Study of driven lower-hybrid waves in the Alcator A tokamak using CO2 laser scattering

Measurements are presented of the properties of lower-hybrid waves excited at the edge of plasmas in the Alcator A tokamak by a two-waveguide array operating at microwave powers up to 70 kW. The waves are studied directly in the plasma by measuring the small angle scattering of CO2 laser radiation. A broad N∥ spectrum is observed in the range 2⩽N∥≲8, with more intensity at lower N∥ (here, N∥ is the parallel index of refraction). The waves are distributed throughout the minor cross section and do not appear to be localized in resonance cones. The wave amplitude is linear in net microwave power into the plasma. A power-independent frequency width of several mega-Hertz and a downshift in the frequency of the waves of the order of 1 MHz are observed. Finally, the wave amplitude is observed to be independent of the relative phase of the exciting, two-waveguide array. Most of these results are explained by taking into account the scattering of the lower-hybrid waves from the large-amplitude, low-frequency density fluctuations previously observed near the plasma edge.