Abstract
Extraordinary mode interferometry with the frequency lower than the electron cyclotron frequency has been applied to the core plasma of the JIPP TII-U tokamak. The system is operated as a reflectometer when the density exceeds the cutoff for the extraordinary mode. The measured fringe shift is in good agreement with the calculated fringe shift using the density profile obtained by a six-chord HCN laser interferometer. Advantages and disadvantages of extraordinary mode interferometry are shown numerically and experimentally. To overcome the disadvantages, a dual (ordinary and extraordinary) mode interferometer is proposed as a divertor diagnostic. There are two attractive choices for this scheme. One choice double the operational density range but does not compensate for changes in the length of the transmission line. The other can compensate for transmission line changes, but with a density range limited by the O-mode cutoff.
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