Different scenarios of transitions into improved confinement modes

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Abstract
This review addresses transitions into regimes with improved confinement. The paradigm is tested on the example of L - H transitions. It is demonstrated that L - H transitions may emerge either due to an amplification of the diamagnetic drift term caused by enhanced pressure gradient or due to an increased poloidal rotation velocity at the separatrix. In general, it is asserted that the emergence and dynamics of transitions are very sensitive to fine details of prelude plasma profiles. The L - H transition occurs provided the radial electric field changes dramatically within a few poloidal gyro-radii from the separatrix. The adoption of a significant power flux to the separatrix as the trigger of the L - H transition appears from the theoretical point of view to be a simplistic `brute force' solution to the issue of confinement control. The propagation of the front of the barrier is very fast. The dynamics of interest consists of threshold conditions for barrier formation, barrier propagation speed and profile steepening rates, barrier limits and mechanisms for their relaxation and termination.