The influence of impurities on ion temperature measured by a retarding field analyzer

Abstract

The retarding field analyzer (RFA) is considered to be one of the most effective tools to measure ion temperature (Ti) in the plasma boundary. However, there are some inaccuracies in the measured Ti by RFA caused by impurities in the plasma. In this paper, the influence of impurities on the measured Ti by RFA have been analyzed in hydrogen isotope plasmas with both low-Z and high-Z impurities. The analytical results show that Ti is underestimated in H+ (also D+ and T+) plasma when the fuel ion charge is taken as the charge number and the analyzed Ti in H+ plasma are all larger than that in D+ and T+ plasmas, exhibiting the isotope effect. With the concentration of low-Z impurity (ηl) increasing, the analyzed Ti decreases and is underestimated by about 20% with ηl ∼ 12%. Following a comparison of different charge numbers and masses of low-Z impurities, it is found that larger charge numbers and smaller masses of low-Z impurities lead to greater underestimation of the analyzed Ti. High-Z impurities make almost no impact on the Ti, due to their small current contribution. Moreover, the Ti is measured by RFA and edge rotational diagnostic (ERD) systems through experiments. The results show good agreement with the analytical evaluations.