THEORY AND SIMULATION OF ROTATIONAL SHEAR STABILIZATION OF TURBULENCE

Numerical simulations of ion temperature gradient (ITG) mode transport with gyrofluid flux tube codes first lead to the rule that the turbul ence is quenched when the critical ExB rotational shear rate gamma E_c rit exceeds the maximum of ballooning mode growth rates gamma(0) witho ut ExB shear [Waltz, Kerbel, and Milovich, Phys. Plasmas 1, 2229 (1994 )]. The present work revisits the flux tube simulations reformulated i n terms of Floquet ballooning modes which convect in the ballooning mo de angle. This new formulation avoids linearly unstable ''box modes'' from discretizing in the ballooning angle and illustrates the true non linear nature of the stabilization in toroidal geometry. The linear ei genmodes can be linearly stable at small EXB shear rates, yet Floquet mode convective amplification allows turbulence to persist unless the critical shear rate is exceeded. The flux tube simulations and the gam ma(E_crit)approximate to gamma(0) quench rule are valid only at vanish ing relative gyroradius. Modifications and limits of validity on the q uench rule are suggested from analyzing the finite relative gyroradius ''ballooning-Schrodinger equation'' [R. L. Dewar, Plasma Phys. Contro lled Fusion 39, 437 (1997)], which treats general ''profile shear'' (x variation in gamma(0)) and ''profile curvature'' (x(2) profile variat ion). (C) 1998 American Institute of Physics.