"On Energetic-Particle Excitations of Low-Frequency Alfvén Eigenmodes in Toroidal Plasmas"

 

 

 

It is well-known that, within the ideal magnetohydrodynamic (MHD) description, there exists three branches of low-frequency continuous spectra in toroidal plasma devices, such as tokamaks. The corresponding accumulation frequencies, in the case of finite magnetic shear considered here, are the beta-induced Alfvén eigenmode (BAE) frequency, the ion-sound wave (ISW) frequency, and the zero frequency.  To form localized discrete eigenmodes, the plasma must be ideal MHD unstable.  The zero-frequency branch then corresponds to the ideal MHD unstable discrete mode; while the BAE and ISW discrete eigenmode frequencies could be significantly shifted away from the accumulation frequencies.  Energetic-particle (EP) effects can be analyzed and understood based on the generalized fishbone linear dispersion relation (GFLDR).  In particular, for an ideal MHD stable plasma, EPs could play the roles of both localization and destabilization.  It can then be shown that EPs preferentially excite the BAE branch over the ISW branch. The zero-frequency branch, meanwhile, becomes the well-known fishbone dispersion relation; giving rise to energetic-particle modes (EPM).  Modifications due to kinetic thermal-ion description will also be discussed.

 

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