Comparative measurement of electron density and temperature profiles in low-temperature ECR discharges by a lithium atom beam and Thomson scattering

Abstract

In this paper a method to determine spatially-resolved profiles of the electron temperature Te and density ne in an electron-cyclotron-resonance (ECR) discharge is presented. This technique is based on the observation of line emission from a neutral Li atom beam, which is injected into the plasma and excited by electron collisions. A collisional-radiative model valid for the injected Li atoms is used to predict the emission intensities as function of ne and Te for several lines theoretically. In contrast to the electron temperature regime representative for the edge of tokamak discharges (Te>5 eV), the ECR discharge offers a Te range where selected line intensity ratios strongly depend on the electron temperature. Therefore, a comparison of the measured ratios with the calculated ones yields Te profiles for the first time. The ne measurement is performed as in tokamaks by observing the attenuation of the beam due to ionization in the plasma. We present radial profiles of Te and ne for discharges in argon and xenon under different operating conditions. These results are compared with results obtained by Thomson scattering. Our measurements give evidence for a satisfying agreement between the two methods.