Physics considerations for the FED limiter

The physics of the scrapeoff plasma and its interaction with a pumping limiter is reviewed. The governing equations for heat and particle transport in the scrapeoff are solved analytically with several simplifying assumptions. The assumed scrapeoff model is widely used and is presented here for completeness. From the solution, a shape for the limiter is derived which has a uniform heat loading on the front surface. The assumed crossfield transport coefficient is varied to examine the sensitivity of the heat loading to variations in plasma scrapeoff thickness. The heat flux to the limiter leading edge is an extremely sensitive function of variations in scrapeoff thickness, and we argue that uncertainties in the plasma crossfield transport preclude derivation of a believable limiter shape. We also discuss the problems involved with an edgeless, perforated limiter and with a global limiter. Finally, a solution is proposed using a flat-plate limiter with the heat flux to the leading edge controlled by major radius motion of the plasma.