Paper PS-ThP11
Mass Spectroscopy of Metastable Atomic Species in Gas Analysis and Processing Plasmas at High Spectrometer Source Pressures

Thursday, November 3, 2011, 6:00 pm, Room East Exhibit Hall

The availability of particle counters which may be operated at ambient pressures of up to 2x10^-4 Torr is highly desirable in many current research applications. In one field in particular, it allows mass spectroscopy of gas analysis and processing plasmas to be performed using pressures of this order in the quadrupole mass spectrometer (QMS). These pressures are much closer to those of many processing plasmas so that the sampling of neutral species, in particular, from the plasma is improved. The sampling of ion and neutral species from magnetron plasmas is a good example. The particle counter used in the present investigation, could be mounted axially so as to have a direct line-of-sight view of the sampling orifice of the QMS. Consequently, energetic neutral species such as metastable atoms of helium, which are produced in helium plasmas and have long lifetimes against spontaneous de-excitation, may travel to the detector and have sufficient energy to be counted there. The detection of metastable helium may be of importance in the study of reaction processes during hydrogen (H/HD)-deuterium (D₂) plasma fusion, of which the residual fusion byproduct is helium ash. Furthermore, collision processes in the ionisation source of the QMS, (including Penning ionisation), which are insignificant at the more usual source pressures of below 1x10^-5 Torr, generate product ion species whose study helps the interpretation of the processes occurring in the plasma reactor. Typical data from neutral gases and plasmas in a range of gas mixtures which include helium or argon with molecular gases such as hydrogen and nitrous oxide are shown. The majority of the data presented consist of electron impact threshold ionisation efficiency curves obtained by scanning the energy of the electrons in the QMS source. The results presented are discussed in terms of processes which include collisions between metastable species such as He (2¹S and 2³S) at 20.61eV and 19.82eV respectively^[1] having radiative lifetime states of 20±2ms^[2], and also other plasma constituents. The measurements suggest new avenues of research into gas analysis and processing plasmas in which energetic, long-lived, metastable species may have important roles.

[1] Laborie. P, Rees, J.A. Electronic Cross Sections and coefficients, Hydrogen and rare gases, Dunod, 1968

[2] Van Dyck, Robert,S et al. Phys. Rev. A 4, 1327–1336 (1971)