Evaluation of an Omegatron mass spectrometer for use in the plasma exhaust processing of a fusion reactor

The plasma exhaust processing plant of fusion reactors require on-line analysis of gas streams to monitor various gas compositions. The resolution of an Omegatron increases with decreasing mass and it can easily discriminate between all the 20+ species of ions between masses 1 and 9, except between T+ and He3+ ions. The Omegatron is a candidate device for gas analysis, it is small, simple to operate and relatively inexpensive. The disadvantages are the upper pressure limit of about 1.0E−6 mbar and low output currents, typically less than 1.0E−11 A, which give slow response times. The Omegatron at the Tritium Laboratory Karlsruhe is well established and has been used successfully for qualitative analyses of tritiated gas mixtures. Quantitative measurements on known gas mixtures up to mass 28 have revealed several anomalies, particularly when helium is present. Work to explain this effect is reported. The gas in the mass spectrometer has to be a representative sample and be introduced in a reasonable timescale. A potential pumping and expansion arrangement is described. The Omegatron is highly suited to computer control as the cyclotron frequencies of the various ions are precisely known and techniques to control the Omegatron and evaluate the results are proposed.