Clean source of soft X-ray radiation formed in supersonic Ar gas jets by high-contrast femtosecond laser pulses of relativistic intensity

In this work, we optimized a clean, versatile, compact source of soft X-ray radiation (Ex-ray similar to 3 keV) with an yield per shot up to 7 x 10(11) photons/shot in a plasma generated by the interaction of high-contrast femtosecond laser pulses of relativistic intensity (I-las similar to 10(18)-10(19) W/cm(2)) with supersonic argon gas jets. Using high-resolution X-ray spectroscopy approaches, the dependence of main characteristics (temperature, density and ionization composition) and the emission efficiency of the X-ray source on laser pulse parameters and properties of the gas medium was studied. The optimal conditions, when the X-ray photon yield reached a maximum value, have been found when the argon plasma has an electron temperature of T-e similar to 185 eV, an electron density of N-e similar to 7 x 10(20) cm(-3) and an average charge of Z similar to 14. In such a plasma, a coefficient of conversion to soft X-ray radiation with energies Ex-ray similar to 3.1 (+/- 0.2) keV reaches 8.57 x 10(-5) , and no processes leading to the acceleration of electrons to MeV energies occur. It was found that the efficiency of the X-ray emission of this plasma source is mainly determined by the focusing geometry. We confirmed experimentally that the angular distribution of the X-ray radiation is isotropic, and its intensity linearly depends on the energy of the laser pulse, which was varied in the range of 50-280 mJ. We also found that the yield of X-ray photons can be notably increased by, for example, choosing the optimal laser pulse duration and the inlet pressure of the gas jet.