Plasma Diagnostics by Thomson Scattering of a Laser Beam

Abstract

The use of Thomson scattering of a laser beam as a diagnostic technique for laboratory plasmas is discussed. Several potential sources of difficulty in the use of this technique are investigated. Included are simple signal-to-noise considerations based upon photon counting statistics and noise due to plasma radiation, Rayleigh or Raman scattering from neutral particles in the plasma, and scattering from walls, apertures, etc. in the apparatus. Also discussed are other effects which may affect the state of the plasma being measured, namely, free-free absorption which leads to electron heating and free-bound absorption or photo-ionization which can produce additional electrons. Experimental determinations of electron density and temperature in a steady-state hollow-cathode arc plasma, by Thomson scattering of a ruby laser beam, are reported and discussed. These measurements refer primarily to argon plasmas over an electron density range 1013–1014 cm−3 and electron temperature range 3–8 eV. Essential agreement of these measurements with measurements of the same quantities by Langmuir probes is obtained, except for some interesting explainable differences.