Simple catalytic cell for restoring He leak detector sensitivity on vacuumsystems with high D-2 backgrounds

The DIII-D National Fusion Facility at General Atomics (GA) focuses on plas ma physics and fusion energy science. The DIII-D tokamak is a 35 m(3) toroi dal vacuum vessel with over 200 ports for diagnostic instrumentation, cryog enics, microwave heating, and four large neutral beam injectors. Maintainin g vacuum in the 10(-8) Torr range is crucial for producing high performance plasma discharges. He leak checking the DIII-D tokamak and the neutral bea mlines has historically been difficult. D-2 is used as the fuel gas in most plasma discharges and neutral beams. After plasma operations, D-2 outgassi ng from the torus walls and internal beamline components can exceed 10(-4) std cm(3)/s. The mass of the D-2 molecule (4.028 u) is indistinguishable fr om that of the He atom (4.003 u) to a standard mass spectrometer leak detec tor. High levels of D-2 reduce leak detector sensitivity and effectively ma sk the He trace gas signal rendering normal leak checking techniques ineffe ctive. A simple apparatus was developed at GA to address these problems. It consists of a palladium based catalyst cell and associated valves and pipi ng placed in series with the leak detector. This reduces the D-2 throughput by a factor greater than 10000, restoring leak detector sensitivity. This article will briefly discuss the development of the cell, the physical proc esses involved, the tests performed to quantify and optimize the processes, and the operational results at DIII-D. (C) 1999 American Vacuum Society. [ S0734-2101 (99)08304-9].