Controlling ELM Dynamics: From Bursting Instabilities to Continuous Turbulence

Robust control of Edge Localized Modes (ELMs) while preserving high confinement is critical for ITER and future fusion reactors. Recent BOUT++ simulations of DIII-D hybrid plasmas reveal a novel physics mechanism governing the transition from large ELM bursts to continuous turbulence-driven transport. Central to this discovery is the separatrix-to-pedestal density ratio (n_e,sep/n_e,ped), identified as a powerful actuator for edge stability. Elevated separatrix densities stabilize global peeling-ballooning (P-B) instabilities while triggering localized high-n ballooning modes near the separatrix. Nonlinear simulations uncover a delicate interplay: increased separatrix density and outward-shifted pedestal density gradients suppress global modes, fostering continuous small-amplitude turbulence rather than discrete ELM bursts. The competition between drift-Alfvén instabilities (DAI) and localized ballooning modes is decisive in determining whether turbulence-driven transport persists or reverts to ELM bursts. A novel diagnostic, the post-crash pressure fluctuation peak (δP_rms), accurately captures this nonlinear threshold, enabling predictive real-time control of edge conditions. Comprehensive parameter scans in highly shaped double-null configurations highlight SOL density, gradient location, and resistivity as critical factors influencing edge dynamics. These insights link fundamental plasma turbulence to macroscopic stability, offering actionable strategies for achieving small/no-ELM regimes compatible with detached divertor operation. This work significantly advances predictive capabilities and practical edge control strategies, essential for ITER and next generation burning plasma devices.