Development of Fast Helium Beam Emission Spectroscopy for Tokamak Plasma Density- and Temperature Diagnostics
Optical emission from energetic lithium beams has been successfully used as a diagnostics of tokamak plasma density. Electron temperature as well as plasma density can be measured with thermal helium diagnostic beams. However, both Beam Emission Spectroscopies (BES) are limited in range to the outer plasma region because of small penetration depth of the injected neutral particles. More energetic (20 keV) helium atoms penetrate much deeper into the plasma and therefore offer the prospect of locally measuring electron temperature and plasma density over a much wider range. Of particular interest are such measurements with good spatial resolution extending over H-mode and internal transport barriers. For developing electron density and -temperature diagnostics based on fast He Beam Emission Spectroscopy (fast He-BES), we have performed in recent years different proof-of-principle experiments at ASDEX Upgrade (AUG) in Garching and JET in Culham. Thereby measured HeI emission profiles showed fair agreement with simulated ones for given plasma density-, temperature- and impurity distributions, utilizing a collisional-radiative model and atomic collision data supplied by the ADAS group. Sufficiently intense He diagnostic beams are now obtained by puffing a small amount of helium into a standard deuterium heating beam ion source, such producing a "doped" diagnostic He beam. The experiments made use of on-site beam emission spectroscopy systems which provided suboptimal geometry for observing the Doppler-shifted HeI line emission from the injected He diagnostic beam.