Particle control issues of a compact stellarator with external vacuum vessel

The Quasi-Poloidal Stellarator (QPS) [J.F. Lyon, S.P. Hirshman, D.A. Spong, et al., 30th EPS Conference on Plasma Phys. Control. Fusion, St. Petersburg, 2003] has a very low plasma aspect ratio (R/a ∼ 2.7) and is a new confinement approach that could ultimately lead to a high-beta compact stellarator reactor. QPS is being developed to test key features of this approach. In QPS, the modular stellarator coils and other components are inside the vacuum vessel, requiring special attention with respect to particle control. We have analyzed two specific aspects: (1) The casings of the internal coils will assume elevated temperatures during operation whereas the vacuum vessel will be at room temperature; we have analyzed the consequences for adsorbed gas layers. (2) The plasma is surrounded by a large gas reservoir that might cause problems for density control. We have constructed a model to predict plasma and neutral densities as functions of fueling, divertor baffling, wall adsorption, and particle exhaust. This model predicts that plasma and neutral densities can be controlled.