Measurement of the optical depth at the third electron cyclotron harmonic in Tore Supra

Electron cyclotron absorption of extraordinary waves at the third harmonic has important applications for heating in toroidal plasmas, in particular for devices working at low magnetic field and in high density regimes. Experimental transmission measurements have been performed on Tore Supra, using a vertically propagating extraordinary mode at variable frequency. In contrast with horizontal propagation, the use of vertical propagation allows a detailed comparison with the relativistic theory of wave absorption and ray propagation. Direct experimental measurements of the optical depth have given a global confirmation of the relativistic theory of wave absorption at the 3rd electron cyclotron harmonic. (K.A.) 14 refs.; 6 figs