Wisconsin In Situ Penning (WISP) gauge: A versatile neutral pressure gauge to measure partial pressures in strong magnetic fields

Abstract

A new type of in-vessel Penning gauge, the Wisconsin In-Situ Penning (WISP) gauge, has been developed and successfully operated in the Wendelstein 7-X (W7-X) island divertor baffle and vacuum vessel. The capacity of the quantitative measurements of the neutral reservoir for light impurities, in particular helium, is important for tokamaks as well as stellarator divertors in order to avoid fuel dilution and radiative energy loss. Penning gauges assisted by spectroscopy are a powerful tool to obtain the total neutral pressure as well as fractional neutral pressures of specific impurities. The WISP gauge is a miniaturized Penning gauge arrangement, which exploits the ambient magnetic field of magnetic confinement fusion experiments to establish the Penning discharge. Then, in-situ spectroscopy is conducted to separate the fractional neutral pressures of hydrogen, helium and possibly also other impurities. The WISP probe head was qualified using the magnetic field of the Magnetized Dusty Plasma Experiment, MDPX, at Auburn University between 0.25 T and 3.5 T [E. Thomas, Journal of Plasma Physics 81 ( 2015)]. The in-depth quantitative evaluation for hydrogen and helium will be shown as well as an exploration of nitrogen, argon and neon. A power law scaling between current I and pressure p: I = f (Gas,V) • p n(Gas, B) was shown. The factor f is gas and anode potential dependent, while n is gas and magnetic field strength dependent. Pressure measurements from 0.1 mbar and down to 1 × 10 −5 mbar were achieved, demonstrating a reliable operation range for relevant pressure levels in the divertor and main vessel regions in current and future fusion devices, with a time resolution of up to 1 kHz. The lowest achievable pressure measurement increases with increasing B and can be shifted with the anode potential V .At Wendelstein 7-X the WISP probe head was mounted on an immersion tube set up that reaches through the cryostat and places the probe head close to the plasma. Two probe heads were positioned in the different divertor pump gaps, top and bottom, and one close to the plasma on the mid-plane in one module. The gauges were in-situ calibrated together with the ASDEX pressure gauges [G. Haas and H. Bosch, Vacuum 51, 39 (1998)]. Data was taken during the entire operation phase 1.2b (OP1.2b) and measurements were coherent with other neutral gas pressure gauges. For the spectroscopic partial pressure measurements, channels of a spectroscopic detection system based on photo-multipliers, so-called Filterscope [R. Colchin, Review of scientific instruments 74, 2068 (2003)] provided by Oak Ridge National Lab (ORNL) were used.

Physics basis for the Wisconsin HTS Axisymmetric Mirror (WHAM)

“…The Wisconsin in situ Penning gauge will provide fast measurements of neutral gas pressure near the plasma edge (Kremeyer et al. 2020).…”

Analysis of hydrogen fueling, recycling, and confinement at Wendelstein 7-X via a single-reservoir particle balance

“…However this only applies to the main species at low impurity concentrations where nH ≈ ne . Partial neutral pressures of H are also available in three poloidal locations through the Wisconsin in situ penning gauges and can be used for a neutral effective confinement time [47].…”

Improvements on the Diagnostic Residual Gas Analyzer at Wendelstein 7-X

“…There exists, however, a Penning gauge optimized for spectroscopic analysis in fusion divertor conditions, and the Wisconsin In situ Penning (WISP) gauge developed for subdivertor partial pressure analysis at W7-X [13]. Aimed at divertor-relevant pressures, with an experimentally observed lower limit 1 × 10 −2 Pa in W7-X OP1.2, the current WISP setup cannot directly be used in the P-DRGA OGA, which operates at about 1 × 10 −3 Pa.…”

“…In principle, a combination of a lower magnetic field at the OGA, e.g., via a permanent magnet clamped onto the outside of the cathode, and a high anode potential can be envisaged to extend the operation regime down to lower pressures. At low magnetic fields, mode switching has been observed [13] but seems to be avoidable by proper field strength selection. Finally, the magnetic field arrangement has to be such that magnetic leakage is minimized (the OGA plasma source in the DRGA analysis station arrangement is in close proximity to the TMP) and also minimally perturbed by external magnetic fields, since magnetic shield may be impractical for the location of the OGA.…”