Simple catalytic cell for restoring He leak detector sensitivity on vacuum systems with high D2 backgrounds

Abstract

The DIII–D National Fusion Facility at General Atomics (GA) focuses on plasma physics and fusion energy science. The DIII–D tokamak is a 35 m3 toroidal vacuum vessel with over 200 ports for diagnostic instrumentation, cryogenics, microwave heating, and four large neutral beam injectors. Maintaining 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 beamlines has historically been difficult. D2 is used as the fuel gas in most plasma discharges and neutral beams. After plasma operations, D2 outgassing from the torus walls and internal beamline components can exceed 10−4 std cm3/s. The mass of the D2 molecule (4.028 u) is indistinguishable from that of the He atom (4.003 u) to a standard mass spectrometer leak detector. High levels of D2 reduce leak detector sensitivity and effectively mask the He trace gas signal rendering normal leak checking techniques ineffective. A simple apparatus was developed at GA to address these problems. It consists of a palladium based catalyst cell and associated valves and piping placed in series with the leak detector. This reduces the D2 throughput by a factor greater than 10 000, restoring leak detector sensitivity. This article will briefly discuss the development of the cell, the physical processes involved, the tests performed to quantify and optimize the processes, and the operational results at DIII–D.