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

Direct measurements of the plasma potential with high temporal and spatial resolution are only possible with special probes whose floating potential becomes equal to the plasma potential. One of these probes is the emissive probe which can be heated to electron emission. This leads to the effect that the plasma electron current towards the probe is compensated by an equal electron emission current flowing from the probe into the plasma which shifts the probe floating potential to the value of the plasma potential. While emissive probes are usually realized by heatable loops of wires of a refractory metal like tungsten or thoriated tungsten, we have developed various types of probes whose tip can be heated by an infrared laser beam. This offers several advantages like higher emission, longer lifetime and faster time response. Here we report on the latest development of a laser-heated emissive probe and on measurements with it in a magnetized helicon plasma.

Section: Introductionmentioning

confidence: 99%

New Results on a Laser‐Heated Emissive Probe

Ioniţă

Balan

Windisch

et al. 2008

“…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.…”

Section: Introductionmentioning

confidence: 99%

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

Koepke

Demidov

Finnegan

et al. 2006

Self Cite

Key words Electric probe, probe array, magnetized plasma, potential and temperature oscillations. PACS 52.70.-m An array of baffled probes for the real-time monitoring of oscillating values of space potential, electric field and electron/ion temperature in magnetized plasma is assembled and tested. The multi-probe array samples a region with dimensions on the order or less than an ion gyroradius, essentially a single point, and does not require voltage sweeps. We refer to an array of probes contained within such a single point as a probe cluster. Here, we describe two versions of the baffled-probe cluster. In the first cluster, four independent coils of collection wire are mounted at 90-degree intervals around the circumference of a short, thin, cylindrical, four-bore ceramic tube. This tube slides into a larger-diameter, single bore, ceramic tube having four azimuthally separated masks (baffles), each lining up with a coil-coil gap on the 4-bore tube, and four baffle-to-baffle gaps, each lining up with a coil on the 4-bore tube. The second cluster uses a single four-bore ceramic tube, with four longitudinal slots to allow side access to each bore, and probes made from a single strand of collection wire. With the tubes oriented perpendicular to the magnetic field, the baffled-probe cluster is rotated around its axis in order to regulate the ratio between electron and ion currents to the baffled probes and, thereby, change the probes' relative sensitivity to space potential and temperature oscillations. This probe-cluster design has the advantages of realtime monitoring of magnetized-plasma fluctuations, post-acquisition determination of electron-temperature or ion-temperature time series, post-acquisition determination of cross-phase and coherence between fluctuations of individual plasma parameters, compact size, minimal plasma perturbation (probes are non-emissive), and relative insensitivity to alignment uncertainty. The clusters have been used for distinguishing temperature fluctuations from potential fluctuations in plasma of the WVU Q-machine.

“…Unfortunately, the plug probe must be aligned precisely with respect to the magnetic field and this is difficult to maintain. This paper evaluates the ability of the so-called baffled probe [16] to distinguish space-potential and electron/iontemperature oscillations. The baffled probe, like the plug probe, is an ion-sensitive probe, but is constructed to have a wider range of orientation angles.…”

Section: Introductionmentioning

confidence: 99%

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

Demidov

Finnegan

Koepke

et al. 2004

Self Cite

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.