DIFFERENT SCENARIOS OF TRANSITIONS INTO IMPROVED CONFINEMENT MODES

This review addresses transitions into regimes with improved confineme nt. The E x B paradigm is tested on the example of L-H transitions. It is demonstrated that L-H transitions may emerge either due to an ampl ification of the diamagnetic drift term caused by enhanced pressure gr adient or due to an increased poloidal rotation velocity at the separa trix. In general, it is asserted that the emergence and dynamics of tr ansitions are very sensitive to fine details of prelude plasma profile s. The L-H transition occurs provided the radial electric field change s dramatically within a few poloidal gyro-radii from the separatrix. T he adoption of a significant power flux to the separatrix as the trigg er of the LH transition appears from the theoretical point of view to be a simplistic 'brute force' solution to the issue of confinement con trol. The propagation of the front of the barrier is very fast. The dy namics of interest consists of threshold conditions for barrier format ion, barrier propagation speed and profile steepening rates, barrier l imits and mechanisms for their relaxation and termination.