A far-infrared Michelson interferometer for Tokamak electron density measurements using computer-generated reference fringes

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Abstract
A simple far-infrared interferometer which uses the 394 mu m laser line from optically-pumped formic acid vapour to measure tokamak electron density is described. This interferometer is unusual in requiring only one detector and a single probing beam since reference fringes during the plasma shot are obtained by computer interpolation between the fringes observed immediately before and after the shot. Phase modulation is provided by a rotating mirror modulator. Electron density has been measured with a phase resolution corresponding to +or-1/20 wavelength fringe shift, which is equivalent to a central density resolution of +or-0.1*1019 m-3 for an assumed parabolic density distribution in a plasma of diameter of 0.2 m, and with a time resolution of 0.2 ms.