Role of Nonlinear Coupling in Quiescent H-Mode Development in DIII-D

Nonlinear coupling between edge harmonic oscillation (EHO) components in quiescent H-mode (QH-mode), predicted by MHD simulations, has been confirmed experimentally. We have also observed nonlinear coupling of the EHO to core-resonant tearing modes (TM), which disturbs the EHO and restricts long-lived QH-mode. QH-mode maintains an ELM-free H-mode pedestal, attractive for ITER, via a coherent EHO that enables particle transport. Simulations with JOREK show that nonlinear coupling between harmonics saturates the EHO via an inverse cascade. We consider a DIII-D marginal QH-mode plasma that transitions to ELMy H-mode. The EHO is observed with the Radial Interferometer-Polarimeter (RIP) diagnostic, which detects magnetic fluctuations from the core to the edge, and by beam emission spectroscopy in the edge. Bicoherence analysis reveals that the first three EHO components are coupled throughout QH-mode, reflecting the cascade. After ELMy H-mode arises, we detect with RIP, but not external sensing coils, a transiently returning EHO nonlinearly coupled to two core-resonant TMs. Coupling to TMs interrupts coupling within the EHO. The EHO becomes intermittent, while ELMs persist. TM control, a priority in ITER to avoid disruptions, can also reduce TM-EHO interactions to make QH-mode more robust. The TIP diagnostic on ITER, which resembles RIP, may be able to detect core-edge coupling and inform controls.



Supported by DOE under DE-FC02-04ER54698; DE-SC0019003; DE-SC0019004; SCGSR (ORISE, DE‐SC0014664).