Description
Electromagnetic microinstabilities are expected to dominate core transport in the high-$\beta_e$ conditions targeted by next-generation fusion pilot plants (FPPs) such as STEP [1], where $\beta_e$ denotes the ratio of electron thermal pressure to magnetic pressure. In the absence of strong sheared flow, nonlinear local gyrokinetic simulations indicate that unstable electromagnetic modes can drive heat fluxes exceeding the available heating power. Predicting transport in this regime is challenging, as such simulations often fail to attain a robust steady state. Instead, as $\beta_e$ increases, the turbulence becomes more electromagnetic and undergoes a transition from a low-transport state dominated by strong zonal flows to a high-transport state in which zonal-flow regulation is weakened and transport rises sharply.
This work addresses these challenges in the context of STEP. Using the stress-balance framework of Zhang et al. [2] and Kennedy et al. [3], we provide a predictive description of this transition in terms of the zonal-flow torque balance. We confirm that the onset of extreme transport correlates with a critical value of $q^{2}\beta_{e}$ (equivalently a limit in $\beta_{\mathrm{pol}}$), where $q$ is the safety factor, which can lie well below any relevant linear stability boundary in $(q, \beta_e)$ space. Extensive nonlinear simulations show that the transition is governed by a competition between electrostatic and magnetic-flutter zonal torques: below threshold, the net torque reinforces zonal flows and supports saturation, whereas above threshold electromagnetic torques counteract the electrostatic drive, suppress zonal-flow regulation, and enable electromagnetic runaway. We further show that access to a second-stable regime at sufficiently high $\beta^{\prime}$ permits re-saturation at larger $\beta_e$, with the ideal ballooning mode threshold providing a useful proxy for delineating this region, with direct implications for flux-driven simulations.
This talk will review these results with a primary focus on their implications for next-generation FPPs. We discuss how the predicted transport regimes constrain viable high-$\beta_\mathrm{pol}$ operating scenarios and examine multiple high-performance MAST-U discharges to assess how close present-day experiments approach the predicted $q^{2}\beta_{e}$ threshold. Implications for scenario design, including tailoring the $q$-profile, achieving sufficient heating power, or exploiting profile control to avoid the transition, are discussed.