Opacity is one of the important factors in the formation of intensity and shape of x-ray line spectra of high-temperature and high-density plasmas produced by powerful laser pulses. On dependence of plasma properties opacities may cause complicated non-symmetrical deformations of line contours, especially in expanding inhomogeneous plasmas. In this work, we present the results demonstrating that in the presence of relative microscopic motions of plasma components, the influence of self-absorption could be reduced if, due to the Doppler effect, the shift of the absorption and the emission line centers will be comparable or larger than the intrinsic width of the line.
The experiment was performed at J-KAREN-P laser facility [2] on two-side interaction of high contrast ultra-relativistic intensity of ~ 10²¹ W/cm² pulses with thin (1-2 mm) 4-Chlorostyrene foils. The rear side of the target was heated by 1/100 of the main pulse power picked up with controlled a few-nanosecond time interval prior to the main pulse irradiating the front side of the target. The x-ray emission of highly charged Cl ions was measured by a high-resolution x-ray spectrometer from the rear side of the target. At such observation, the spectra represent the emission of the hot front-side plasma after its self-absorption and absorption after passing through the back-side preplasma with T_e of ~ few hundred keV. In opposite to the expected increase of opacity of preplasma for the He-like line, the decrease of absorption was observed, indicated by the narrowing of He_b line and the decreasing of intensity ratio He_b /Ly_a.
  One of the possible explanations for observed plasma transparency is a high relative speed of downstream (from front) and upstream (from rear) plasma expansion. The result is interesting. We will discuss in detail experimental conditions, spectral features, method of spectral analysis and argue our conclusions.

 

high power lasers,x-ray spectroscopy,,Relativistic Intensity,laser-produces plasma