Investigation on the harmonic currents in an asymmetric double Langmuir probe when AC voltage is applied

Double Langmuir probes are mainly used when the plasma is electrically isolated. When an AC voltage (v₀) is applied to the double probe, harmonic currents flow due to the nonlinearity of the current-voltage curve. In the case of a double probe with the same area, odd harmonic currents (I_ω, I_3ω, I_5ω...) flow. Electron temperature can be measured from the ratio of harmonic currents. However, when v₀/T_e ~ 0.5, the ratio of I_3ω to I_ω is about 1/50, which is very small, so it is difficult to obtain accurate electron temperature measurement. In this study, an asymmetric double probe is used. I_ω and I_2ω are used to obtain the electron temperature and ion density. It is found that the magnitude of I_2ω is given as a function of the area ratio of the two probes and the applied AC voltage. There is an area ratio at which I_2ω is maximum at a given applied voltage. In the case of using a large driving voltage, we will present a way for measuring the accurate electron temperature and ion density.