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JUNE 26 MONDAY MORNING
WS-18-MoM-OR.1 MASS SPECTOMETRIC ANALYSIS OF COMPLEX GAS
MIXTURES BY USING A CRYOGENIC TRAP: APPLICATION TO H2:CH4:N2 PLAS-
MAS. Jose A. Ferreira and Francisco L. Tabarés. Laboratorio Nacional de Fusión por Confinamiento
Magnético, CIEMAT, Avda Complutense 22, Madrid
The interpretation of mass spectra from gas mixtures showing strong overlapping of the cracking
contributions from their constituents has been always challenging, and it is particularly problematic
in carbon film deposition plasmas, with strong contribution from several kinds of hydrocarbons. In
the present work we describe the use of a cryogenic trap, suitable for thermal desorption studies
above liquid nitrogen temperatures, for the discrimination of species with overlapping cracking patterns.
The cryogenic trap is a simplified model of that described by C. Leitao et al. [1]. A DC glow
discharge with a total pressure of 10 mtorr was produced in a mixture of H 2 :CH 4 :N 2 with 80, 10 and
10 percent respectively. These types of plasmas are of particular interest in fusion research in relation
to a new technique being developed for the mitigation of tritium trapping by codeposition in the
next step fusion reactor [2]. The complex reactions present in the plasma lead to the formation of
numerous stable species that are difficult to identify by mass spectrometry alone. Thus, for example,
the presence of N 2 and CH 4 prevents the use of 28, 29, 16, 15 mass peaks in the deconvolution of the
spectra. Some volatile compounds like acetylene, hydrogen cyanide, ethylene etc. [2] are produced in
this kind of glow, conveying important information about the reactive processes taking place in the
plasma. However, their cracking peaks are strongly masked by the main plasma species. Many of
these compounds can be readily condensed at the liquid nitrogen temperature, 78 K. Once exposed to
the plasma products, the cryogenic sample can be ramp-heated to desorb the volatile species, therefore
obtaining different mass peaks that evolve with the temperature ramp depending on the dew
point of the different species. The obtained data are then compared to previous studies in this type of
mixtures.
[1] Carlos M.M. Leitao et al. Vacuum, Vol 52 (1999) 23-26
[2] F. L. Tabarés, V. Rohde et al. Plasma Phys. Control. Fusion, Vol 46 (2004) B381-B395
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