The quest for a divertor effect in TJ-II

Neutral beam injection (NBI) heating in the TJ-II stellarator faces the particular challenge of unwanted particle sources, which for relatively low injected powers drive the plasma to collapse at relatively low values of line density. This effect is aggravated by an enhancement of particle confinement that occurs as density increases. At present, candidate magnetic topologies that make use of intrinsic islands at the plasma edge are being investigated experimentally. This concept uses magnetic configurations having a rotational transform with a rational value at the edge, i.e., an island divertor (ID). Recently, impurity injection experiments have been performed for ID and limiter configurations to investigate the divertor effect in TJ-II. Indeed, the enhanced screening of injected impurities, as well as the low intrinsic plasma contamination found in these ID plasmas, points to such magnetic configurations as being good candidates for the NBI experimental program in TJ-II.