A Double Hemispherical Probe for Characterizing andMinimizing the Self-Wake Effects on ProbeMeasurements

摘要

Space-borne Langmuir probes generally have a self-wake behind themselves due to supersonic relative velocities (Mach numberM >1) between the spacecraft and ambient plasma ions. Such a wake may create difficulties for correctly measuring plasma characteristics. Here we present a new technique-the Double Hemispherical Probe (DHP) to characterize and minimize the self-wake effects on probe measurements. The DHP consists of two hemispheres that are simultaneously swept with a bias voltage to obtain two independent current-voltage (I-V) curves, which are used to identify the wake effects of the probe. A laboratory DHP model is inserted in plasma flows (M >10) created in the Colorado Solar Wind Experiment (CSWE) chamber. In this case, the ion current is the ram current collected by the upstream hemisphere of the DHP, leaving an ion wake behind the downstream hemisphere. A wide range of the Debye ratio R_D (ratio of the probe radius to electron Debye length) is tested, and it is found that (1) when R_D 1, the electron current collected by the downstream hemisphere becomes lower than the upstream, indicating a reduced electron density in the probe's wake due to the ambipolar electric field effect. In this case, the electron density measured by traditional single Langmuir probes will be underestimated. With the DHP, we can use the upstream hemisphere measurements to minimize the self-wake effects.