A promising operational regime of future fusion reactors is characterized by an edge transport barrier, i.e. a localized steepening of density and temperature gradients. Typically, such a barrier is unstable and relaxes quasi-periodically. In this work, we show that complete barrier relaxation cycles can be reproduced by three-dimensional turbulence simulations. In these simulations, a barrier forms due to an imposed E × B shear flow. This barrier relaxes intermittently on the confinement time scale, even if fluctuations of the E × B flow are suppressed. It is found here that if the E × B shear increases faster than linearly with heating power, the relaxation frequency decreases with power. A relaxation event has a complex dynamical behaviour, characterized by the intermittent growth of a mode at the barrier centre. A crucial ingredient in this non-linear dynamics is a time delay for an effective E × B velocity shear stabilization.
Transition to an enhanced internal transport barrier