Measuring the ionization balance of gold in a low-density plasma ofimportance to inertial confinement fusion

Abstract

Charge state distributions (CSDs) have been determined in low-density (≈1012 cm–3) gold plasmas having either a monoenergetic beam (EBeam = 2.66, 3.53, 4.54, 5.35, 5.85, and 6.35 keV) or experimentally simulated thermal electron distributions (Te = 2.0, 2.5, and 3.0 keV). These plasmas were created in the Livermore electron beam ion traps, EBIT-I and EBIT-II. Line emission and radiative recombination features of K to Kr-like gold ions were recorded in the X-ray region with a crystal spectrometer and a photometrically calibrated microcalorimeter. The CSDs in the experimentally simulated thermal plasmas were inferred by fitting the observed 4f → 3d and 5f → 3d lines with synthetic spectra from the Hebrew University Lawrence Livermore Atomic Code (HULLAC). Additionally, the CSDs in the beam plasmas were inferred both from fitting the line emission and fitting the radiative recombination emission to calculations from the General Relativistic Atomic Structure Program. Despite the relatively simple atomic physics in the low-density plasma, differences existed between the experimental CSDs and the simulations from several available codes (for example, RIGEL). Our experimental CSD relied upon accurate electron impact cross sections provided by HULLAC. To determine their reliability, we have experimentally determined the cross sections for several of the n = 3 → 4 and n = 3 → 5 excitations in Ni to Ga-like Au and compared them to distorted wave calculations. Cross-section calculations by flexible atomic code (FAC) and HULLAC were found to be very consistent. Recent Au spectra recorded during experiments at the OMEGA laser facility are presented and compared with those recorded from EBIT-I and EBIT-II. This comparison shows that spectra from the two sources are surprisingly similar despite a 10 order of magnitude difference in their respective plasma densities. PACS Nos.: 52.50.Fs, 52.25.Jm, 34.80.Kw, 34.80.Lx