PLASMA-DENSITY MEASUREMENTS ON COMPASS-C TOKAMAK FROM ELECTRON-CYCLOTRON EMISSION CUTOFFS

Electron cyclotron emission (ECE) is a standard diagnostic in present day tokamak devices for temperature measurement, When the plasma densi ty is high enough the emission at some frequencies is cut off. Of thes e cutoff frequencies, the first frequency to cut off depends on the sh ape of the density profile. If the density profile can be described by a few parameters, in some circumstances, this first cutoff frequency can be used to obtain two of these parameters. If more than two parame ters are needed to describe the density profile, then additional indep endent measurements are required to find all the parameters. We descri be a technique by which it is possible to obtain an analytical relatio n between the radius at which the first cutoff occurs and the profile parameters. Assuming that the shape of the profile does not change as the average density rises after the first cutoff, one can use the cuto ffs at other frequencies to obtain the average density at the time of these cutoffs. The plasma densities obtained with this technique using the data from a 14 channel ECE diagnostic on COMPASS-C tokamak are in good agreement with those measured by a standard 2 mm interferometer. The density measurement using the ECE cutoffs is an independent measu rement and requires only a frequency calibration of the ECE diagnostic . (C) 1996 American Institute of Physics.