A MODEL FOR EDGE TRANSPORT BARRIER FORMATION DURING L-H TRANSITION INA TOKAMAK

Based on the edge turbulent layer (ETL) model, a combined ASTRA-ETL nu merical code is developed, which makes it possible to simulate the for mation of the transport barrier just within the separatrix during the L-H transition in a tokamak. The combined model allows both fast (on a time scale of 50 mu s) and slow (on a time scale of 50 ms) GH transit ions to be described. These transitions are characterized by the appea rance of a pedestal on the ion and electron temperature profiles and g eneration (due to turbulence) of a sheared E x B poloidal rotation tha t leads to the suppression of turbulence and a significant reduction o f the turbulent transport; as a result, the energy confinement time in creases. By using the combined model, the dependence of the threshold power for the GH transition on both the toroidal magnetic field and th e plasma density is studied. Agreement of the results from ASTRA-ETL s imulations with the data from the DIII-D experiments shows that the co mbined model correctly describes at least one of the possible mechanis ms for the GH transition and can be used to calculate the scenarios of the tokamak operation in improved confinement regimes.