Cavity-enhanced Thomson scattering measurements of electron density and temperature in a hollow cathode discharge.

A cavity-enhanced Thomson scattering (CETS) diagnostic has been developed to perform electron density and temperature measurements in low-density weakly ionized discharges. The diagnostic approach is based on generating a high-power beam in an optical build-up cavity and using the beam as a light source for Thomson scattering from plasma housed within the cavity. In our setup, a high-power (∼5  W) fiber laser at 1064 nm allows an intra-cavity power of 11.7 kW in a two-mirror cavity for measurements in the plume of a BaO hollow cathode discharge. A study of plasma density and temperature was performed at various operating conditions. Electron densities and temperatures in the range of ∼10  cm and ∼3  eV were measured, respectively. The high signal-to-noise ratio (SNR) of the present measurements (SNR=1100) suggests the ability to measure significantly lower density plasmas in the range of ∼3×10 to 3×10  cm, thereby extending current laser Thomson scattering diagnostic capabilities.