Detecting Non-Maxwellian Electron Velocity Distributions at JET by High Resolution Thomson Scattering
The present work is motivated by a long standing discrepancy between the electron temperature measurements of Thomson Scattering (TS) and Electron Cyclotron Emission (ECE) diagnostics for plasmas with strong auxiliary heating observed at both JET and TFTR above 6-7keV, where in some cases the ECE electron temperature measurements can be 15-20% higher than TS measurements. Recent analysis based on ECE results at JET have shown evidence of distortions to the Maxwellian electron velocity distribution and a correlation with the TS and ECE discrepancy has been suggested. In this paper, a technique to determine the presence of non-Maxwellian behaviour using TS diagnostics is outlined. The difficulties and limitations of modern TS system designs to determine the electron velocity distribution are also discussed. It is demon- strated that small deviations such as those suggested by previous ECE analysis can be detected, depending on the spectral layout of the TS polychromators. The spectral layout of the JET HRTS (High Resolution Thomson Scattering) system is such that it could be used to determine these deviations between 1-6keV and the results presented here indicate that no evidence of non-Maxwellian behaviour is observed in this range. In this paper, a modifcation to the current polychromator design is proposed allowing Maxwellian distortions to be detected up to at least 10keV.