Experimental investigation of current jumps in linear geometry hot cathode ionization gauges in strong magnetic fields

Abstract

In magnetic confinement nuclear fusion, neutral gas in the plasma edge region plays an important role for plasma fuelling and plasma wall interaction and thus influences the overall plasma behaviour. The ASDEX pressure gauge is a diagnostic instrument designed to provide local and temporally highly resolved measurements of the neutral gas flux density within the strong magnetic fields of fusion devices. It is a hot cathode ionization gauge with a linear geometry. Its function principle relies on the measurement of an ion current, caused by oscillating electrons in the gauge which ionize the neutral gas. Anomalous jumps in the ion current of up to 30% have been observed during gauge operation in the tokamak ASDEX Upgrade and the stellarator Wendelstein 7-X, which introduces an error in the measurement. This paper describes for the first time the characterization of these jumps over the broad operational space. Parameter dependencies on pressure, electron current, electrode potentials and magnetic field angle are discussed. The appearance of electrostatic oscillations with frequencies in the range of 160MHz to 350MHz which are associated with the current jumps has been discovered. The frequency of these oscillations was found to scale with the oscillation frequency of the electron motion inside the gauge. Potential physical mechanisms to explain the jumps are discussed.