Mechanism of the energy transfer across the magnetic field by Alfvén waves in toroidal plasmas

 

M. H. Tyshchenko, Ya. I. Kolesnichenko, Yu. V. Yakovenko

 

Institute for Nuclear Research, Kyiv, Ukraine

 

Destabilized magnetohydrodynamic (MHD) eigenmodes can transfer the energy and momentum from the region where particles (e.g., fast ions) drive the plasma instability to another region, where the destabilized waves are damped. This phenomenon named “spatial channeling” (SC) was predicted in [1,2]. The classical Alfven waves are known to propagate along the magnetic field, so that their transverse group velocity in a homogeneous plasmas in a homogeneous magnetic field vanishes. Therefore, the physical mechanism of the SC produced by GAE, EAE and other Alfvén eigenmodes was not clear. Nevertheless, the conclusions of [1] and [3] were based on the assumption that Alfvén modes are capable of transferring energy across the magnetic field, although no analysis of the mechanism of this transfer was carried out.

This work is aimed at understanding how Alfvén modes transfer energy across the magnetic field. It is found that, in contrast to the classical Alfvén waves in infinite plasmas, the Alfvén waves in toroidal systems produce plasma compression due to coupling with fast magnetoacoustic waves, which provides the energy transfer. The radial group velocities of the traveling waves constituting the Global Alfvén Eigenmodes (GAE) and Toroidicity-induced Alfvén Eigenmodes (TAE) are calculated. It is shown that equations for Alfvén eigenmodes derived in the approximation of vanishing wave field along the equilibrium magnetic field reproduce the longitudinal magnetic field of the wave and lead to correct transverse energy flux. The obtained results explain how Alfvén eigenmodes can provide the spatial energy channeling.

 

1.       Ya. I. Kolesnichenko, Yu. V. Yakovenko, and V. V. Lutsenko, Phys. Rev. Lett. 104, 075001 (2010)

2.       Ya. I. Kolesnichenko, Yu. V. Yakovenko, V. V. Lutsenko, R. B. White, and A. Weller, et al., Nucl. Fusion 50, 084011 (2010)

3.       Ya. I. Kolesnichenko and A. Tykhyy, Phys. Lett. A 382, 2689 (2018).