Laboratory tests of pulsed CO₂ laser collective Thomson scattering for measurements of divertor ion temperature

Measuring the ion parameters is essential for divertor operations and for validation of divertor simulation codes. Collective Thomson scattering (CTS) is a diagnostics method that measures the ion temperature of a plasma. CTS measurements are challenging because of the inherently small Doppler shift and scattering signals that are difficult to detect. In this work, a heterodyne detection scheme is implemented to measure spectrum broadening of less than a few GHz, which is expected from the ion temperature of divertor plasma. A unique optical collection scheme that is designed to collect the scattered light efficiently using an Axicon lens is demonstrated. The axicon lens is used to collect the scattered light emitted within an annular cross-section from the scattering volume. The efficiency and performance of the heterodyne detection scheme and annular collection approach were demonstrated using a Transverse Excited Atmospheric-pressure (TEA) CO₂ laser with a pulse energy of 160 mJ at = 10.59 mm. Using a pulsed CO₂ laser, feasibility of the heterodyne detection system is demonstrated. *Supported by the U.S. DOE under Contracts DE-SC0019079.