In situ neutral pressure measurements in a compact high‐field tokamak, Alcator C‐Mod

The Alcator C‐Mod advanced divertor program requires fast in situ, absolutely calibrated neutral pressure measurements. The concept of a linear‐geometry ionization gauge, first implemented at Princeton and Garching, was further developed at the MIT Plasma Fusion Center, to meet Alcator C‐Mod requirements (high field, compact design). In order to correctly interpret experimental data, the gauge was subjected to a series of in situ calibration tests. It has been found that the gauge sensitivity increases with increasing parallel (to gauge axis) magnetic field. In the high‐pressure, high‐field limit the basic assumption of the operation of ionization gauges, I i /I e ≪1, no longer holds. Consequently a modified expression for the gauge sensitivity, which is a strong function of the primary electron current, is necessary to linearize gauge response over a broad pressure range. Representative pressure measurements from the Alcator C‐Mod divertor are included.