Development of cavity enhanced Raman and Thomson scattering diagnostics

Improved measurement capabilities of electron density (ne) and temperature (Te) in low density (∼1010–10n cm−3) plasmas would greatly benefit the study of processing plasmas, electric propulsion thrusters, atmospheric pressure plasmas for combustion and flow control, and other weakly ionized systems. Electron properties can be measured via physical probes or with a non-intrusive optical technique, such as laser Thomson scattering (LTS). LTS has seen widespread use for measurements in high-density plasmas but extension of the technique to the low-density regime has been extremely challenging owing to weak LTS signals and interferences from Rayleigh and elastic background scattering. We present the development of a cavity enhanced Thomson scattering (CETS) diagnostic that seeks to capitalizes on a high-power (10–100 kW) intra-cavity beam achieved by frequency locking a narrow line width laser source to a high finesse optical cavity. The technique should result in a much higher average power light source for LTS measurements and much lower peak power in comparison to pulsed laser sources, which may be advantageous for reducing plasma perturbations. The initial proof-of-concept CETS instrument has been built using a low-power (20 mW) fiber laser and a moderate cavity finesse of F ≈21,000. The Pound-Drever-Hall locking technique is used to maintain a frequency overlap between the laser and cavity. An intra-cavity power of 7 W is generate from an incident laser power of 5 mW, which corresponds to a power build-up factor of 1400. The instrument has been used to measure Rayleigh scattering and rotational Raman spectra in N2, O2, and CO2. Current work is moving to a higher power configuration based on a 5 W laser source and cavity with finesse of F>100,000. The status and challenges of the high power system are discussed.