A TALE OF TWO (VISCO)CITIES Electromagnetic Turbulence and Transport Bifurcations: Implications for Next- Generation Fusion Power Plants
The nonlinear transition to large heat fluxes in local gyrokinetic simulations of electromagnetic turbulence in STEP (a conceptual spherical tokamak power plant) is investigated. Consistent with results reported elsewhere, the onset of enhanced transport is shown to depend on a critical value of q2βe and appears closely connected to a limit on βpol. A broad Set of simulations demonstrates that the transition is mediated by the competition between E × B and magnetic-flutter momentum fluxes. Access to a second-stable regime allows a return to more modest fluxes at higher β′. The ideal ballooning mode (IBM) threshold is identified as a useful proxy for charting these regions of parameter space. These findings establish a predictive framework for recognising “no-go” zones in local gyrokinetics and provide new insight into the saturation physics of electromagnetic turbulence in STEP and potentially in other high-q2βe devices.