ON BA+ PRODUCTION IN THE CRIT-II EXPERIMENT

Analysis of particle data from the CRIT II experiment, studying Alfven 's critical ionization velocity (CIV) effect, shows that the density o f newly created ions (presumably Ba+ from the shaped-charge beam) is c onsistent with the increase in total plasma density measured by the in dependent RF plasma probe on board (Swenson et al., 1990) at the most active time period. We model this ion production using the measured el ectron flux data and the neutral barium model of Stenbaek-Nielsen et a l. (1990a). To identify the main source mechanisms which may contribut e most to the barium ionization, a simple model for barium ion density at the payload location is developed based on Liouville's theorem. We estimate that the electron impact ionization is responsible for 90% o f the barium ion production observed by GRIT II in the first release a nd up to 45% in the second release. By employing a ''two-state approxi mation'' calculation (Rapp and Francis, 1962), the Ba-O+ charge exchan ge cross section is found to range from about 2.0 x 10(-17) cm(2) at a velocity of 4 km/s to 2.0 x 10(-15) cm(2) at a velocity of 20 km/s. T his result suggests that the Ba-O+ charge exchange is probably dominan t among all the non-CIV ionization processes. By considering the charg e exchange process in our density model, the barium ion densities are calculated for the two releases on GRIT II. The comparison between the model results and the observed data is found to be reasonably consist ent if the cross sections, as calculated above, are multiplied by 0.3 for the first release and 1.0 for the second release. Our result sugge sts that the charge exchange process could be the most important non-C IV ionization mechanism in the CRIT II experiment and it should be con sidered carefully case by case in CIV experiments.