A spatially scanning far-infrared interferometer for the measurement of tokamak electron density profiles

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Abstract
A far-infrared interferometer, illuminated with an optically pumped formic acid vapour laser, which uses a rotating mirror assembly spinning at high speed to scan the probing beam radially across a tokamak plasma, is described. This interferometer has been used on the University of Sydney's TORTUS tokamak, where the data recorded during each scan has been processed to yield radial electron density profiles of the 200 mm diameter plasma. The beam sweeps across the plasma in 1.0 ms, in which time the plasma electron density does not change significantly. Scans are obtained every 8.3 ms throughout the lifetime of the plasma. The phase shift due to the plasma is measured with an accuracy of +or-1/20th wavelength fringe shift, corresponding to an uncertainty in the electron density of +or-1*1018 m-3 and a spatial resolution of 20 mm for the 433 mu m wavelength used.