Study of x-ray generation from noble gases mixture jets irradiated by unr FS-Leopard laser with different pulse contrast

Abstract

Some aspects of physical phenomena occurring when an intense laser pulse with sub-ps duration and intensity > 1018-1019 W/cm2 heats under-dense plasma in supersonic clusters/gas jet from gas-puff system were studied to determine the relative contribution of these processes to soft and hard x-ray emission from debris-free plasmas. Experiments were performed at the UNR Leopard laser operated with 15J / 350 fs and different pulse contrast (107 or 10−5). The supersonic linear (elongated) nozzle generated Ar and Kr clusters/monomer jets as well as jets with Ar/Kr or Ar/Kr/Xe mixtures with density > 1019 cm3. In prior laser heating experiments, all jets were probed with optical interferometry and Rayleigh scattering to measure jet density and cluster distribution parameters. The elongated jet provides the capability to study laser beam self-focusing in plasma that is interconnecting with efficient x-ray generation. The laser radiation flux density in the focal spot was up to 2 × 1019 W/cm2. Plasma diagnostics included x-ray diodes, pinhole cameras and spectrometers. Jet signatures of x-ray emission from pure Ar carrier gas as well as from mixture with lower Kr concentration and Ar/Kr mixture with adding Xe gas, were found to be very different. The most intense x-ray emission was observed in a wide spectral region 1–9 keV from these gas mixtures compared to pure Ar or pure Kr carrier gases. Also, this x-ray emission was strongly anisotropic depending on the direction of laser beam polarization. Non-LTE models have been implemented to analyze the x-ray spectra. It was shown that Ar/Kr jet plasma was hotter than that from pure Kr. X-ray spectra of Ar carrier gas (only “cold” Ka line) were similar to the spectra obtained early from subsonic tube nozzle (no clusters in jet). It can be explained by the fact that Ar in Ar/Kr mixture did not generate clusters as opposed to Kr. Evidence of electron beam generation in supersonic jets' plasma was found. Influence of fs laser pulse contrast level on x-ray emission characteristics is discussed. Future research will focus on the study of cluster formation in linear supersonic jets with Ar carrier gas and Kr and Xe impurities, varying its parameters for optimization of x-ray yield and power. Another goal is to determine optimal conditions for possible application of x-ray source with gas mixture jets at larger sub-ps laser facilities.