He leak detection in the presence of deuterium background in tokamak vacuum systems

Helium leak detection systems for magnetic fusion devices present several unique design problems because of the large dynamic range required and the high partial pressures of D2 encountered. We describe the design and operation of a He leak detector system for the PDX tokamak. The system consists of a differentially‐pumped, low resolution, He mass spectrometer which is interfaced to the foreline of one of the torus turbomolecular pump lines. The He detector has a minimum throughput sensitivity of 10−10 Torr⋅l/s, and the torus‐integrated system has been designed for detection of torus leaks over the range of 10−7 to 10 Torr⋅l/s. Minimum leak‐rates on the 38 m3 PDX vessel which have been quantified using this system are ∠3×10−8 Torr⋅l/s. When PDX is operated with D2 plasmas it is necessary to reduce the partial pressure of D2 by a factor of 100 within the mass spectrometer to maintain this sensitivity in the presence of the torus D2 outgassing. We have designed and incorporated a D2 filter which employs a Z...