Fast ion-induced transport barriers in global gyrokinetic simulations

In this contribution, we discuss global gyrokinetic simulations predicting a possible new high-confinement regime in fusion plasmas, sustained via a novel type of transport barrier called F-ATB (fast ion-induced anomalous transport barrier) [1]. More specifically, we performed global GENE [2, 3] simulations with realistic ion-to-electron mass ratio, collisions, electromagnetic effects, and fast ions modelled with realistic background distributions and profiles of a properly designed ASDEX Upgrade plasma discharge with substantial ion-cyclotron-resonance-heating (ICRH). The trigger mechanism responsible for the F-ATB generation is shown to be a recently discovered wave-particle resonance interaction between supra-thermal particles and ion-scale plasma turbulence [4, 5, 6]. This interaction can strongly affect the free energy content of the ITG instabilities and lead to a transport barrier [1]. Signatures of these beneficial fast ion effects have been observed at ASDEX Upgrade [1], JET [7] and are also expected for W7-X [6] and during the ramp-up phase of an ITER [4] standard scenario. 

To gain further physics understanding and to envisage possible experimental actuators for future experiments, we present results exploring the parameter space and physical conditions for the F-ATB formation by performing a systematic study with a series of global GENE simulations. Here, a particular emphasis is given to the transport barrier width and its localization by scanning over different energetic particle profiles. 

The last part of this contribution concerns an exploration study on the existence of F-ATB in SPARC H-mode plasmas at reduced field and with 25 MW of ICRH heating. This is accomplished with a ladder-like approach made of linear and nonlinear GENE simulations. These findings can therefore be considered an important step along with understanding and optimizing nowadays magnetic confinement based fusion experiments as well as improving predictions for future devices.