Probe measurements of TFTR edge plasmas at neutral beam heating powers up to 20 MW

Abstract

This paper presents radial profiles in the scrape-off layer of neutral beam heated plasmas in TFTR at powers up to 20 MW. The measurements have been made with, among other, a novel “fast probe” which moves quickly through the scrape-off during the discharges to provide radial profiles of electron density, electron temperature, and power flux parallel to field lines. The dependence of the edge on changes in plasma current and density during beam heating has been measured. A number of heating scenarios have been investigated, including current ramping, power ramping, and variation in beam injection direction. Scrape-off scale lengths of density, temperature and power during ohmic and beam phases are found to be similar: λ n = 4.5 cm, λ T = 5 cm, λ P = 2.5 cm . The power to the ion side is consistently a factor 2–4 higher than that to the electron side, and there are differences in scale-length of up to 1 cm between the two sides. At low core electron densities, the edge electron density during counter-injection is found to exceed that during co-injection by more than a factor of ten.