Excitation and light production processes in gas discharge lamps
are the result of inelastic collisions between atoms and free
electrons in the plasma. Therefore, knowledge of the electron
density ne and temperature Te is essential for a
proper understanding of such plasmas. In this paper, an
experimental system for laser Thomson scattering on a
low-pressure, inductively-coupled gas discharge lamp and
measurements of ne and Te in this lamp are
presented. The experimental system is suitable for low electron
temperatures (down to below 0.2 eV) and employs a triple grating
spectrograph for a high stray light rejection, or equivalently a
low stray light redistribution (Reff≈7×10-9 nm-1 at 0.5 nm from the laser wavelength). The
electron density detection limit of the system is ne≈1016 m-3. The modifications to the lamp that
were necessary for the measurements are described, and results are
presented and compared to previous work and trends expected from
the electron particle and energy balances. The electron density
and temperature are about ne≈1019 m-3
and Te≈1 eV in the most active part of the
plasma; the exact values depend on the argon filling pressure, the
mercury pressure and the position in the lamp.
This paper discusses the possibility of determining, at the same time, both the electron density and temperature in a discharge produced at atmospheric pressure using the Stark broadening of lines spontaneously emitted by a plasma. This direct method allows us to obtain experimental results that are in good agreement with others previously obtained for the same type of discharge. Its advantages and disadvantages compared to other direct methods of diagnostics, namely Thomson scattering, are also discussed.
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