Dual laser interferometer for plasma density measurements on large tokamaks

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Abstract
A two‐wavelength plasma density interferometer utilizing a CO2 laser and an HeNe laser is described. The interferometer is being designed for use on Doublet III, a large noncircular cross‐section tokamak. The use of the two wavelengths allows the distinction between fringe shifts due to plasma density and fringe shifts due to mirror vibrations. Plasma density fringe shifts of 1/10 of a fringe shift can be measured in the presence of mirror vibrations which cause several fringe shifts. A simple digital phase comparing electronic system is used.