The Mega Amp Spherical Tokamak (MAST) was a low aspect ratio device (R/A = 0.85/0.65 ~ 1.3) with similar poloidal cross-section to other medium-size tokamaks. The physics programme concentrates on addressing key physics ___________________________________________________________________________
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.
Direct measurements of the helium (He) fractional neutral pressure in the neutral gas around fusion devices is challenging because of the small mass difference between the abundant D molecules and the He ash which will be produced by deuterium-tritium fusion. To study He exhaust, an in situ Penning gauge system is being developed at UW-Madison that is optimized for good pressure and high spectroscopic sensitivity. Three different anode geometries have been studied regarding their vacuum electrostatic fields, light output, and ion current. The light output of the two new anode configurations are at least one order of magnitude above the currently available designs, hence improving the spectroscopic sensitivity at similar total neutral pressure resolution.
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