DESCRIPTION OF TURBULENT TRANSPORT IN TOKAMAKS BY INVARIANTS

In general, turbulent transport drives a plasma toward a state of turb ulent equipartition, in which Lagrangian invariants are uniformly dist ributed. Different invariants decay with different rates, and in tokam aks the frozen-in law of particles in the poloidal magnetic field surv ives longer than the corresponding law for the toroidal field, assumin g that the trapped particles dominate the turbulent transport. Therefo re, the plasma profiles depend on the safety factor q(r), and the cond ition for convection of trapped particles is that the shear dq/dr is p ositive. There are two ways to suppress this convection and thereby en hance confinement. The first one is to reverse the magnetic shear. The energy of typical trapped particles then increases outward instead of inward, which suppresses instabilities. The second method is to elimi nate the trapped ions by poloidal rotation, and thereby create a trans port barrier. (C) 1997 American Institute of Physics.