A model for edge transport barrier formation during L-H transition in a tokamak

Based on the edge turbulent layer (ETL) model, a combined ASTRA-ETL numerical code is developed, which makes is possible to simulate the formation 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 μs) and slow (on a time scale of 50 ms) L-H transitions to be described. These transitions are characterized by the appearance of a pedestal on the ion and electron temperature profiles and generation (due to turbulence) of a sheared ExB poloidal rotation that leads to the suppression of turbulence and a significant reduction of the turbulent transport; as a result, the energy confinement time increases. By using the combined model, the dependence of the threshold power for the L-H transition on both the toroidal magnetic field and the plasma density is studied. Agreement of the results from ASTRA-ETL simulations with the data from the DIII-D experiments shows that the combined model correctly describes at least one of the possible mechanisms for the L-H transition and can be used to calculate the scenarios of the tokamak operation in improved confinement regimes