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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 ∼10^12  cm^−3 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 ∼3×10^9 to 3×10^10  cm^−3, thereby extending current laser Thomson scattering diagnostic capabilities.

A novel cavity-enhanced laser diagnostic has been developed to perform point measurements of spontaneous rotational Raman scattering. A narrow linewidth fiber laser source (1064 nm) is frequency locked to a high-finesse cavity containing the sample gas. Intracavity powers of 22 W are generated from 3.7 mW of incident laser power, corresponding to a buildup factor of 5900. A triple monochromator and a photomultiplier tube in counting mode are used to disperse and measure the scattering spectra. The system is demonstrated with rotational Raman spectra of nitrogen, oxygen, and carbon dioxide at atmospheric pressure. The approach will allow temporally and spatially resolved Raman measurements for combustion diagnostics and, by extending to higher power, Thomson scattering for diagnostics of low-density plasmas.