Measurement of edge parameters in TEXTOR-94 at the low and high field side with atomic beams

Abstract

In TEXTOR-94, He-atoms are injected on the low and high field side using a collimating hole structure. Additionally a supersonic He-beam, which has a very low divergence, is mounted on top of the vessel. At all locations the radial emission profiles of three lines are observed with a radial (1.2 mm) and temporal resolution (100 ms) over the whole discharge. The He-atoms are excited by the plasma and the profiles of emission light of two singlet lines and one triplet line are observed simultaneously. A comparison of the measured line intensity ratios with those obtained by a collisional-radiative model delivers the radial profiles of n e and T e in the plasma edge. The comparison of edge profiles at different poloidal positions requires a careful alignment procedure in order to resolve asymmetry effects. Time dependent changes of the plasma positions or shape of the plasma cross-section can be identified by the He-beam diagnostic and must be avoided. The edge profiles at the different poloidal locations of TEXTOR-94 are investigated as a function of the central line averaged electron density in ohmic and neutral beam (NBI) heated discharges. T e and n e-values at the last closed flux surface between the low and high field side are compared. As a result of a constant magnetic pressure at one radial position the temperature T e and density n e are inversely dependent on the central density, but the changes, which are different on the high and low field side, indicate a horizontal asymmetry.