Fusion Science and Technology / Volume 62 / Number 2 / October 2012 / Pages 322-332
The coupling of lower hybrid waves from the two multijunction-type antennas installed on Tore Supra is investigated. At low power good agreement between the measurement of the power reflection coefficient and the computation with the ALOHA code is found for most cases. Details on electron density measurements, documented from embedded Langmuir probes, are discussed. At high power, departure from the linear theory is clearly seen in many cases. Ponderomotive forces depleting the electron density in a thin plasma layer in front of the antennas are likely to be responsible for the increase of power reflection coefficients measured at the input of the antennas. The decrease of the antenna directivity resulting from weaker wave coupling accounts for the experimental reduction of current drive efficiency.