Langmuir probe array for edge plasma study on the compact helical system heliotron/torsatron

Abstract

In order to study plasma behaviors and transport in edge plasma region, a triple probe array is installed on the compact helical system (CHS) heliotron/torsatron. The probe array is designed to measure the radial profiles of electron temperature, electron density, plasma potential and these fluctuations, simultaneously. The standard triple probe technique does not supply reliable results unless all electrodes are on the same magnetic surface. Then we also use the modified triple probe technique based on five electrodes, which provides precise fluctuations of electron temperature. In edge plasma region of neutral beam injection (NBI) heated plasmas on CHS, plasma profiles and their fluctuations are measured by the newly installed triple probe array. In the region of ρ≃0.95∼1.1, Boltzman relationship ñe/〈ne〉≃Ṽs/〈Te〉 is roughly satisfied and temperature fluctuation T̃e/〈Te〉 is by a factor of 3 smaller than density fluctuation ñe/〈ne〉. In sawtoothing plasmas, we also measure these plasma parameters, their gradients, and plasma fluctuations. Electron temperature and the gradient transiently increase associated with each sawtooth crash, but electron density and the gradient show the opposite behavior. Radial electrical field and the gradient are also modulated by sawtooth crashes. Correlation between the increased radial electrical field gradient and fluctuations, however, is unclear in these plasmas.