Physics – Plasma Physics
Scientific paper
Nov 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aps..dpp.rt101s&link_type=abstract
American Physical Society, 41st Annual Meeting of the Division of Plasma Physics, November 15-19, 1999 Seattle, WA, abstract #RT
Physics
Plasma Physics
Scientific paper
Plasmas are at the heart of modern high-efficiency general-purpose light sources: fluorescent lamps and high-intensity discharge lamps. In fluorescent lamps a weakly ionized positive column discharge in a mixture of a rare-gas (few torr) and mercury vapor (few mtorr) converts electrical power into mercury atomic radiation (254 and 185 nm) with an efficiency around two-thirds. The atoms and ions remain near room temperature, while the electrons are non-Maxwellian with an average energy near 1 eV. A phosphor then downconverts the mercury radiation into a spectrum of visible light. The monochromatic yellow low-pressure sodium lamps used in street lighting have an analogous neon-sodium discharge that emits directly into the visible on the sodium D lines. In high-intensity discharge (HID) lamps visible light is generated directly by a weakly ionized arc where all species are in approximate thermal equilibrium, at least near the arc core. Typical total gas pressures are 0.5--50 atm and typical peak temperatures are 5000 K. Mercury accounts for the overwhelming majority of the atoms in the vapor, but the visible light is produced by comparatively small numbers of other metals such as sodium, scandium, thallium, indium, tin, and some of the lanthanides. In many lamps a significant fraction of the emitting species can be ionized, but the presence of a large ``buffer'' gas background means that ionization fractions are typically less than 10-3. Current topics of potential interest to this audience include breakdown and lamp starting; plasma-wall interactions (which are nonequilibrium regions even in HID arcs); plasma-electrode sheaths (which can be fully ionized); induction drive (electrodeless lamps); and radiation transport.
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