Unveiling the structure and dynamics of peeling mode in quiescent high-confinement tokamak plasmas

Quiescent high-confinement mode plasmas with edge-harmonic oscillations do not exhibit the explosive instabilities associated with edge-localized modes. Instead, an additional means of enhanced transport is considered to maintain the plasma edge under conditions just below the boundary of the peeling mode instability. Although the potential of the peeling mode has been widely recognized in plasma physics, no direct evidence for this mode has been revealed previously because decisive diagnostics were lacking. Herein, we report evidence of the structure and dynamical steady state of peeling mode in quiescent high-confinement mode. Edge-harmonic oscillations are dominated by fundamental mode at both the low- and high-field sides. Edge perturbations are confirmed to have kink parity and exhibit the frozen-in-condition predicted by linear stability analysis. The envelope signal of the fundamental mode exhibits repeated cycles of growth and damping in association with minor changes in the edge gradient. Results from this study are quantitatively consistent with limit-cycle-oscillation model. Unveiling the properties of edge harmonic oscillation is one of the major pursuits to understand complex systems in plasma physics. The authors present measurements at the JT-60U tokamak that show direct observations of peeling modes in quiescent high-confinement tokamak plasmas, offering insights into the "peeling nature" of both laboratory plasmas and astronomical plasmas.