Comparison between Laser Thomson Scattering and Spectroscopic Measurements in Low Temperature Helium Plasmas in Divertor/Edge Simulator MAP-II

Abstract

Electron temperature and density measurements of low temperature Helium plasmas are performed in the divertor/edge plasma simulator MAP-II, by means of Laser Thomson Scattering (using a ND:YAG Laser, a double monochromator and an ICCD detector) and optical emission spectroscopy (using a spectrometer and a CCD detector). The recent upgrades of our LTS system (reduction of the level of stray light and reduction of the band width of the notch filter) allowed the measurement of temperatures as low as 0.1 eV and the investigation of Electron Ion Recombination (EIR) processes in He plasma. Spatial profiles of electron temperature and density along the plasma column have been taken moving the plasma recombination front across the measurement point by controlling the gas pressure from 80 to about 145 mTorr. A comparison between LTS results and spectroscopic analysis based on a He I CR model including radiation trapping is shown in order to confirm the consistency of the diagnostics. CR model results, obtained fitting the excited state populations with principal quantum number n = 3,4, are consistent with those from LTS. The discrepancies are found to be attributable to the mixing of the ionization and recombination regimes over the line of sight of the optics.