Study of drift-wave turbulence by microwave scattering in a toroidal plasma

A study of drift-wave turbulence by miowave scattering technique was carried out in a toroidal plasma confinement device, the FM-1 spherator. The principal results are: (a) Observation of the linear dispersion relation of drift waves in the high-magnetic-shear condition. The linear dispersion relation is followed up to the weak turbulence state (γ ≲ ω). (b) An experimental demonstration of the stabilization of the drift waves with an increase of the shear strength. In particular, the ion-mass dependence of the observed marginal stability criterion for drift waves (ω≈ω*) was in reasonably good agreement with theoretical expectations. Further, very-low-frequency (ω≪ω*) fluctuations were enhanced when the shear strength was lowered, (c) A detailed study of the behaviour of drift waves in a weak shear condition where a state of isotropic turbulence was observed. The turbulent density fluctuations showed a strong dependence on frequency, but no dependence on wave number (in sharp contrast to theoretical expectations). The possible dependence of the anomalous loss process on the observed drift-wave turbulence is also discussed.