Spectroscopic Studies of Atomic and Molecular Processes in the Edge Region of Magnetically Confined Fusion Plasmas
- School of Physics, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban 4000 (South Africa)
- Institut fuer Plasmaphysik, Forschungszentrum Juelich, Association EURATOM-FZJ, Trilateral Euregio Cluster (Germany)
Edge plasma studies are of vital importance for understanding plasma-wall interactions in magnetically confined fusion devices. These interactions determine the transport of neutrals into the plasma, and the properties of the plasma discharge. This presentation deals with optical spectroscopic studies of the plasma boundary, and their role in elucidating the prevailing physical conditions. Recorded spectra are of four types: emission spectra of ions and atoms, produced by electron impact excitation and by charge-exchange recombination, atomic spectra arising from electron impact-induced molecular dissociation and ionisation, visible spectra of molecular hydrogen and its isotopic combinations, and laser-induced fluorescence (LIF) spectra. The atomic spectra are strongly influenced by the confining magnetic field (Zeeman and Paschen-Back effects), which produces characteristic features useful for species identification, temperature determination by Doppler broadening, and studies of chemical and physical sputtering. Detailed analysis of the Zeeman components in both optical and LIF spectra shows that atomic hydrogen is produced in various velocity classes, some related to the relevant molecular Franck-Condon energies. The latter reflect the dominant electron collision processes responsible for production of atoms from molecules. This assignment has been verified by gas-puffing experiments through special test limiters. The higher-energy flanks of hydrogen line profiles probably also show the influence of charge-exchange reactions with molecular ions accelerated in the plasma sheath ('scrape-off layer') separating limiter surfaces from the edge plasma, in analogy to acceleration in the cathode-fall region of gas discharges. While electron collisions play a vital role in generating the spectra, ion collisions with excited atomic radiators act through re-distribution of population among the atomic fine-structure sublevels, and momentum transfer to the atomic nuclei via ion-induced dipole collisions with the bound electrons. The ions are thus important in randomising and equilibrating the velocity distribution of atomic products of molecular dissociation.
- OSTI ID:
- 20898638
- Journal Information:
- AIP Conference Proceedings, Vol. 876, Issue 1; Conference: 23. summer school and international symposium on the physics of ionized gases, National Park Kopaonik (Serbia), 28 Aug - 1 Sep 2006; Other Information: DOI: 10.1063/1.2406033; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-243X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
CHARGE EXCHANGE
CHARGE-EXCHANGE REACTIONS
DOPPLER BROADENING
ELECTRON COLLISIONS
EMISSION SPECTRA
FLUORESCENCE
HYDROGEN
ION COLLISIONS
IONIZATION
MOLECULAR IONS
MOMENTUM TRANSFER
PLASMA
PLASMA CONFINEMENT
PLASMA DIAGNOSTICS
PLASMA SCRAPE-OFF LAYER
PLASMA SHEATH
THERMONUCLEAR DEVICES
VISIBLE SPECTRA
WALL EFFECTS
ZEEMAN EFFECT