Verification of lifetime and mechanical load aspects for ITER diagnostic pressure gauges based on ZrC emitter

Abstract

Hot cathode ionization gauges will measure the neutral gas pressure in the vacuum vessel of ITER. Overall, 52 gauge heads based on the concept of the ASDEX pressure gauge but using novel ZrC emitters are located in the divertor, in equatorial ports and in pumping ducts. The initial lifetime tests of the novel gauge design have raised the concern that the pyrolytic graphite used to indirectly heat the ceramic ZrC emitter erodes too fast during operation. This effect might limit the lifetime of the gauges below the limits acceptable according to the inherent availability (ignoring operational delays) requirement of 99.8% for the diagnostic system over the lifetime of ITER. Long term operation tests have been performed using 0.1 mm thick W, Ir, and Mo foils as an interlayer between the pyrolytic graphite and the ZrC within a constant atmosphere of 20 Pa H2. The longest lifetime of 860 h has been achieved with the Ir foil. Subsequent vibration tests applying accelerations on a shaker with magnitudes typical for seismic and ITER disruption loads demonstrated that the emitter can survive such demanding loads even after far-progressed erosion of the pyrolytic graphite. Additional ON/OFF cycle tests demonstrated that the gauge is still fully operational. In combination with a revision and consolidation of the operational plan and the reliability, accessibility, maintainability, and inspectability (RAMI) analysis, ITER’s inherent availability requirement for the diagnostic system could be demonstrated successfully.