Understanding And Prediction Of Internal Transport Barriers In Tokamaks Using Integrated Modeling

Abstract

Internal transport barriers (ITBs) are regions of plasmas in tokamak discharges with strongly reduced anomalous transport. The ITB formation can lead to the improvement of energy and particle confinement. Some advanced tokamak scenarios rely on the ITB formation in the plasma core. In particular, the high beta poloidal scenario that has been recently studied on the DIII-D and EAST tokamaks depends on the ITB formation at large radii 1. It is generally believed that ITBs are triggered by a large ExB flow shear and by a small or reversed magnetic shear. However, the analysis of DIII-D experimental data shows that the ExB flow shear plays a small role in the ITB formation in high $\mathrm {b}_{p}$ discharges. Using the integrated modeling approach, we demonstrate that the ITBs in these discharges require a combination of large Shafranov shift and reversed magnetic shear. The ExB flow shear indeed does not play a role in the ITB triggering. In addition, it is demonstrated that the ITB formation depends on the q profiles. An increased plasma current can result in a disappearance of ITB. The dependences of ITB on the Shafranov shift, magnetic shear, and q-profiles can be explained from the analysis of the anomalous transport in high beta poloidal discharges. Theory-based Weiland transport model 2 that predict the anomalous transport driven by ion-temperature gradient driven modes, trapped electron modes and some other MHD modes shows a significant quenching effect from the Shafranov shift. The q-profiles are also found important for the formation of transport barriers in the high beta poloidal DIII-D discharges. The gyro-kinetic GTC simulation confirms the anomalous transport trends observed with the theory-based Weiland model. The observed requirements for the ITB formation can be used in the integrated predictive modeling to optimize the high beta poloidal discharge scenarios in DIIID, EAST and other tokamaks.