Study On The Influence Of Plasma Sheath Capacitance On Fast Sweep Double Probe Current-voltage Curve

Plasma,as the fourth state of matter,requires various diagnostic to collect and analyze its parameters and to determine the state and properties.As one of the most common and basic plasma diagnostic method,Langmuir probe is by far one of the most cost and maintenance friendly plasma diagnostics to operate.These characteristics facilitates the wide-spread applications of the Langmuir probe.Langmuir probes were generally used to measure steady-state plasma parameters,but the recent interest in dynamic phenomena like plasma turbulence has renewed the need for time-resolved probe diagnostics,most notably via fast-sweep Langmuir probe techniques.However,sheath capacitance,a fundamental quantity accompanied with the sheath formation process,perturbs the signal received via producing an added displacement current into the I-V characteristics.From this issue rises the need to study in depth the effects of sheath capacitance to the I-V characteristics of a fast-sweep Langmuir probe.To date,most relevant studies focus on the techniques and the physical phenomena associated with a fast-sweep single Langmuir probe,and works on fast-sweep double Langmuir probes were rather limited,especially on systems of the asymmetric fast scanning double-probe,which has a wide application prospect in the case of complex boundary such as finite particle loss surface,high particle collision and magnetized plasma.Therefore,in this paper,an asymmetric double probe fast scanning system is built and studied in a uniform,quiescent plasma provided by a multi-dipole confined hot cathode discharge,and different sizes of fast scanning asymmetric double probes are designed for experimental research on the influence of capacitance effect on measurement results.Meanwhile,the traditional double circuit technology used for fast scanning single probes is applied to the double probes,and the scanning frequency can reach 500kHz.In this work,it is found that although the displacement currents caused by sheath capacitance effects with different probe area ratios appeared to be similar,they were actually caused by two different underlying processes:zero crossing of sheath thickness and probe bias change.The influence of the area ratio of the fast scanning double-probe on the measured results is deduced and analyzed.The results of the asymmetric double Langmuir probe with different area ratios and different frequencies under the influence of ion saturation flow and sheath current are also compared and analyzed in this paper.