CO2 laser interferometer array for Big Dee

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Abstract
A 12 channel interferometer array is planned for obtaining electron density profiles on Big Dee. Each channel consists of coaxial CO2 and HeNe laser beams. The reference beam is formed by splitting off half of the laser power at each wavelength by using acousto‐optic modulators which introduce a 40‐MHz frequency shift in the reference beam. The detected interference signals from room‐temperature visible and IR detectors are mixed with the 40‐MHz drive for the acousto‐optic cells. The quadrature mixer signals result in phase information due to vibration and phase‐velocity variation in the plasma. Simultaneous solution of the two equations describing the phase shift at the two wavelengths yields the phase shift due only to the plasma effects. The arithmetic operations will be performed digitally in real time by a dedicated microprocessor in each channel. The execution speed of the microprocessor is expected to be sufficient for measuring line integral density in the presence of vibrations of 1‐mm amplitude at 1 Hz or 10‐μm amplitude at 100 Hz. Phase resolution for each channel is 2π/64 and time between samples is 25 μs. Density profiles are derived from chordal line integral density data by computer tomography.

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