In vessel pressure measurement in nuclear fusion experiments with ASDEX gauges

Two crucial aspects of the operation of large existing and future fusion experiments are strongly influenced by the neutral gas environment of the plasma. High energy confinement and good performance of H-mode plasmas call for an as low as possible neutral gas density at the surface of the main plasma. The reduction of power load and erosion on the collector plates call for an as high as possible neutral gas density in the divertor. Both conditions can be fulfilled simultaneously only by appropriate mechanical structures and operational scenarios. The scenarios are close to operational limits. For safe operation, the neutral gas has to be controlled carefully and with high time resolution. At MPI fur Plasmaphysik, Garching, a special type of hot cathode ionization gauge has been developed the so-called ASDEX gauge, which can cope with the conditions inside the discharge vessels of fusion oriented plasma experiments:strong magnetic fields with direction changing by up to +/- 25 degrees and high levels of noise and background signals. The gauges are used successfully at several tokamaks and stellarators. The design of the gauges will be shown and the influence of the magnetic field on the operation of the gauge will be discussed. (C) 1998 Elsevier Science Ltd. All rights reserved.