Description
In magnetic fusion devices, ITG turbulence can be suppressed by Shear Alfvén Wave eigenmodes. These interact non-linearly to produce low-frequency zonal flows, which shear apart turbulent eddies and reduce transport in the core. Secondary instability calculations have been undertaken for this process, but quantitative predictions of the transport suppression require a theoretical understanding of the fully-developed turbulent system.
As a first step, coherently-forced weak reduced-magnetohydrodynamic turbulence is studied analytically and numerically in a homogeneous plasma. Zonal-flow analogues - modes that are constant along the magnetic field and at zero frequency - are produced at the second level of non-linearity, with saturated amplitudes related analytically to those of the pump waves. Contrary to previous beliefs, this indicates that zonal flows can be generated by Shear Alfvén Waves without compressibility, kinetic effects or complex geometry. This new understanding may help to quantify the transport suppression by Alfvén Eigenmodes in future fusion devices.