Current-mode triple and quadruple Langmuir probe methods with applications to flowing pulsed plasmas

A current-mode method for triple and quadruple Langmuir probes was developed and implemented in flowing, pulsed, collisionless plasmas. The current-mode method involves biasing all probe electrodes, and requires the measurement of probe currents providing the electron temperature, the electron density, and the ratio of ion speed to most probable thermal speed. The current-mode theory is developed for a single species, two-temperature, collisionless plasma. The current collection model for a probe aligned with the flow and radius to Debye length ratios of 5≤r_p/λ_D≤100 accounts for finite-sheath effects while for r_p/λ_D>100, current collection is based on the thin-sheath assumption. The ion current to the perpendicular probe assumes a thin-sheath and is given as a function of the ion speed ratio. The numerical procedure for the solution of the nonlinear current-mode equations, as well sensitivity and uncertainty analysis are presented. The plasma source used in the experiments is a laboratory Teflon pulsed plasma thruster, operating at discharge energies of 5, 20, and 40 J, with a pulse duration of 10-15 μs, ablating 20-50 μg/pulse. Current-mode triple and quadruple probe measurements obtained at various locations in the plume of the plasma source are presented. Extensive comparisons between double probe and current-mode probe measurements validate the new method.