Initial measurement of impurity production and hydrogen energy distribution from neutral beam injectors

Abstract

Impurity production and hydrogen energy distributions for neutral beam injectors (NBI) developed by the Plasma Technology Section of Oak Ridge National Laboratory's (ORNL) Fusion Energy Division have been measured by exposing silicon samples to beam pulses and analyzing them by nuclear microanalysis and secondary ion mass spectrometry (SIMS) techniques. From the three major energy components arising from the formation of H +, H + 2, and H + 3 in the ion source only the full-energy component (40 keV) is easily separated in the depth analysis, but the E/2 and E/3 components overlap, mainly because of range straggling. Analysis by SIMS is used to determine the depth profiles of the implanted hydrogen, and the three energy components are determined from this depth profile. Nuclear microanalysis is used to analyze for heavy impurities. Initial results have shown that oxygen is the major atomic impurity and is present at levels of 1.5 × 10 −3 O/H. Carbon is the next most abundant impurity, and copper is at or below the sensitivity of the analysis technique. The neutral hydrogen species power distribution deduced from the implant profiles results in relative abundances of 67:17:16 for the energy components E:E/2:E/3, respectively. These data do not agree with three other independent measurements at ORNL and Princeton Plasma Physics Laboratory that are in general agreement with each other and yield a neutral hydrogen species power distribution of 80:13:7. At present this discrepancy is unaccounted for, but migration effects due to heating of the Si crystal are under investigation. The present technique can be easily applied to study other neutral beam injectors on test stands or on plasma devices.