Utility of a Baffled Probe for Measurements of Oscillations in Magnetized Plasma

Abstract

Key words Electric probe, magnetized plasma, space potential and temperature oscillations. PACS 52.70.-m A baffled probe for distinguishing the oscillations of space potential and temperature in magnetized plasma is tested. The probe consists of a tungsten wire tip, oriented 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 the ion Larmor radius is comparable to or larger than the probe radius. The probe uses the same principles as Katsumata and plug probes, but has the advantage of convenient, realtime controllability of the ratio between electron and ion current while electrically floating, and insensitivity to uncertainties in the orientation of the probe tip relative to the direction of the magnetic field. By rotating the baffle configuration around the probe tip, the ratio between electron and ion probe current and consequently the relative sensitivity of the probe to oscillations of space potential and electron/ion temperature, can be adjusted. Thus, measurements of ac floating potential with different ratio of electron and ion currents allow us to distinguish oscillations of space potential from electron/ion temperature fluctuations. Experiments have been conducted in a fully ionized, barium plasma in the West Virginia University Q-machine.

Baffled‐Probe Cluster for Simultaneous, Single‐Point Monitoring of Magnetized Plasma Fluctuations

“…As it was shown in Ref. [14], the probe floating potential oscillationsṼ fl can be represented asṼ ). For such isolation, a cluster of at least four probes is required.…”

“…As it was shown in Ref. [14] Subtracting the fluctuating floating potentialsṼ fl (t) obtained from two probes in an array yields a quantity that reflects both the electric field component along the array and the propagation of the temperature fluctuation in the array direction. The effect of the propagating temperature fluctuations on this quantity can be minimized by orienting the probes so that I sat e /I sat i ≈ 1 in which case the quantity can be interpreted as the electric field.…”

“…It was shown [13,14] that a baffled probe is a convenient and simple non-emitting instrument for the real-time monitoring of dc and ac values of both space potential and electron temperature in magnetized plasma. The probe works under the condition that electron Larmor radius is much smaller than the probe radius, and the ion Larmor radius is comparable to or larger than the probe radius.…”

Magnetically insulated baffled probe for real-time monitoring of equilibrium and fluctuating values of space potentials, electron and ion temperatures, and densities

“…Demidov et al 2,8,9 appears to be the first to suggest using similar probes in a floating regime for measurements of Ṽ s , electric field Ẽ s , and T i in a magnetized plasma, which dramatically increases temporal resolution with respect to fast sweeping probes, and this paper summarizes its potential and achieved outcomes. It was demonstrated that, for plasmas with Maxwellian charged particle distributions, if the probe electron saturation current I sat e is approximately equal to the ion saturation currant I sat i , then the probe floating potential For practical measurements of Ṽ s and Ẽ s the plug probes 10 and the baffled probes 11 have been used.…”

“…Later a novel ion-sensitive probe, which is basically a combination of the Katsumata probe and ion energy analyzer, was used by Ochoukov et al 14 Measurements of T i have not been reported yet, but it is demonstrated that appropriate regime when I sat i ӷ I sat e is possible. 9 The principles of operation of all these probes are similar and based on the dependence of the voltage drop in the plasma-probe sheath on the direction of the local magnetic field. When the magnetic field is parallel to the probe surface, the electron-repelling sheath can be significantly reduced as the magnetic field also impedes the cross-field electron flow and therefore, a smaller sheath voltage is needed to maintain the zero current balance to the floating probe.…”