Electron energy distribution function in the divertor region of the COMPASS tokamak during neutral beam injection heating

Abstract

This paper presents the results from swept probe measurements in the divertor region of the COMPASS tokamak in D-shaped, L-mode discharges, with toroidal magnetic field BT = 1.15 T, plasma current Ip = 180 kA and line-average electron densities varying from 2 to 8×1019 m−3. Using neutral beam injection heating, the electron energy distribution function is studied before and during the application of the beam.The current-voltage characteristics data are processed using the first-derivative probe technique. This technique allows one to evaluate the plasma potential and the real electron energy distribution function (respectively, the electron temperatures and densities).At the low average electron density of 2×1019 m−3, the electron energy distribution function is bi-Maxwellian with a low-energy electron population with temperatures 4-6 eV and a high-energy electron group 12-25 eV. As the line-average electron density is increased, the electron temperatures decrease. At line-average electron densities above 7×1019 m−3, the electron energy distribution function is found to be Maxwellian with a temperature of 6-8.5 eV.The effect of the neutral beam injection heating power in the divertor region is also studied.