@article{e4e04c0385ee42d7ac1216a45b18b0bf,

title = "Axisymmetric global Alfv{\'e}n eigenmodes within the ellipticity-induced frequency gap in the Joint European Torus",

abstract = "Alfv{\'e}n 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 Alfv{\'e}n 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 Alfv{\'e}n continuum, eigenmode structure, and continuum damping. For zero ellipticity, a single mode exists at a frequency below the Alfv{\'e}n 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 Alfv{\'e}n continuum agrees with the numerical modelling.",

author = "H.J.C. Oliver and S.E. Sharapov and B.N. Breizman and L.J. Zheng and X. Litaudon and S. Abduallev and M. Abhangi and P. Abreu and M. Afzal and K.M. Aggarwal and T. Ahlgren and Ahn, {J. H.} and Leena Aho-Mantila and N. Aiba and Markus Airila and R. Albanese and V. Aldred and D. Alegre and E. Alessi and P. Aleynikov and A. Alfier and A. Alkseev and M. Allinson and B. Alper and E. Alves and G. Ambrosino and R. Ambrosino and L. Amicucci and V. Amosov and Sund{\'e}n, {E. Andersson} and M. Angelone and M. Anghel and C. Angioni and L. Appel and C. Appelbee and Antti Hakola and A. J{\"a}rvinen and J. Karhunen and H.T. Kim and H.S. Kim and Seppo Koivuranta and A. Lahtinen and Jari Likonen and Y. Liu and T. Makkonen and H. Nordman and Antti Salmi and M.I.K. Santala and Paula Sir{\'e}n and Tuomas Tala and {JET Contributors}",

note = "Funding Information: 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. Funding Information: 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. Publisher Copyright: {\textcopyright} 2017 EURATOM.",

year = "2017",

month = dec,

day = "1",

doi = "10.1063/1.5005939",

language = "English",

volume = "24",

journal = "Physics of Plasmas",

issn = "1070-664X",

publisher = "American Institute of Physics (AIP)",

number = "12",

}