Analysis ofKα line emission from aluminum plasmas created by intense proton beams

Abstract

X-ray satellite line emission from targets irradiated by intense-light ion beams can be used to diagnose plasma conditions and beam properties. We present results from our analysis of spectroscopic measurements of x-ray K\ensuremath{\alpha} satellites emitted from a target irradiated by an intense-light ion beam. In this experiment, performed on the Particle Beam Fusion Accelerator-II device at Sandia, an Al target was irradiated with a 4--6 MeV proton beam with a peak power density of 1--2 TW/${\mathrm{cm}}^{2}$. Up to 15% of the beam electrical current was in the form of carbon contaminants. A time-integrated spectrum was obtained with a resolution of \ensuremath{\lambda}/\ensuremath{\Delta}\ensuremath{\lambda}>1200. In our analysis, collisional radiative equilibrium (CRE) calculations were performed to study the dependence of the Al K\ensuremath{\alpha} emission spectrum on plasma and beam properties. Good agreement is obtained between calculated satellite spectra and the emission features observed in the experimental spectrum. We find that excited states with M-shell spectator electrons contribute significantly to K\ensuremath{\alpha} satellite emission spectra, exhibiting relatively broad lines at wavelengths blueshifted with respect to those originating from ground-state configurations. Because of opacity effects, it is likely that only radiation emitted from the outer skin of the target plasma was seen by the detector in the experiment. It is also shown that resonant self-absorption can skew the spectrum towards apparent higher ionization stage. Our results suggest that emission spectroscopy of x-ray satellite lines from thin tracer layers offers a potentially valuable technique for determining plasma conditions in intense-light-ion-beam experiments.