Gas pressure in the end plug regions of the TMX‐U thermal barrier experiment

We consider briefly the upper bound on background pressure (0.5 to 1.0×10−6 Torr) that is required for operation of the thermal barrier end plugs of TMX‐U and propose a rate equation for pressure buildup that includes terms for neutral beam injection and wall reflux. During early operation of TMX‐U the background pressure exceeded this upper bound; in addition, axial end plugging of plasma losses seemed limited by the excessive pressure. Modifications to the vacuum system to decrease this pressure to the desired range are described. Furthermore, wall reflux is shown to depend linearly on pressure. We conclude with an example of the dramatic increase in time duration of axial plugging of plasma losses with the reduced background pressure.