Pedestal Turbulence: Microtearing in the Presence of RMPs

One of the key turbulence regimes in present-day tokamak pedestals is driven by the microtearing (MT) instability, allowing density and temperature profiles to evolve independently, thus distinguishing it from MHD-like regimes, where profiles are coupled. Based on the identification of quasi-coherent modes on DIII-D as signatures of MT turbulence, we investigate by means of nonlinear gyrokinetic simulations the impact of external resonant magnetic perturbations (RMPs) on this regime, applying a constant-in-time perturbation at the experimentally relevant length scale. We separate the dynamical impact on the MT from the island-induced flutter. Results are compared to scenarios where RMPs interact with electrostatic modes, in particular trapped-electron mode turbulence.

Additionally, we present progress on collisionless microtearing theory – where access to free energy is provided by the ∇B curvature resonance rather than collisionality – with a range of applications, including reversed-field pinches and the Pegasus spherical tokamak, where MT turbulence has been identified as an important player [D.R. Smith, TTF 2018].