Plasma-surface interactions in large tandem mirror devices - MFTF-B

Present experiments on TMX-U and modeling of MFTF-B indicate that plasma-surface interactions can be controlled in MFTF-B. The MFTF-B configuration uses a hot electron population created by ECRH and a sloshing-ion population created by neutral beams in the thermal barrier region to create a potential that confines the central cell ions. Neutral beams and ICRH are used to heat the central cell ions. Plasma-surface interactions can be minimized at radial surfaces by control of the axial confinement of the edge plasma. The thermal barrier configuration is sensitive to the background neutral density, and requires low wall reflux and efficient shielding by the edge plasma. Glow discharge cleaning, titanium gettering, and control of the gas from neutral beams will be used to provide wall conditioning and to reduce the background gas pressure. The shielding efficiency of the plasma edge has been modeled in MFTF-B by comparing computer codes with current experimental measurements. In addition, it is very important to reduce high-energy neutral-beam-injected impurities; this is accomplished by using gettering or magnetic separation in the injector systems. Plasma-edge scrapers, diverter-like devices, and direct-conversion equipment will be located in the end region. Major disruptions are not anticipated. Finally, MFTF-B will also test some technological issues that are revelant to reactors: superconducting magnet systems and nearly steady-state (30-s) operation