Local recycling coefficients and wall equilibration in tokamaks

The recycling coefficient of the first wall of a fusion device is a space- and time-dependent quantity. In present analyses, it is often taken as one single parameter, without taking account of the vast variations across the surface areas of the vessel. The present paper is an analysis of the spatially dependent recycling coefficient as a function of time, using the DIII-D vacuum vessel as an example. In the first step, an analytical formula is constructed to simulate the recycling coefficient as a function of the trapped particle flux, based on computer simulations (W. Eckstein, Calculated Trapping Curves of D in C, Si, Garching Report IPP 9/33, October 1980). In the second step, the vessel walls are subdivided into a computational grid of 123 segments and for each segment the incident particle flux during a discharge is computed with the B2 plasma and DEGAS neutral codes. The incident particle fluxes and the recycling coefficient formula are then used to analyze the local and temporal recycling behavior of the wall.