Performance of ITER pressure gauges during deuterium operation in the large helical device

Abstract

During deuterium campaigns on the heliotron large helical device (LHD), ITER pressure gauges with different cathode materials were used to measure the neutral pressure in the sub-divertor region. Throughout these campaigns, it was observed that the performance of the LaB6 cathodes was unsatisfactory, credited to the generation of neutrons impacting the gauges. Conversely, measurements taken with pressure gauges with a ZrC cathode performed well throughout the deuterium pulses. The ITER pressure gauge with the ZrC cathode could be operated with a very high electron current of 800 µA, thus improving the lower detection limit of the neutral pressure in LHD. With this design it was also possible to avoid jumps in the ion current within strong magnetic fields, improving the accuracy of the measurement from 15% uncertainty to 5%. These features allowed very precise neutral pressure measurements to be made in a fusion device with magnetic confinement. The total running time in the magnetic field was around 60 hours, less than the cathode life time of 350 hours.