Tokamak Temperature Measurement using Electron Cyclotron Emission at the Third Harmonic

The use of electron cyclotron emission from a tokamak plasma as a means for measuring the electron temperature spatial profile has become popular since the possibility was pointed out by Engelmann and Curatolo [1] in 1973. Generally one uses the fundamental or the second harmonic because the emission from a thermal plasma at these frequencies is very nearly at the black-body level, so that the intensity is directly proportional to the electron temperature and conveniently independent of the electron density. Use of the third harmonic has not been considered attractive because the plasma is optically thin at this frequency so the emission is dependent on the density [2] as well as the wall reflectivity. Unfortunately, as tokamak researchers succeed in creating plasmas of higher densities, the lower harmonic emission can be cut off, so that it does not propagate to the receiving antenna. Under such conditions, the third harmonic becomes more interesting.