April 15-17

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Author: Henry J C Oliver
Requested Type: Pre-Selected Invited
Submitted: 2019-02-22 13:47:51

Co-authors: S. E. Sharapov, B. N. Breizman, A. K. Fontanilla, D. A. Spong, D. Terranova

Contact Info:
University of Texas at Austin
2515 Speedway Stop C1600
Austin, Texas   78712
USA

Abstract Text:
Alfvén eigenmodes driven by energetic particles are ubiquitous in tokamak plasmas. These modes consist of poloidal harmonics coupled by inhomogeneity in the magnetic field. Further coupling can be introduced by 3D inhomogeneities in the plasma density during the assimilation of injected pellets. The material deposited by the pellet temporarily breaks the toroidal and poloidal symmetry of the density profile, causing additional coupling of poloidal and toroidal harmonics. This additional coupling modifies the Alfvén continuum and discrete eigenmode spectrum.

In JET, we observe significant changes to Alfvén eigenmodes during pellet injection [1]. From observed changes in the frequency of the Alfvén eigenmodes, information about the changes in the plasma density due to a pellet can be inferred. To use Alfvén eigenmodes for MHD spectroscopy of pellet injected plasmas, the 3D MHD codes Stellgap [2] and AE3D [3] were generalised to incorporate 3D density profiles. A simple model for the expansion of the pellet wake along a magnetic field line was derived from the fluid equations. Thereby, we obtain the time evolution of the Alfvén eigenfrequency. From the change in mode frequency, we estimate the initial density of the pellet wake and the timescale for poloidal homogenisation of the wake density. The enhancement of the mode amplitude by the time-dependent frequency will be discussed.

References
[1] S. E. Sharapov et al., Nucl. Fusion, 58, 082008 (2018)
[2] D. A. Spong et al., Phys. Plasmas, 10, 3217 (2003)
[3] D. A. Spong et al., Phys. Plasmas, 17, 022106 (2010)

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