Description
Hot cathode ionization gauges are used at Wendelstein 7-X for total pressure measurements in the ultra-high vacuum environment of the stellarator. These gauges operate by generating an electron beam through the acceleration of electrons emitted from a heated cathode, which then ionize residual gas molecules. The resulting ion current is used as a measure of the total pressure.
The cathode design has evolved significantly over time. Early implementations used tungsten wire cathodes [0], which were later replaced by linear LaB₆ emitters [1] to improve stability and lifetime. More recently, ZrC cathodes have been adopted in the ITER ionization gauge design [2]. Each of these developments aimed to increase durability and robustness under plasma-relevant conditions.
To assess lifetime and reliability, the ITER ZrC gauge was subjected to a dedicated cycling test that mimics expected tokamak operation, consisting of 3000 periodic on/off cycles. A linear LaB₆ cathode assembly was subjected to similar conditions but in residual gas, and failed with drastically reduced electron emission towards the end of the test. In contrast, operational experience at Wendelstein 7-X demonstrates significantly better performance: up to 18 linear LaB₆ cathode assemblies routinely endure multiple experimental campaigns, each comprising well over 1500 plasma pulses, without comparable degradation. Furthermore, they were subjected to prolonged exposure to atmosphere during maintenance phases, without notable effect on performance.
These observations indicate that linear LaB₆ cathodes are well suited for hot cathode gauge applications in neutron-free environments such as Wendelstein 7-X. At the same time, the discrepancy between test-stand results and in-machine performance suggests that the applied cycling test may not reliably predict real-world behavior of cathode assemblies under actual operating conditions.
[0] G Haas, HS Bosch, Vacuum, 1998
[1] U Wenzel et al, RSI 2019
[2] H Meister et al, RSI 2024