LOCAL TRANSPORT BARRIER FORMATION AND RELAXATION IN REVERSE-SHEAR PLASMAS ON THE TOKAMAK FUSION TEST REACTOR

The roles of turbulence stabilization by sheared E x B flow and Shafra nov shift gradients are examined for Tokamak Fusion Test Reactor [D. J . Grove and D. M. R Meade, Nucl. Fusion 25, 1167 (1985)] enhanced reve rse-shear (ERS) plasmas. Both effects in combination provide the basis of a positive-feedback model that predicts reinforced turbulence supp ression with increasing pressure gradient. Local fluctuation behavior at the onset of ERS confinement is consistent with this framework. The power required for transitions into the ERS regime are lower when hig h power neutral beams are applied earlier in the current profile evolu tion, consistent with the suggestion that both effects play a role. Se paration of the roles of E x B and Shafranov shift effects was perform ed by varying the EX B shear through changes in the toroidal velocity with nearly steady-state pressure profiles. Transport and fluctuation levels increase only when E x B shearing rates are driven below a crit ical value that is comparable to the fastest linear growth rates of th e dominant instabilities. While a turbulence suppression criterion tha t involves the ratio of shearing to linear growth rates is in accord w ith many of these results, the existence of hidden dependencies of the criterion is suggested in experiments where the toroidal field was va ried. The forward transition into the ERS regime has also been examine d in strongly rotating plasmas. The power threshold is higher with uni directional injection than with balance injection. (C) 1997 American I nstitute of Physics.