H α measurements and modeling of plasma confinement in TEXTOR with the ALT-II pump limiter

Abstract

H α emission in TEXTOR is measured at several locations in order to study recycling and global particle confinement with the Advanced Limiter Test-II (ALT-II) pump limiter. The measurements are modeled with the neutral transport code DEGAS for ohmic density scans in deuterium plasmas with carbonized vacuum vessel walls. The core efflux, particle confinement time, diffusion coefficient D ⊥, boundary flow asymmetry, and profiles of the core fueling rate by particle recycling are determined for line-averaged densities n ¯ e = ( 1.4 − 3.7 ) × 10 13 c m − 3 . Measured distributions of core plasma density and temperature, together with scrape-off layer (SOL) plasma parameters from the scanning probe, the neutral lithium beam diagnostic, and the Langmuir probes in the ALT-II scoops, describe the plasma environment for the DEGAS H α simulations. In particular, it is found that radial profiles of ion flux in the SOL, derived from scoop Langmuir probe measurements, give excellent fits to the H α data. A tangency point particle flux asymmetry favoring the ion drift side of the blade of 2.25 to 1.24 for the density range (1.4–3.7)× 10 13 cm −3 is indicated by both the H α simulations and the edge plasma diagnostics. The particle confinement time varies from approximately 50 to 80 ms for this ohmic density scan.