Spectroscopic studies of intense ion beam propagation in the PBFA-II gas cell

We are studying the physics of ion beam transport on PBFA II by measuring time-resolved visible spectral emission from the gas cell. The initial goals are to perform a spectral survey, to measure the spectral-line intensities and widths, and to evaluate the potential for using this emission as a diagnostic for conditions in the gas-cell, plasma. We have acquired spectra in the 4000 Å to 7000 Å regime excited by the transport of proton and carbon beams through 5 Ton-argon gas. The nominal ion beam energy and current density at the spectroscopic line of sight are 6–7 MeV protons, 12–21 MeV carbon, and 10–50 kA/cm
²
. The emission is dominated by n=4 to n=4 transitions in Ar II, with FWHM values typically 1–2 Å. We are investigating a variety of mechanisms which might contribute to the line profiles, including plasma Stark broadening, Stark and Zeeman splittings and shifts due to residual electric and magnetic fields, and opacity effects. The relative contributions of ion-impact and plasma electron collisional excitation are being evaluated using an LTE atomic model. Measured plasma properties will be incorporated into cur understanding of beam transport.