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 \textbf{60}, 056004 (2020).
[2] G.~Birkenmeir et al., Nucl. Fusion \textbf{56}, 086009 (2016).
[3] E.~R.~Solano et al., Nucl. Fusion \textbf{57}, 022021 (2017).
[4] O.~Grover et al., Nucl. Fusion \textbf{64}, 026001 (2024).