Abstract
Hydrogen‐ and deuterium‐fueled glow discharges are used for the initial conditioning of magnetic fusion device vacuum vessels following evacuation from atmospheric pressure. Hydrogenic glow discharge conditioning (GDC) significantly reduces the near‐surface concentration of simple adsorbates such as H2O, CO, and CH4, and lowers ion‐induced desorption coefficients by typically three orders of magnitude. The time evolution of the residual gas production observed during hydrogen‐glow discharge conditioning of the carbon first‐wall structure of the TFTR device is similar to the time evolution observed during hydrogen GDC of the initial first‐wall configuration in TFTR, which was primarily stainless steel. Recently, helium GDC has been investigated for several wall‐conditioning tasks on a number of tokamaks including TFTR. Helium GDC shows negligible impurity removal with stainless steel walls. For impurity conditioning with carbon walls, helium GDC shows significant desorption of H2O, CO, and CO2; however, the total desorption yield is limited to the monolayer range. In addition, helium GDC can be used to displace hydrogen isotopes from the near‐surface region of carbon first‐walls in order to lower hydrogenic retention and recycling.
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