Hybrid simulations of sub-cyclotron compressional and global Alfven Eigenmode stability in spherical tokamaks

Lestz, J. B. ; Belova, E. V. ; Gorelenkov, N. N.
Issue date: 2021
Creative Commons Attribution 4.0 International (CC BY)
Cite as:
Lestz, J. B., Belova, E. V., & Gorelenkov, N. N. (2021). Hybrid simulations of sub-cyclotron compressional and global Alfven Eigenmode stability in spherical tokamaks [Data set]. Princeton Plasma Physics Laboratory, Princeton University. https://doi.org/10.11578/1814953
  author      = {Lestz, J. B. and
                Belova, E. V. and
                Gorelenkov, N. N.},
  title       = {{Hybrid simulations of sub-cyclotron comp
                ressional and global Alfven Eigenmode st
                ability in spherical tokamaks}},
  publisher   = {{Princeton Plasma Physics Laboratory, Pri
                nceton University}},
  year        = 2021,
  url         = {https://doi.org/10.11578/1814953}

A comprehensive numerical study has been conducted in order to investigate the stability of beam-driven, sub-cyclotron frequency compressional (CAE) and global (GAE) Alfven Eigenmodes in low aspect ratio plasmas for a wide range of beam parameters. The presence of CAEs and GAEs has previously been linked to anomalous electron temperature profile flattening at high beam power in NSTX experiments, prompting further examination of the conditions for their excitation. Linear simulations are performed with the hybrid MHD-kinetic initial value code HYM in order to capture the general Doppler-shifted cyclotron resonance that drives the modes. Three distinct types of modes are found in simulations -- co-CAEs, cntr-GAEs, and co-GAEs -- with differing spectral and stability properties. The simulations reveal that unstable GAEs are more ubiquitous than unstable CAEs, consistent with experimental observations, as they are excited at lower beam energies and generally have larger growth rates. Local analytic theory is used to explain key features of the simulation results, including the preferential excitation of different modes based on beam injection geometry and the growth rate dependence on the beam injection velocity, critical velocity, and degree of velocity space anisotropy. The background damping rate is inferred from simulations and estimated analytically for relevant sources not present in the simulation model, indicating that co-CAEs are closer to marginal stability than modes driven by the cyclotron resonances.

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