Speaker
Henry James Oliver
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.1056.pdf
Modification of Alfvén eigenmodes in tokamaks by pellet injection
H. J. C. Oliver1,2, S. E. Sharapov2, B. N. Breizman1, D. A. Spong3, and JET contributors*
1
Institute for Fusion Studies, University of Texas, Austin, Texas, 78712, USA
2
CCFE, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, UK
3
Oak Ridge National Laboratory, Oak Ridge, Tennessee, 37831-6169, USA
*
See the author list of “X. Litaudon et al., Nucl. Fusion, 57, 102001 (2017)”
Alfvén eigenmodes driven unstable by energetic particles are routinely observed in tokamak
plasmas. The most frequently observed Alfvén eigenmodes are gap modes, where two
poloidal harmonics are coupled by geometric effects. Further coupling of harmonics can
transiently result from the injection of frozen deuterium pellets. Once injected, pellets break
down on timescales of several milliseconds. In JET, we observe a significant change in the
Alfvén eigenmode spectrum during this short period. This phenomenon may be explained by
the transient breaking of the toroidal and poloidal symmetry of the density profile by the
material deposited by the pellet. These inhomogeneities couple additional poloidal and
toroidal harmonics, modifying the Alfvén continuum and eigenmode spectrum.
We have generalised the 3D MHD codes Stellgap [1] and AE3D [2], which characterise
Alfvén waves in stellarators, to incorporate 3D density profiles generated from analytical
expressions for pellet deposition profiles. We thereby obtain the Alfvén mode spectrum and
structure in tokamak plasmas with pellet injection. We compare these calculations with
analytical predictions of mode coupling due to density inhomogeneities.
These results complement the ongoing efforts to use Alfvén eigenmodes for MHD
spectroscopy [3]. From changes in the mode frequency and amplitude – both of which are
affected by the density-related coupling of harmonics – information can be inferred about the
pellet deposition dynamics and fast particle response to pellet injection.
This work has received funding from the U.S. Department of Energy under Grant No. DE-FG02-04ER54742
(IFS) and DE-AC05-00OR22725 (ORNL).
References
[1] D. A. Spong, R. Sanchez, and A. Weller, “Shear Alfvén continua in stellarators”, Phys. Plasmas, 10, 3217
(2003); [2] D. A. Spong, E. D’Azevedo, and Y. Todo, “Clustered frequency analysis of shear Alfvén modes in
stellarators”, Phys. Plasmas, 17, 022106 (2010); [3] S. E. Sharapov, H. J. C. Oliver, B. N. Breizman, M.
Fitzgerald, L. Garzotti, and JET contributors, “MHD spectroscopy of tokamaks with pellets via Alfvén
Eigenmodes”, submitted to Nucl. Fusion (2018)
