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

In order to study plasma behaviors and transport in edge plasma region, a t riple probe array is installed on the compact helical system (CHS) heliotro n/torsatron. The probe array is designed to measure the radial profiles of electron temperature, electron density, plasma potential and these fluctuat ions, simultaneously. The standard triple probe technique does not supply r eliable results unless all electrodes are on the same magnetic surface. The n we also use the modified triple probe technique based on five electrodes, which provides precise fluctuations of electron temperature. In edge plasm a region of neutral beam injection (NBI) heated plasmas on CHS, plasma prof iles and their fluctuations are measured by the newly installed triple prob e array. In the region of rho similar or equal to 0.95 similar to 1.1, Bolt zman relationship (n) over tilde(e)/[n(e)] similar or equal to (V) over til de(s)/[T-e] is roughly satisfied and temperature fluctuation (T) over tilde (e)/[T-e] is by a factor of 3 smaller than density fluctuation (n) over til de(e)/[n(e)]. In sawtoothing plasmas, we also measure these plasma paramete rs, their gradients, and plasma fluctuations. Electron temperature and the gradient transiently increase associated with each sawtooth crash, but elec tron 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, h owever, is unclear in these plasmas. (C) 1999 American Institute of Physics . [S0034-6748(99)62901- X].