Experimental exploration of the upper measuring limit of pressure gauges for ITER by variation of instrumental parameters

Hot cathode ionization gauges are intended to be the only in-situ diagnostic for neutral gas pressures in the vacuum vessel of ITER. The development bases on the well established ASDEX pressure gauge. The upper measuring limit for ITER is required to be at least 20 Pa in hydrogen at a magnetic flux density of up to 8 T. The objective of the presented study is to find parameter and design settings that allow to satisfy this specification. At the same time, the sensitivity shall be high and only weakly dependent on the magnetic field strength. Gauge parameters, specifically the electron emission current, the electrode potentials and the transparency of the acceleration grid, were varied consecutively to assess their impact on the calibration characteristics. The ratio of ion to electron current as a function of pressure and magnetic flux density was obtained for each parameter set. A monotonic progression of the signal in dependence of the pressure was proven even up to 30 Pa. This was achieved by a low grid transparency and high electric field strength at the cathode. While the former leads to a lower sensitivity, which is unfavorable for measurements in the low pressure range, this can be compensated by a higher electron current.