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

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