Effects of neutral-beam co- and counter-injection on impurity radiation from ISX-B plasmas

Abstract

In addition to heating, neutral beam injection modifies the impurity transport in ISX-B plasmas. Radiometric data show the effects of co- and counter-injection on impurity radiation profiles to be significantly different. Data from an array of twelve collimated detectors and an uncollimated monitor (all pyroelectric) show the radial distribution of volume impurity emission to be peaked at the centre of the discharge with counter-injection and at the plasma edge with co-injection, especially for Pinj > 1 MW. At high co-injection power, only 20% of the total input power is lost as impurity radiation. Also, co-injection-heated discharges reach an equilibrium shortly after beam turn-on, whereas counter-injection-heated discharges do not reach equilibrium and usually terminate disruptively. It is conjectured that the disruption is due to modification of the plasma current distribution resulting from the extreme cooling at the plasma core.