Development of an Interferometer Diagnostic for the Spheromak Turbulent Physics Experiment (STPX)

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Laser interferometry is a proven method for measuring electron density in fusion plasmas. Typically, far-infrared interferometers are used because of the large phase shift due to the change in the index of refraction of the plasma and their relative insensitivity to vibrations. A two-color (CO2: λ=10.6 μm, HeNe: λ=633 nm) interferometer has been designed for installation on STPX. The interferometer is a Mach-Zehnder configuration with a single-chord, double pass probe beam. The laser beams copropagate along a 10-meter path for vibration subtraction along with frequency modulation for heterodyne detection. Detection is carried out using a HgCdTe photoconductor for the CO2 and a Si APD for the HeNe. The capabilities and challenges with the installation of the interferometer on STPX will be discussed.