Statistical fitting analysis of Stark-broadened optically thick ArII spectra measured in ion beam transport experiments

We present a method for fitting a multi-line spectrum with an emission mode l that accounts for multiple line-broadening mechanisms and the effects of finite optical depth. This technique includes an analysis of the joint-prob ability distribution of the model parameters that allows one to determine s tatistically valid confidence limits on the fitted parameters. We apply thi s method to a time series of optical Ar II spectra produced when an intense lithium beam (kinetic energy of 9 MeV and current density of 22 kA cm(-2)) was injected into an argon gas cell. We show that line optical depth effec ts are important in these data, and by including finite optical depth as we ll as Stark and Doppler broadening in the model-fitting procedure, we are a ble to place meaningful constraints on the time-dependent Ar II level popul ations and the electron density in the gas cell. The values we derive for t hese quantities are in reasonably good agreement with detailed collisional- radiative models that include the effects of non-thermal electrons and beam ions. Understanding the time-dependent conditions in the gas cell is criti cal for efficient beam transport. (C) 2000 Elsevier Science Ltd. All rights reserved.