Description
An intermediate plasma state, with better confinement than the L-mode but not yet a fully developed H-mode and free of type-I ELMs, has been routinely reported during the L-H transition at several fusion devices (JET, AUG, DIII-D, EAST, etc), providing an interesting option for ITER operation at marginal heating power~[1]. Low-frequency ($\sim 1$ kHz) axisymmetric ($n = 0, m = \pm 1$) magnetic oscillations are usually detected at the top of the pedestal by a variety of diagnostics for this specific confinement state, which is referred to as I-phase at AUG~[2] and M-mode at JET~[3]. The precise nature of these low-frequency oscillations (LFOs) is still elusive, regardless of several experimental scaling laws~[2,3,4] and theoretical models~[3,4] being recently proposed to predict their frequency values. In this work, a local dispersion relation, originally introduced by analogy with hydrodynamic internal waves propagating in stratified neutral fluids~[3], is rigorously derived within the framework of ideal MHD theory for a simplified slab geometry. It predicts Alfvén waves oscillating at frequencies much smaller than the typical value $\omega_A = k_\parallel v_A$ (i.e., of magnetic-shear waves, with $v_A$ and $k_\parallel$ the Alfvén speed and the parallel wave vector, respectively) that depend strongly on the local characteristic length scales of the sharply inhomogeneous magnetic field, density, and temperature profiles at the plasma pedestal. The conditions for the propagation or damping of such waves are discussed, along with their relevance for the interpretation of the LFOs observed in fusion experiments.
[1] D. I. Réfy et al., Nucl. Fusion 60, 056004 (2020).
[2] G. Birkenmeir et al., Nucl. Fusion 56, 086009 (2016).
[3] E. R. Solano et al., Nucl. Fusion 57, 022021 (2017).
[4] O. Grover et al., Nucl. Fusion 64, 026001 (2024).