Effect of the transmission line on the ion temperature measured by the retarding field analyzer

Due to the presence of capacitance in the transmission line, the current collected by the retarding field analyzer (RFA) becomes asymmetrical (i.e., hysteresis) during voltage scanning, and it further affects the measurement of the ion temperature (Ti). In this article, we first analyze the hysteresis of the RFA’s I–V characteristics measured in the edge plasma of the J-TEXT tokamak, and we found that the ion temperature fitted from the falling side of the collector current is about 14% higher than that fitted from the rising side. An analytic model is built and verified by experimental results. Based on the model, the influence of the capacitance of transmission lines and the frequency of the scanning voltage on the hysteresis is investigated. It is found that the hysteresis becomes more remarkable with increasing capacitance or scanning frequency. Besides, the impact of the capacitance and the scanning frequency on the Ti measurement is also studied, which indicates that Ti is overestimated due to the hysteresis. The temperature error is linearly dependent on the capacitance and the scanning frequency. Furthermore, the analytical results are expected to be used in the prediction of scanning frequency for RFA applications.