An ionization pressure gauge with LaB6 emitter for long-term operation in strong magnetic fields

Abstract

We report here on a potentially significant improvement in the design of neutral pressure gauges of the so-called ASDEX-type which were first used in the Axially Symmetric Divertor EXperiment (ASDEX). Such gauges are considered state-of-the-art and are in wide use in fusion experiments, but they nonetheless suffer from a relatively high failure rate when operated at high magnetic field strengths for long times. This is therefore a significant concern for long-pulse, high-field experiments such as Wendelstein 7-X (W7-X) and ITER. The new design is much more robust. The improvement is to use a LaB 6 crystal instead of a tungsten wire as the thermionic emitter of electrons in the gauge. Such a LaB 6 prototype gauge was successfully operated for a total of 60 hours in B = 3.1 T, confirming the significantly improved robustness of the new design, and qualifying it for near-term operation in W7-X. With the LaB 6 crystal, an order of magnitude reduction in heating current is achieved, relative to the tungsten filament based gauges, from 15-20 A to 1-2 A. This reduces the Lorenz forces and the heating power by an order of magnitude also and is presumably the reason for the much improved robustness. The new gauge design, test environment setup at the superconducting magnet, and results from test operation are described.

Gas exhaust in the Wendelstein 7-X stellarator during the first divertor operation

“…APGs are equipped with tungsten cathodes [12] while CCPGs have crystal cathodes, improving the robustness of the gauge in long pulse operation. The CCPGs used in W7-X were equipped with cathodes made from single crystalline LaB 6 rods [17]. While APGs suffered from severe cathode problems [19], no such problems were observed with the CCPGs during the operation phase OP1.2b [18].…”

Mechanical Stability of Filaments for ITER Diagnostic Pressure Gauges Relating to Creep and Fatigue

“…crystal emitters with indirect heating, which require significantly less heating current for the same electron emission. At present pressure gauges with such emitters are in operation at W7-X [8].…”

D-Mag — a laboratory for studying plasma physics and diagnostics in strong magnetic fields

“…To provide some examples of the functionality and experimental capabilities of the D-Mag laboratory, we will present some of the results from the latter two topics mentioned in more detail. For the deeper physical understanding on Langmuir probes and crystal cathode pressure gauges (CCPG), the reader is referred to [20] and [9,14], respectively.…”