Measurement of Electron Density and Temperature and Their Fluctuations Using Modified Triple Langmuir Probe Grounded through Finite Resistance

Abstract

In the triple Langmuir probe (T-LP) method, the electron temperature (T_e) and density (n_e) can be simultaneously derived from potential measurements of electrode of T-LP and the ion saturation current (I_is) where no current flows in the electrodes for potential measurements. In the case of aiming at measuring high-frequency fluctuations, however, the smaller load resistance of electrode is required for high frequency response. Then the finite current can flow in the measurement circuits of the floating potential (V_f) and the plus-biased potential (V_p). When the current becomes comparable to I_is, the T_e derived from measured V_f and V_p without the current considerably deviates from an actual value. This would be significant for fairly low density plasma of the n_e <∼ 5 × 10¹⁷ m^-3 , and the correction of the finite current is necessary. A new relationship between T_e and potential signals (V_f and V_p) where the finite current in the electrodes for V_f and V_p measurements is taken into account was derived, and experimentally confirmed the validity in the experiments of the Compact Helical System.