Thomson scattering on a low-pressure, inductively-coupled gas discharge lamp

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 n_e and temperature T_e 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 n_e and T_e 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 (R_eff≈7×10^-9 nm^-1 at 0.5 nm from the laser wavelength). The electron density detection limit of the system is n_e≈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 n_e≈10¹⁹ m^-3and T_e≈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.