Measurement of electron density and temperature and their fluctuations using modified triple Langmuir probe grounded through finite resistance

In the triple Langmuir probe (T-LP) method, the electron temperature (Te) and density (ne) can be simultaneously derived from potential measurements of electrode of T-LP and the ion saturation current (Iis) 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 (Vf) and the plus-biased potential (Vp). When the current becomes comparable to Iis, the Te derived from measured Vf and Vp without the current considerably deviates from an actual value. This would be significant for fairly low density plasma of the ne < ≅5 x 1017 m-3, and the correction of the finite current is necessary. A new relationship between Te and potential signals (Vf and Vp) where the finite current in the electrodes for Vf and Vp measurements is taken into account was derived, and experimentally confirmed the validity in the experiments of the Compact Helical System. (author)