Propagation and Damping of Lower Hybrid Fast Waves in a Tokamak Plasma.

The propagation and damping of the Lower Hybrid Fast Waves (LHFW) have been studied in detail in the UCLA Continuous Current Tokamak (CCT) in both test wave and high power current drive experiments. Anomalous damping of LHFW launched by conventional antennas has been observed in CCT over a wide range of frequencies and magnetic fields during high power Fast Wave Current Drive (FWCD) experiments. The fast wave damping is 10-100 times larger than expected from either Landau or collisional damping. In order to identify the wave damping mechanisms, insertible magnetic dipole antennas have been used to launch test waves locally. The damping length of the LHFW was determined to be ~30cm in CCT using detailed poloidal field maps at various distances from the antennas. Lorentzian shaped sidebands, observed to be excited strongly near the fundamental and harmonics of the driver frequencies are very well correlated with the local density fluctuation spectra. At low power levels, the sideband spectra are independent of power and carrier frequencies suggesting that scattering rather than parametric processes are responsible for its generation. We have chosen the wave scattering theory developed by S. N. Antani and D. J. Kaup to compare with our experiments mainly because of its simplicity. There are however, several problems in their paper: (1) An incorrect approximation using the expression Im(n|) ~ (Im(n| ^2)) ^{1/2} has resulted in the overestimation of the wave damping by a factor of 10^3-10^5 in the final solution. Upon correction we are able to get good agreement between the theory and the experiment. (2) At small n_parallel, their fast wave approximation is no longer valid. The original theory has been examined in detail and we have generalized it to a full-wave formalism.