Baffled probe for real-time measurement of space potential in magnetized plasma

Abstract

A probe for measurements of space potential in magnetized plasma is tested in a fully ionized, barium, Q-machine plasma. The probe consists of a tungsten wire tip, situated perpendicular to the magnetic field, that is partially shielded by ceramic baffles (masks). The probe works under the condition that the electron Larmor radius is much smaller than the probe radius, and that the ion Larmor radius is comparable to or larger than the probe radius. By rotating the baffle configuration around the probe tip, the ratio between the electron and ion probe current can be adjusted. The probe uses the same principles as Katsumata and plug probes [V. I. Demidov et al., Rev. Sci. Instrum. 73, 3409 (2002)], but has the advantage of convenient control of the ratio between the electron and ion current, and is not sensitive to uncertainties in the orientation of the probe tip relative to the direction of the magnetic field. Measurements of potential are made while the probe floats electrostatically. When saturated electron and ion currents have comparable magnitudes, accurate, real-time measurements of space potential can be acquired.