U.S. Department of EnergyOffice of Scientific and Technical Information
Hybrid simulations of sub-cyclotron compressional and global Alfven Eigenmode stability in spherical tokamaks
Abstract
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 relevantmore »
- Authors:
-
- Princeton University (PPPL)
- Publication Date:
- DOE Contract Number:
- AC02-09CH11466
- Research Org.:
- Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States)
- Sponsoring Org.:
- USDOE Office of Science (SC)
- Subject:
- Compressional Alfven Eigenmodes; Doppler-shifted cyclotron resonance; Fast ion driven instabilities; Global Alfven Eigenmodes; Hybrid simulations
- OSTI Identifier:
- 1814953
- DOI:
- https://doi.org/10.11578/1814953
Citation Formats
Lestz, J B, Belova, E V, and Gorelenkov, N N. Hybrid simulations of sub-cyclotron compressional and global Alfven Eigenmode stability in spherical tokamaks. United States: N. p., 2021.
Web. doi:10.11578/1814953.
Lestz, J B, Belova, E V, & Gorelenkov, N N. Hybrid simulations of sub-cyclotron compressional and global Alfven Eigenmode stability in spherical tokamaks. United States. doi:https://doi.org/10.11578/1814953
Lestz, J B, Belova, E V, and Gorelenkov, N N. 2021.
"Hybrid simulations of sub-cyclotron compressional and global Alfven Eigenmode stability in spherical tokamaks". United States. doi:https://doi.org/10.11578/1814953. https://www.osti.gov/servlets/purl/1814953. Pub date:Thu Apr 01 00:00:00 EDT 2021
@article{osti_1814953,
title = {Hybrid simulations of sub-cyclotron compressional and global Alfven Eigenmode stability in spherical tokamaks},
author = {Lestz, J B and Belova, E V and Gorelenkov, N N},
abstractNote = {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.},
doi = {10.11578/1814953},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Thu Apr 01 00:00:00 EDT 2021},
month = {Thu Apr 01 00:00:00 EDT 2021}
}