The effect of shear on turbulent diffusion in a microwave discharge plasma in a toroidal stellarator

A toroidal l=3 stellarator with large shear (thetasub(max)<0.25) is used to study the oscillations of a steady-state microwave discharge plasma, and the values of the correlation coefficient between density and electric field fluctuations and turbulent plasma fluxes are measured by the correlation method. It is shown that the oscillations observed are due to the drift-dissipative instability. The influence of shear on the amplitude of drift oscillations, on the cross-correlation coefficient and on the turbulent fluxes due to drift instabilities is analysed. A threshold value of shear (thetasub(cr) approximately 0.05) is found above which a substantial reduction in the oscillation level ((deltansub(e)2)sup(1/2)/nsub(e) approximately thetasup(-1.5)) and in the magnitude of turbulent plasma fluxes (anti GAMMA approximately theta-3) can be noted. (author)