Studies of hydrogen recycle from the walls in tokamaks using a plasma wall ineraction simulator

The recycle of hydrogen from walls and limiters plays a fundamental role in the operation of tokamaks for several reasons: (1) the average proton confinement time in the plasma is short with respect to plasma length, therefore each proton is, on the average, recycled several times to and from the walls; (2) the walls may elastically or inelastically reflect hydrogen but, according to measurements, half or more of the hydrogen incident on the walls may be buried in the wall, thermalized, and most will return to the plasma volume on a time scale which is on the order of 10-1s. The authors have studied details of the hydrogen recycle phenomena from 304L stainless steel using a plasma-wall interaction simulator. Low energy (30-200 eV) atomic and moecular hydorgen ions were used. The evolution rate from surface bombarded with a flux of 6 x 1015 ions/cm2.s has been measured for several conditions and appears to be limited by complex diffusion phenomena and perhaps, in some cases, by surface recombination rates. The evolution rate reveals effects due to diffusion from the highly supercharged near surface region. Several mechanisms were discussed but it is not possible to identify which are responsible for the prompt desorption processes. (Auth.)