, the properties of the deuterium-tritium (DT) implosion target X-ray source driven by 30-180 kJ laser pulses were explored using a radiation hydrodynamics code.We focus on laser pulse energies of 30-180 kJ at 351 nm wavelength to match the range of the OMEGA laser on the low end and the SG-Ⅲ laser on the high end. The laser pulse parameters are scaled with the target size in identical fashion to that of the OMEGA laser and the ignition designs of the National Ignition Facility to maintain the same irradiance on the surface of the capsule.The temporal and spatial evolution of the implosion targets was calculated using Multi-1D, a one-dimensional radiation hydrodynamics code. The emergent X-ray spectrum is calculated by post-processing from the time histories of the temperature and density profiles output by the Multi-1D code. We adjusted the laser absorption fraction to ensure neutron yield in accordance with OMEGA's 1D simulation results.It shows that the rapid increase of density and temperature at stagnation time develops a 150 ps point X-ray flash with approximately 100 μm size. The dominant X-ray emission comes from the inner layer of the dense compressed shell, which should be the focus of future efforts to improve the X-ray emission. Softer X-rays below 30 keV carry most of the energy due to the exponentially decaying spectral profile of implosion X-ray source. Opacity of the dense compressed shell plasma can markedly reduce the very softer X-ray emission of 1-3 keV. DT fusion reactions can enhance the share of harder X-rays above 30 keV greatly, while show negligible effect on the brightness of the implosion X-ray source. Thus higher-Z plastic target or glass target may be a better choice in generating the implosion X-ray source.