Testing of ITER diagnostic pressure gauges in preparation of final design review

Abstract

Hot cathode ionization gauges will measure neutral gas pressures in the vacuum vessel of ITER. Overall 52 gauge heads are distributed in the divertor, the equatorial ports and pumping ducts. Present design and prototype testing of diagnostic pressure gauge (DPG) heads is presented. Focus is on the development of a novel type of cathode, which is able to overcome major issues that emerged during the preliminary design phase. In comparison with the formerly employed yttria coated tungsten–rhenium filament, the operational stability of the present cathode, which uses an indirectly heated plate of zirconium carbide as the electron emitter, is enhanced in terms of needed heating current and accuracy. A major issue of filament deflection by electromagnetic forces was overcome through the rigid setup, which still allows for the compensation of differential thermal expansion and provides a high reproducibility in manufacturing. Reliable functioning for long periods was proven successfully in a superconducting magnet, covering the required pressure range. Prototype testing revealed an issue during permanent operation in hydrogen at high pressures: erosion can lead to limited life-time of the cathode. The introduction of interlayer foils can retard but not completely inhibit the process. A possible mechanism based on reaction and diffusion of carbon is proposed.