Transport barriers at the plasma edge are key elements of high confinement modes (H-modes) in fusion devices. These barriers, characterized by a local steepening of density and temperature gradients, are strongly linked to shear flows. The latter reduce significantly turbulent heat and particle transport. During a transition from low to high confinement (L-H transition), an edge transport barrier builds up spontaneously. A barrier can also be produced by an externally driven ExB shear flow via biasing techniques.
In the most promising operational regime of future reactors, the edge transport barrier is not stable but relaxes quasi-periodically. During such fast relaxation events, turbulent transport through the barrier increases strongly and the pressure inside the barrier drops. Thereafter, the barrier builds up again on a slow, collisional time scale. These relaxation oscillations are linked to so called edge localized modes (ELMs) which are believed to be magneto-hydrodynamical (MHD) modes driven by the edge pressure gradient (ballooning) and/or the edge current (peeling). The basic physical mechanism underlying these kind of relaxation oscillations is not yet understood. In particular, there is no explanation why the plasma, instead of remaining in a statistically stationary state close to pressure gradient or edge current stability limits, crosses these limits quasi periodically.
In the present work, we investigate the relaxation dynamics of transport barriers at the tokamak edge using a reduced model as well as 3D simulations of resistive ballooning turbulence, where the transport barrier is generated by an externally imposed sheared flow. Quasi periodic relaxation oscillations of transport barriers associated to strong peaks in the turbulent flux are observed in these simulations. These oscillations persist even when the poloidal flow profile is frozen (i.e. with a stationary mean flow and no zonal flows).