Physics of Stimulated $L\to H$ Transitions

We report on model studies of stimulated $L\to H$ transitions. These studies use a novel reduced mesoscale model. Studies reveal that $L\to H$ transitions can be triggered by particle injection into a subcritical state (i.e., $P<P_{\mathrm{Thresh}}$). Particle injection alters the edge mean flow shear via changes of density and temperature gradients. The change of edge mean flow shear is critical to turbulence collapse and the subsequent stimulated transition. For low ambient heating, strong injection is predicted to trigger a transient turbulence collapse. We predict that repetitive injection can maintain the turbulence collapse and so sustain a driven $H$-mode-like state. The total number of particles required to induce a transition by either injection or gas puffing is estimated. Results indicate that the total number of injected particles required is much smaller than that required for inducing a transition by gas puffing. Thus, we show that internal injection is more efficient than gas puffing of comparable strength. We also observe that zonal flows do not play a critical role in stimulated transitions.