Abstract
Observations on the National Spherical Torus Experiment (NSTX) indicate that externally applied non-axisymmetric magnetic perturbations (MP) can reduce the amplitude of toroidal Alfvén eigenmodes (TAE) and global Alfvén eigenmodes (GAE) in response to pulsed n = 3 non-resonant fields. From full-orbit following Monte Carlo simulations with the one- and two-fluid resistive MHD plasma response to the magnetic perturbation included, it was found that in response to MP pulses the fast-ion losses increased and the fast-ion drive for the GAEs was reduced. The MP did not affect the fast-ion drive for the TAEs significantly but the Alfvén continuum at the plasma edge was found to be altered due to the toroidal symmetry breaking which leads to coupling of different toroidal harmonics. The TAE gap was reduced at the edge creating enhanced continuum damping of the global TAEs, which is consistent with the observations. The results suggest that optimized non-axisymmetric MP might be exploited to control and mitigate Alfvén instabilities by tailoring the fast-ion distribution function and/or continuum structure.
Highlights
As the fusion program develops in support and preparation of burning plasma operation, the role of fast ions is becoming central for scenario development
[25] are thought to be global Alfvén eigenmodes (GAE) [26]. Both GAEs and compressional Alfvén eigenmodes (CAE) [27] can be present in National Spherical Torus Experiment (NSTX) in the 0.5–2.0 MHz frequency range with very similar experimental characteristics but CAEs need a potential well [28] which is not present at the exper imental parameters of the observed mode and we identify those modes as GAEs
In [50] the con tinuum damping for toroidal Alfvén eigenmodes (TAE) is calculated in 3D magnetic field configurations at negligible β which might be extended to high β plasmas which are routinely created in NSTX
Summary
As the fusion program develops in support and preparation of burning plasma operation, the role of fast ions is becoming central for scenario development. During periods of fast-wave heating, reduction or suppression of Alfvénic activity has been reported in NSTX [10,11,12] While some of these observations could be interpreted as scatter of coherent phase space structures [6, 7, 10], it is extremely challenging to draw conclusions about the mechanisms responsible for the observed behaviour. In this paper we further extend the investigation of the fast-ion drive in these scenarios with the inclusion of the resistive MHD plasma response to the applied 3D magnetic perturbation. Possible effects of changes of the distribution function and toroidal mode coupling associ ated with the MP on the AE stability are studied in section 4 while a summary of the results and a perspective for the use of this technique in fusion devices is presented in the concluding section
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
