Spectroscopic measurements on low-pressure hydrogen microwave plasmas

  • R.G. Wijtvliet

Student thesis: Master

Abstract

Spectroscopic temperature measurements are done on microwave plasmas at 0.36 mbar, excited by a waveguide surfatron. The following gases are used: pure hydrogen, water vapour, and mixtures of various percentages of hydrogen in argon and in helium. The rotational temperature of the hydrogen molecules is determined from a Boltzmann plot of the intensity of the Fulcher-alpha band lines. They have a similar range in all mixtures: 900-1500 K. The (Doppler) widths of the helium and oxygen lines indicate temperatures in that same range, and it is assumed that this is a good approximation of the gas temperature. The temperature of the hydrogen atoms in the n = 4...8 states is determined fromthe Doppler broadening of the Balmer lines. In pure hydrogen and He/H2 it is found that the temperature increases with the upper level quantum number: the atoms at highest electronic levels are hottest. Several hypotheses are formulated that might be causing this observation; more research is necessary. In Ar/H2 the trend is absent and in water vapour it is also less pronounced; an excitation transfer mechanism is identified as a probable cause. The highest temperatures measured are in the order of 5000 K (0.4 eV). Additionally, the intensities of the Balmer lines are analysed to compare the population densities of the hydrogen atoms. It is found that in each mixture nq/gq < np/gp for all 3 ? p < q ? 8. The motive for the research is the publication of peculiar measurements in specific mixtures, mainly by Mills, including highly broadened hydrogen lines (indicating temperatures from tens to hundreds of eV) and Balmer population inversion. These results are not reproduced.
Date of Award30 Apr 2008
Original languageEnglish
SupervisorGerrit M.W. Kroesen (Supervisor 1), Sander Nijdam (Supervisor 2), Eddie M. van Veldhuizen (Supervisor 2) & Elena S. Tatarova (External coach)

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