Radiation force exerted by radio frequency waves on density filaments

The scattering of radio frequency (RF) waves by filaments in the edge region of a tokamak plasma has been well studied. In the cold plasma approximation, valid in the scrape-off layer, there exist two distinct modes, either one of which can be a propagating or an evanescent wave. The scattering of an incident wave leads to spatial fragmentation of RF power and coupling of power between the two modes. The RF waves also exert a radiation force on the filament. The full-wave analytical theory formulated for the scattering process [1], in conjunction with the Maxwell stress tensor, is used to determine the radiation pressure. We find that the direction of the force depends on the wave frequency and its polarization, and on the electron densities inside and outside the filament. The force can either pull the filament towards the RF source or push it away. The lower hybrid wave pulls in filaments with higher density (relative to the ambient plasma) and pushes away filaments with lower density. The direction of the radiation force due to ion cyclotron waves varies with the density of the ambient plasma. For a variety of RF frequencies, a detailed analysis of the force will be presented.

[1] A. K. Ram and K. Hizanidis, Phys. Plasmas 23, 022504 (2016).