Neutral depletion in an H− source operated at high RF power and low input gas flow

Abstract

On an RF driven H− ion source the temperature of the H2 background gas was measured via optical emission spectroscopy as a function of flow and applied RF power at two locations of the source. The source operates at an applied RF power of 30–100 kW and at input gas flows of less than 1 Pa m3 s−1. At the same time the pressure was measured by Baratrons connected to the source body via tubes of length much greater than their diameter. Using these two data sets the background gas density and degree of depletion were determined. The neutral gas depletion was found to be relatively high (50–70%) as compared with the gas density before plasma ignition. The results obtained for the temperatures show the background gas is colder (maximum 1200 K) in the area where the RF power is coupled into the plasma than in the expansion region near the extraction grid (maximum 2100 K). The degree of depletion is highest where the power is coupled in, so the background gas density is lowest there.