Analysis of fusion-plasma gases by low-resolution mass spectrometry

Abstract For the analysis of DT fusion-plasma gases, not only the primary components D 2 , T 2 , 4 He, but also H 2 and reaction products like HD, HT and DT have to be considered, and even traces of 3 He may be in the system. For mass spectrometric analyses of these eight gas species, at most six data are usable out of the mass range 1 to 6. Moreover, by also taking into account water and hydrocarbon molecules, the unknowns outnumber by far the available data, making the underlying equation system unsolvable. This problem is overcome by repeatedly measuring the spectrum, but with stepwise changing of the settings of selected parameters of the instrument. Of particular interest are parameters like electron energy E (offering chemical separation of helium and hydrogen) and—in the case of quadrupoles—field axis potential V (allowing us to distinguish between molecular and fragment ions). By choosing two parameters each of E and V , we can get up to 24 linear independent equations out of the mass range 1 to 6, good enough for spectrum decomposition by means of established fit procedures. Details of this method and results from analysing typical gas mixtures are presented.