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

Abstract

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.

Progress in development of ITER diagnostic pressure gauges and status of interfaces with ITER components

“…The presented test results demonstrated that the required upper pressure limit of 20 Pa is achievable [3]. At the same time, the requirement on the measurement accuracy of 20 % shall be fulfilled.…”

“…A prototype of the DPG head was placed in the center of the magnet where the magnetic field is maximum. Detailed description of experimental setup and testing procedure is given in [3]. Measurements of the normalized gauge output in the pressure range from 10 -2 Pa to 30 Pa for hydrogen gas and under variation of the magnetic flux density from 1 T to 7.9 T have been taken.…”

“…Measurements of the normalized gauge output in the pressure range from 10 -2 Pa to 30 Pa for hydrogen gas and under variation of the magnetic flux density from 1 T to 7.9 T have been taken. Studying the influence of the electron emission current, the electrode potentials and acceleration grid transparency led to the conclusion that no parameter set demonstrated absolutely superior performance [3]. The reference configuration defined at system level design phase [5] is suited to fulfil the upper pressure limit of 20 Pa.…”

“…On the one hand, at low pressures the sensitivity of the gauge is drastically enhanced by the application of a magnetic field. On the other hand, with the pressure increase the normalized output of the gauge gets flatter limiting measurement accuracy [3]. In order to ensure compliance of the DPG baseline design, described in Section 2, with ITER high pressure operating conditions at 20 Pa, a dedicated performance test has been performed.…”

Mechanical Stability of Filaments for ITER Diagnostic Pressure Gauges Relating to Creep and Fatigue

“…Filaments were fed by 2 power supplies -1 power supply per 6 prototypes with a heating current of 17 A. This value was chosen based on results of previously performed tests [6,7]. The cycles defined included filament heating and cooling phases, both of the same duration, which was optimized so that filaments were in a thermal equilibrium at the end of each phase: The baseline current of 0.5 A serves to measure the voltage drop across the filaments during cooling phase to get a value of total resistance which is used for rough estimation of the mean filament temperature.…”