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.

A powerful tool to quantitatively detect tiny amounts of 4He in a deuterium rich background for fusion research

“…An on line filter, based on a Zr-Al getter alloy to selectively reduce D2 content during leak testing of the PDX tokamak vacuum vessel, has been demonstrated to reduce the deuterium throughput to the mass spectrometer by a factor of 100 -250, depending on the getter temperature [4]. Better results have been achieved during leak testing of the DIII-D tokamak vacuum vessel and of the neutral beam injectors, using Pd catalyst filters, which can reduce the D2 background level by a factor 104, due to the catalyzed reaction of D2 and 02 to produce D20 [5]. These two methods share the further advantage, as compared to cryo-sorption pumps, to preserve helium.…”

“…In figure 3 we have plotted, for each sample, the measured number of 4He atoms versus the number of D2 molecules of the corresponding gas pre-load (partitioning was accounted for). The slope of the linear fit, obtained by the last-squares method, 5 No. of D2 molecules in the sample, ND2 gives a precise estimation of the concentration of 4He, which turns out to be 0.247 + 0.006 ppm, consistently with the expected value V. CONCLUSIONS The results above demonstrate that the proposed sample purification method, coupled with a conventional QMS, allows detection of 4He/D2 peak ratios as low as 2.5 x10-7.…”

“…However, these analyzers feature a rather slow response time and require large and heavy magnets (0.3-0.5 T) in order to achieve the necessary resolution. Quadrupole mass spectrometers or standard helium-leak detectors would be by far more convenient; however, to cope with the above issues, they require appropriate filters, to be placed at their inlet, capable of achieving strong selective attenuation of D2 content [3,4,5]. Deternination of tritium activity, by 3He in-growth method, is a further relevant issue in fusion research, which requires a mass spectrometric analysis system capable to detect small increments of 3He in a tritium background.…”