An experimental study of extraordinary mode absorption at the semi-opaque second and third cyclotron harmonic frequencies has been performed on the Alcator C tokamak. A narrow beam of submillimetre laser radiation was used to illuminate the plasma in the horizontal plane, providing a continuous measurement of the onepass, quasi-perpendicular transmission. Experimental electron cyclotron absorption (ECA) data have been found to agree with theoretical results for lowest significant order in finite density and finite Larmor radius. The ECA data have been used together with data for the tokamak electron density and magnetic field to determine local electron temperatures in the range 75 ≤ Te(eV) ≤ 3300, with a spatial resolution of ≤ 1 cm. Transmission during lower hybrid radiofrequency (LHRF) heating and current drive remained unaffected by suprathermal electrons because of their low density and the relativistic downshift of the absorbing harmonics away from the laser frequency. Therefore, this permitted the ECA technique to be used to measure the bulk plasma temperature during LHRF heating. A density dependent non-resonant attenuation was observed which, when present, was taken into account in the data analysis.
Fokker–Planck studies of high power electron cyclotron heating in tokamaks
Electron cyclotron wave propagation and absorption in the Compact Ignition Tokamak