Secondary electron emission in DC operation of the retarding field analyzer

The DC operation of a retarding field analyzer (RFA) has been proposed for the direct measurements of the ion temperature fluctuations in the plasma scrape-off-layer [1]. The method is based on the relation for the RFA current-voltage (I-V) characteristic resulting from a common RFA model of shifted Maxwellian distribution of the analyzed ions and the measurements of two points on the exponentially decaying region of the I-V characteristic with two differently DC biased RFA electrodes. It was mentioned that the secondary electron emission (SEE) from the RFA electrodes could be a serious problem for the correct application of the method. From the experiments on the tokamak ISTTOK (R = 0.46 m, a = 0.085 m, B = 0.5 T) in ref. [1], the SEE yield for hydrogen ions was measured as gamma similar to 0.2, estimating similar to 10% contribution to the average ion temperature and assuming no influence on the measurements of ion temperature fluctuations. However, in the followed investigations it was observed that the SEE yield could be up to gamma similar to 0.5. In this paper, the influence of the secondary electron emission on the ion temperature fluctuations measurements in RFA DC mode of operation is analyzed and detection scheme improving the measurements is proposed.