Boundary asymmetries and plasma flow to the ALT-II toroidal belt pump limiter

Abstract

The ALT-II toroidal belt pump limiter is constructed with 8 radially adjustable limiter blade segments located 45 ° below the outboard horizontal midplane of TEXTOR. An array of Langmuir probes installed in the plasma scoops measures the total ion current to each neutralizer plate, including asymmetries in flow incident from above and below the belt. Recent measurements indicate that 10–50% more current is collected by neutralizers with flux tubes connecting to the electron drift side, on the outboard side of ALT-II. The source of the particle collection asymmetry is deduced from profile measurements in the scrape-off layer (SOL). On the outboard midplane, a scanning double probe measures λ n = 1.4 cm , while profiles obtained with a Li-beam diagnostic located at the bottom of TEXTOR show λ n = 0.9 cm . The primary source of the asymmetric particle collection at the neutralizers is therefore the difference between e-folding lengths on the electron and ion drift sides. During ohmic discharges, the electron temperature in the center of the scoops (3 cm into SOL) is 8–12 eV, tending to decrease with rising line average density in the core. Corresponding edge plasma densities are in the range (2–5) × 10 11 cm −3 . In this paper, we report on measurements in the SOL behind ALT-II to emphasize the role of asymmetric decay lengths on particle collection by the ALT-II toroidal belt pump limiter.