X-RAY SPECTROSCOPY OF HOT SOLID DENSITY PLASMAS PRODUCED BY SUBPICOSECOND HIGH-CONTRAST LASER-PULSES AT 10(18)-10(19) W/CM(2)

Analysis is presented of K-shell spectra obtained from solid density plasmas produced by a high contrast (1010:1) subpicosecond laser pulse (0.5 μm) at 1018-1019 W/cm2. Stark broadening measurements of He-like and Li-like lines are used to infer the mean electron density at which emission takes place. The measurements indicate that there is an optimum condition to produce x-ray emission at solid density for a given isoelectronic sequence, and that the window of optimum conditions to obtain simultaneously the shortest and the brightest x-ray pulse at a given wavelength is relatively narrow. Lower intensity produces a short x-ray pulse but low brightness. The x-ray yield (and also the energy fraction in hot electrons) increases with the laser intensity, but above some laser intensity (1018 W/cm2 for Al) the plasma is overdriven: during the expansion, the plasma is still hot enough to emit, so that emission occurs at lower density and lasts much longer. Energy transport measurements indicate that approximately 6% of the laser energy is coupled to the target at 10 18 W/cm2 (1% in thermal electrons with T e≈0.6 keV and 5% in suprathermal electrons with T h≈25 keV). At Iλ2=1018 W μm 2/cm2 (no prepulse) around 1010 photons are emitted per laser shot, in 2π srd in cold Kα radiation (2-9 Å, depending on the target material) and up to 2×1011 photons are obtained in 2π srd with me unresolved transition array (UTA) emission from the Ta target. © 1995 American Institute of Physics.