Description
Turbulence in magnetically confined plasmas is a nonlinear multi-scale phenomenon, the understanding of which requires accounting for various multi-scale interactions. This work presents global gyrokinetic GENE simulations of multi-scale interactions between MHD- and ion-scale fluctuations in current-controlled discharges of a reversed-field pinch (RFP) and a tokamak. In the RFP, MHD-scale tearing modes (TMs) are unstable in the core, while trapped-electron modes (TEMs) dominate near the edge. Nonlinear single-scale TEM simulations without TMs produce strong zonal flows (ZFs) that suppress TEM-driven fluxes to negligible levels. When TMs are included, coupling among core TMs excites smaller-scale, linearly stable edge TMs, erodes the ZFs, and enhances transport. For tokamak simulations, TMs appear radially close to ion-scale electron-temperature-gradient (ETG) modes. Single-scale simulations that isolate TMs or ETGs overestimate fluxes compared to multi-scale simulations incorporating both. ETG modes generate strong electrostatic heat fluxes near the peak of the electron temperature gradient, while coupling with TMs flattens this peak and reduces ETG-driven flux. Conversely, MHD single-scale cases exhibit significant electromagnetic heat flux due to activity of neoclassical tearing modes (NTMs) driven by perturbed bootstrap current. Including ETG dynamics in simulations mitigates this effect by eroding TM-induced corrugations and restoring the temperature profile, thereby reducing NTM-driven fluxes. These findings demonstrate the critical role of multi-scale interactions in accurately describing turbulent transport in confined plasmas.