Shear Alfvén wave continuous spectrum within magnetic islands

Abstract

The radial structure of the continuous spectrum of shear Alfvén waves is calculated in this paper within the separatrix of a magnetic island. Geometrical effects due to the noncircularity of the flux surface's cross section are retained to all orders. On the other hand, we keep only curvature effects responsible for the beta-induced gap in the low-frequency part of the continuous spectrum. Modes with different helicity from that of the magnetic island are considered. The main result is that, inside a magnetic island, there is a continuous spectrum very similar to that of tokamak plasmas, where a generalized safety factor q can be defined and where a wide frequency gap is formed, analogous to the ellipticity induced Alfvén eigenmode gap in tokamaks. The presence of this gap is due to the strong eccentricity of the island cross section. The importance of the existence of such a gap is recognized in potentially hosting magnetic-island induced Alfvén eigenmodes (MiAE). Due to the frequency dependence of the shear Alfvén wave continuum on the magnetic-island size, the possibility of utilizing MiAE frequency scalings as a novel magnetic-island diagnostic is also discussed.

Analytical theory of the shear Alfvén continuum in the presence of a magnetic island

“…This study will be compared to the results of Biancalani for l = 0. 10,11 In particular, secondary resonances are expected at rational Ω surfaces where l 0 − j 0 Ω = 0. Future work will also involve studying the Alfvén spectrum numerically using the SIESTA code.…”

“…The effects of an island on the shear Alfvén continuum have been studied in some detail by Biancalani et al [10][11][12] In Biancalani's work, a shooting method code was used to compute the spectrum. Biancalani showed that an island induces an upshift in the spectrum's minimum continuum accumulation point frequency.…”

Observation of explosive collisionless reconnection in 3D nonlinear gyrofluid simulations

“…Along the whole separatrix, current spikes at scales narrower than the collisionless skin depths have been found. The importance of these effects in causing the reconnection acceleration is under investigation, and a detailed analysis will be published in a dedicated paper [18].…”

Fine structure zonal flow excitation by beta-induced Alfvén eigenmode

“…Meanwhile, as we will show later, it is easier to derive the EE contribution using equation (5). Since the resonant EEs effect of interest here can only be important during the linear growth stage of eBAE, the ZFZF generation is a forced driven process [10,18,40], and the feedback of ZFZF to eBAE will not be considered here.…”

“…In quantitative estimating the ZFZF amplitude and comparing with numerical results, e.g., Ref. [40], k Z ≡ −2i∂ r ln Φ 0 ∼ O( √ βk θ,0 ) should be used.…”