Axisymmetric global Alfvén eigenmodes within the ellipticity-induced frequency gap in the Joint European Torus

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Abstract

Alfvén eigenmodes (AEs) with toroidal mode number n = 0 (i.e., axisymmetric) have been observed in the ellipticity-induced frequency range in the Joint European Torus. The axisymmetric modes are of interest because they can be used to diagnose fast particle energy distributions at the mode location. The modes were identified as global Alfvén eigenmodes (GAEs), with the ellipticity of the plasma cross-section preventing strong continuum damping of the modes. The MHD codes CSCAS, MISHKA, and AEGIS were used to compute the n = 0 Alfvén continuum, eigenmode structure, and continuum damping. For zero ellipticity, a single mode exists at a frequency below the Alfvén continuum branch. This mode has two dominant poloidal harmonics with poloidal mode numbers m = ±1 that have the same polarity; therefore, it is an even mode. For finite ellipticity, the continuum branch splits into two branches and the single GAE splits into two modes. An even mode exists below the minimum of the top continuum branch, and the frequency of this mode coincides with the experimentally observed AE frequency. The other mode is found below the lower continuum branch with opposite signs between the two poloidal harmonics (an odd mode structure). This mode was not excited in our experiment. Analytical theory for the n = 0 GAE in an elliptical cylinder shows the n = 0 Alfvén continuum agrees with the numerical modelling.

Original languageEnglish
Article number122505
JournalPhysics of Plasmas
Volume24
Issue number12
DOIs
StatePublished - 1 Dec 2017
Externally publishedYes

Bibliographical note

Publisher Copyright:
© 2017 EURATOM.

Funding

This research was supported by the Office of Fusion Energy Science of the U.S. Department of Energy under Grant No. DE-FG02-04ER54742. AEGIS calculations used resources of the National Energy Research Scientific Computing Center, a Department of Energy Office of Science User Facility supported under Contract No. DE-AC02-05CH11231. This paper benefited greatly from comments by K. G. McClements and M. Baruzzo. We also thank M. Li for advice on modifications to AEGIS. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 under Grant Agreement No. 633053 and from the RCUK Energy Programme [Grant No. EP/P012450/1]. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

FundersFunder number
Department of Energy Office of Science
Euratom research and training programme 2014–2018
Office of Fusion Energy Science
U.S. Department of Energy
Horizon 2020 Framework Programme633053
Research Councils UK

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