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Dual-collector ExB probe measurement of the ion velocity distribution function (IVDF) with analytical and numerical
T Yamauchi1, J Tompkins1, J L Rovey1
1Department of Aerospace Engineering, University of Illinois Urbana-Champaign, 104 S Wright St., Urbana, Illinois 61801, USA.
Abstract:
A dual-collector ExB probe has been developed to enable simultaneous measurements of ion velocity in electric propulsion (EP) testing. In this configuration, a single filter section feeds into two drift tubes of differing lengths, each terminating in an individual collector. This enables the probe to simultaneously measure the ion velocity distribution function (IVDF) in two resolution regimes. The dual-collector ExB probe was tested on a 10 cm gridded ion thruster operating on air, which generates a plume containing O2+, N2+, O+, and N+. The long drift tube configuration resolved all four distinct ion population peaks in the IV curve, whereas the short drift tube configuration merged the four species peaks into two (one for molecular species, the other for atomic species). The IVDFs were estimated from the long ExB probe measurement, the short ExB probe measurement, and both the long and short ExB probe measurements together. The most probable energy, distribution full-width half-maximum (FWHM), and ion species fraction were calculated from these estimated IVDFs. The uncertainty in the most probable energy was mainly contributed by the Wien velocity uncertainty (about 10%), which is consistent regardless of the different ExB probe measurements. When both long and short measurements were used together, the average uncertainty in the FWHM was reduced to 0.040 km/s from 0.063 and 0.211 km/s for individual long and short ExB probe measurements, respectively. By using together the long and short ExB probe measurements, the uncertainty in ion species fraction was reduced to ∼10%, compared to ∼20% and 30% for the individual long and short ExB probe measurements, respectively. This study demonstrates the ExB probe's capability for concurrent IVDF measurements in EP environments, confirms the reduction in the uncertainty in the estimated IVDF and the calculated ion species fraction, and highlights the limitations of purely analytical transmittancy estimates when nonuniform fringe fields induce beam distortion.
