ALT-II toroidal belt pump limiter performance in TEXTOR

Abstract

The Advanced Limiter Test (ALT-II) is a toroidal belt pump limiter in the TEXTOR tokamak. ALT-II is composed of 8 blade segments which form an axisymmetric toroidal belt of 3.4 m 2 exposed surface area, located on the outside of the torus at 45° below the horizontal midplane. Ohmic plasma operation with ALT-II as the main limiter is characterized by a line-averaged density range of 5 × 10 12 to 5.5 × 10 13 cm −3 at B T = 2 T and I P = 340 kA , Z eff = 1.1 to 2 and typically 40 to 95% of the power radiated depending on the plasma density. ICRH heating of the plasma with up to 2.6 MW of incident power has been achieved, which modifies the scrape-off layer (SOL) and the pump limiter performance. The recycling coefficient in TEXTOR is normally close to one, but helium RG conditioning and baking of the limiter at 400 ° C is found to lower the recycling coefficient to 0.8 for the order of 10 shots. Measurements by arrays of probes in the SOL and thermocouples in the limiter tiles indicate the flow to the limiter is toroidally symmetric (taking field ripple into account) and poloidally asymmetric. The asymmetries result in different power and particle fluxes to the ion and electron drift sides of the limiter. The density and power scrape-off lengths are on the order of 1 cm and significantly longer on the outside of the torus (electron drift side). In spite of the flow asymmetry favoring the ion drift side near the tangency point, the longer e-folding lengths on the electron side in the SOL result in equal or higher particle collection by the electron side. The probe arrays indicate that during ohmic heating a total of 15 to 20% of the core efflux is incident on the neutralizer plates located in scoops beneath the blades. More particles are collected during ICRH auxiliary heating due to changes in the SOL profiles and shorter particle confinement times. Based on particle re moval experiments with pumping on one blade, the total exhaust efficiency of the limiter if pumped at all eight blades is 5 to 10%.