Laser Thomson scattering measurements of electron properties in glow discharge plasmas

Thomson scattering is a technique to send a powerful laser into plasmas, and detect scattered spectra and their intensities to yield electron properties, such as electron density and electron temperature (or, more generally, electron energy distribution functions, EEDF). Until recently, the main area of its application has been to high density plasmas, with density of above 10/sup 18/ m/sup 3/ for fusion research, using single or multi-pulse ruby or Nd:YAG lasers as the laser source. In recent years, we have devoted substantial efforts to lower the detection limit for Thomson scattering measurements to below 10/sup 17/ m/sup 3/. This is the density regime which is common for many glow discharges used in industrial applications such as the modification of materials using techniques such as etching and deposition. In high temperature, high density plasmas used for fusion research, a single laser shot is usually used. If this same technique is used in glow discharge plasmas, there is a limit to the number of scattered photons which can be detected. In the steady state in which glow discharges usually operate, this limitation can be overcome by accumulating data for a large number of laser pulses. This has enabled Thomson scattering to be applied to various kinds of glow discharges, such as electron cyclotron resonance (ECR) discharges, radio-frequency inductively coupled plasma (ICP) discharges, a magnetic neutral loop discharges (NLD) and radio-frequency (RF) capacitively coupled discharges.