Fig. 6: Schematic of the peeling-ballooning stability boundary modified by small-scale drift wave turbulence scattering.

This figure illustrates how small-scale drift-wave scattering can expand the ELM-stable operational space. The \(J-\alpha\) stability diagram is calculated for the stationary phase (2270–2370 ms) of QH-mode discharge #173707 using the VARYPED-ELITE suite^38,55, where \(J\) is the peak edge current density and \(\alpha=-2{\mu }_{0}{q}^{2}R{P}^{{\prime} }/{B}^{2}\) is the normalized pedestal pressure gradient. The blue curve marks the standard linear PBM boundary, defined by a growth rate \(\gamma=0.02{\omega }_{A}\), while the green curve represents a hypothetical boundary where small-scale EDW scattering is assumed to weaken PBM drive without considering the self-consistent change in EDW intensity due to profile variation. The yellow square indicates the experimental operating point during the stationary wide-pedestal QH-mode phase. Colored symbols trace an illustrative trajectory approaching an ELM (2352–2373 ms), assuming \(J\) and \({{\rm{\alpha }}}\) scale with the measured electron density gradient evolution (see Supplementary Fig. 3). This trajectory is a schematic representation, not a precise calculation, due to the lack of high-temporal-resolution electron/ion temperature and ion density profiles. The rightmost color bar shows the linear growth rate and the corresponding profile evolution time.