Limiter Biasing Experiments On The Stor-m Tokamak

Abstract

Liniiter biasiiig experiments has been carried out on the STOR-51 tokaiiiak. The liiniter consists of eight stainless steel scgiiieiits in a sandwich structure with equal number of segments on hotli sides of a ceramic base. In the experiments reported here, all four segments on one side are connected togct1ic.r to form a limiter and can be biased either positively (1120 lr, 25 A) or nega.tively (-380 V, -40 A) with respect to tlie tokamak chamber. The segments on the other side are used to measure the plasma floating potential and its fluctuations ( V J ) in the scrape-off-layer (SOL). A movable Langmuir probe at the outer board measures those quantities in both SOL and at the edge (./a < 1) regions. It has been found that floating potential fluctuations in the SOL region depend mainly on the polarity of the limiter potential, not the potential gradient. The potential fluctuations in the SOL were strongly suppressed with a positive limiter potential and enhanced dramatically with a negative limiter potential, despite observations that the potential profile was steeper in the SOL region during negative biasing. The behaviour of Vf may be related to the interchange-flute instability with dissipation caused by fluctuating currents flowing through the Langmuir sheath potential near the surface of the limiter'. At the plasma edge the suppression of the fluctuations is always accompanied by a steeper potential profile, which indicates that the rotational shear layer may be the cause for fluctuation suppression inthat region. Despite different behaviours of Vf during the positive and negative bias, improved confinement has been observed for both bias polarities. The improved confinement is characterized by significant density increase (100% for positive bias and 80% for negative bias) and sudden drop in H, radiation level.