SHAPING OF THE PLASMA FACING SURFACES OF THE ALT-II FULL TOROIDAL BELT LIMITER

The Advanced Limiter Test-II, ALT-II, is a full toroidal belt limiter that began operation on TEXTOR in April 1987. The experimental program investigates the effects of a full toroidal belt on particle confinement and on particle exhaust from the system. ALT-II is designed to operate with 6 MW of plasma heating for three second discharges. For design purposes we assumed that as much as 4 MW is deposited on the limiters. This paper describes how the design conditions were coupled with a detailed description of the tokamak plasma edge region to determine the shapes of the graphite tiles mounted on the ALT-II blades. The front faces of these tiles, the surfaces adjacent to the plasma, were profiled poloidally so that when the energy scrape-off length, lambda-e, is 1.0 cm the active surfaces experience a heat flux of 270 W/cm2. The ALT-II belt consists of eight individually movable blades with a 12.0 cm gap, for diagnostic access, between adjacent blades. Power flow through these gaps produces hot spots on the ends of each blade. Hot spots can also be generated on the ends if one blade is misaligned, protruding further into the plasma, than the adjacent blades. Heat flux loads associated with these hot spots were minimized by providing toroidal shaping to the end tiles in addition to the poloidal, front face, profiling mentioned above. The toroidal profiling is such that a single blade protruding 3 mm further into the plasma than the next nearest blade experiences 1 kW/cm2 on the misaligned edge during full power operation. At this heat load, plasma contamination is maintained at an acceptable level since surface temperatures do no exceed 1800-degrees-C, well below the temperature at which graphite sublimation becomes a serious problem.