Speaker
Description
Axisymmetric (n=0) long-duration (up to $\sim 10$ s) high-frequency oscillations (HFOs, $50-250$ kHz) near the plasma edge have been observed and experimentally described in L-modes and some L-H transition phases at JET and AUG [1,2]. In this work, we show that these HFOs are a general phenomenon observed in various plasma scenarios and compositions (D, H$^1$, D-T, and D-He$^3$) and under different heating schemes. We further show that these HFOs can be explained by MHD theory as Global Alfvén eigenmodes (GAEs) residing below the shear-Alfvén continuum minima, which result from the increase of the safety factor ($q$) and the sharp decrease of the density at the plasma edge. The frequency of the two lowest n=0 GAEs will strongly depend on density, poloidal magnetic field, $q$, and the plasma elongation ($\kappa$), which is responsible for the separation of the lower branches of the continuum due to a coupling mechanism that opens a continuous gap [3]. For multiple discharges, we find a remarkable agreement between experimental HFOs frequencies and those calculated with the ideal MHD codes CASTOR [4] and MISHKA [5] (relative error $\lesssim 10 \%$). Because predicted GAEs’ frequencies are intrinsically related to the edge $q$ and $\kappa$ values, and since their corresponding HFOs are measured over long time windows and across a variety of plasma scenarios, the latter may provide convenient MHD constraints for accurate equilibrium reconstruction.
References
[1] N. Vianello et al., P2.133, 42$^\text{nd}$ EPS Conference on Plasma Physics, Lisbon, Portugal (2015)
[2] D. I. Réfy et al., Nucl. Fusion 60, 056004 (2020)
[3] H. J. C. Oliver et al., Phys. Plasmas 24, 122505 (2017)
[4] W. Kerner et al., Journal of Computational Physics 142, 271 (1998)
[5] A. Mikhailovskii et al., Plasma Phys. Rep. 23, 844 (1997)