Modelling and optimisation of ionisation gauges for magnetic nuclear devices

We have developed a numerical tool that simulates the operation of an ionisation-based pressure gauge in the presence of magnetic field. In this paper we describe the physical model, the numerical implementation and the validation of the code against laboratory experiments using the so-called ASDEX pressure gauge. We then show how this modelling has led to a deeper understanding of processes associated with the presence of magnetic field in this type of gauge: sensitivity enhancement, sensitivity dependence on primary electron current and output saturation at high gas pressure. The combined electric and magnetic fields may trap electrons long enough so that they can spend all their energy in collisions with the neutral gas, thus increasing the sensitivity and leading to early saturation (with respect to the no B-field case). Although it seems difficult to avoid particle trapping with strong B-field, the fraction of trapped electrons and the trapping time can be limited by careful tailoring of the electric potential in the gauge and thus creating E × B drifts. This results in a modified sensitivity and saturation characteristic. Finally we show an example of a modified gauge which has an improved upper pressure limit in view of the next generation nuclear fusion experiment such as ITER.