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_eff7×10^-9 nm^-1 at 0.5 nm from the laser wavelength). The electron density detection limit of the system is n_e10¹⁶ 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_e10¹⁹ m^-3 and T_e1 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.

52.80.Tn Other gas discharges

52.25.Kn Thermodynamics of plasmas

52.20.Hv Atomic, molecular, ion, and heavy-particle collisions

Plasma physics

Issue 12 (21 June 2002)

Received 7 December 2001 , in final form 9 April 2002

Published 31 May 2002