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

Kensaku Kamiya,Naoyuki Oyama,Kimitaka Itoh,Nobuyuki Aiba,Akihiko Isayama,Mitsuru Honda

doi:10.1038/s42005-021-00644-x

Abstract

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.

Highlights

Quiescent high-confinement mode plasmas with edge-harmonic oscillations do not exhibit the explosive instabilities associated with edge-localized modes
The quiescent high-confinement-mode (QH-mode) regime was first discovered in the DIII-D tokamak[1] and was subsequently reproduced in the JT-60U2 and ASDEXUpgrade/JET3 tokamaks
Improved understanding of the common physics of edge-localized modes (ELMs) and solar/stellar flares contributes to our intellectual understanding of plasma physics and can improve the controllability of burning plasmas in self-regulating, combined/complex next-step devices, such as ITER15,16 and DEMO17,18

Summary

Introduction

Quiescent high-confinement mode plasmas with edge-harmonic oscillations do not exhibit the explosive instabilities associated with edge-localized modes. Highconfinement mode with large ELMs (ELMy H-mode) operation (which has a high pedestal pressure) may be impossible without a proper physics understanding of the “peeling” mode (or coupled peeling–ballooning modes) This seems essential for predicting the ELM stability limit because the instability is driven by the pressure gradient and the parallel current (i.e., the so-called bootstrap current)[19,20,21,22]. The envelope signal of the dominant fundamental mode of the EHOs exhibits repeated growth–damping cycles on the order of a few milliseconds These cycles are associated with small changes in the pedestal gradient, and the results are quantitatively consistent with a limit-cycleoscillation model

Methods

Results

Conclusion