A Double Hemispherical Probe for the Characterizing and Minimizing the Self-Wake Effects on Probe Measurements.

Abstract:

Space-borne Langmuir probes generally have a self-wake behind themselves due to supersonic relative velocities (Mach number M > 1) between the spacecraft and ambient plasma ions. This wake can create difficulties for correctly measuring plasma characteristics. The Double Hemispherical Probe (DHP) is intended 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. To test the DHP a laboratory model was inserted in plasma flows created in the Colorado Solar Wind Experiment chamber with M > 10. 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 currents collected by the two hemispheres are similar, indicating a uniform electron density around the probe; and 2) 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 identify the wake effect on electron measurements and use either of the two hemispheres to more accurately characterize ambient plasma electrons.