April 15-17

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Author: C. Z. Cheng
Requested Type: Poster
Submitted: 2019-04-07 17:00:53

Co-authors: G. J. Kramer, M. Podesta, R. Nazikian

Contact Info:
Lehigh University
17 Memorial Dr E
Bethlehem, PA   18015
USA

Abstract Text:
Global Alfven eigenmodes such as TAE, RSAE were discovered in tokamak plasmas from 1980s [1, 2]. Theories and experiments have confirmed that the Alfven eigenmodes can interact with fast particles and cause significant fast particle loss from tokamak core. TAE and RSAE frequencies are above the lowest TAE continuum. However, experiments have also found Beta-induced Alfven Eigenmode (BAE) [3] with frequencies below the TAE gap. The slow-mode approximation [4] of the MHD model has been used to produce the BAE gap and explain the observed BAE modes. However, the slow mode approximation includes the plasma compressibility effect to the Alfven wave by including only the plasma pressure and geodesic curvature effect, but not the slow mode propagation along the field line. Thus, the slow mode approximation does not account for the coupling between the Alfven wave and the slow mode. Here, we present analytical theory of the coupling physics between the Alfven wave and the slow mode and show the formation of Alfven-Sound (AS) frequency gaps in the frequency domain below the TAE gap. We have also discovered new Alfven-Sound Eigenmodes (ASE) with frequencies inside the AS frequency gaps by using the full MHD NOVA code. The ASEs do not suffer continuum damping. The Beta-induced Alfven- Acoustic Eigenmode (BAAE) [5] is one of these ASEs. However, the BAAE can easily interact with Alfven-sound continuum and suffers continuum damping. The existence of the ASEs is robust for normal and reverse shear q-profiles, broad range of plasma pressure profiles and beta values, and plasma shaping.

[1] C. Z. Cheng and M. S. Chance, Phys. Fluids 29, 3695 (1986)
[2] H. Kimura et al., Nucl. Fusion 9, 1303 (1998); B. N. Breizman et al., Phys.
Plasmas 10, 3649 (2003)
[3] W. H. Heidbrink et al., Phys. Plasmas 6, 1147 (1999)
[4] M. S. Chu et al., Phys. Fluids 4, 3713 (1992)
[5] N. N. Gorelenkov et al., Plasma Phys. Controlled Fusion 49, B371 (2007)

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