Interaction of radio frequency waves with cylindrical density filaments -- scattering and radiation pressure

Abstract

The propagation of radio frequency (RF) waves in tokamaks can be affected by filamentary structures present in the edge plasma. The waves are reflected, refracted, and diffracted, leading to a decrease in efficiency of heating and/or current generation. The scattering of RF waves (lower hybrid, helicon, and ion cyclotron waves) by a single cylindrical filament is studied using a full-wave analytical theory. Analysis reveals a variety of common scattering features among the three different RF waves which can be inferred by examining the cold plasma dispersion relation. The physical intuition is useful in understanding experimental observations and results from complex numerical simulations. While a filament can affect the propagation of RF waves, the radiation force exerted by the waves can influence the filament. The force on a filament is determined using the Maxwell stress tensor. In 1905, Poynting was the first to evaluate and measure the radiation force on an interface separating two different dielectric media. For ordinary light propagating in vacuum and incident on a glass surface, Poynting noted that the surface is "pulled" towards the vacuum. In a magnetized cold plasma, there are two independent wave modes. Even if only one of these modes is excited by a RF antenna, a filament will couple power to the other mode, a consequence of boundary conditions. This facet of scattering has consequences on the radiation force that go beyond Poynting's seminal contribution. The direction of the force depends on the polarization of the incident wave and on the mode structure of the waves inside and in the vicinity of a filament. It can either pull a filament towards the RF source or push it away. For all three waves, the force is large enough to impact the motion of a filament and could be measured experimentally. It may be possible to modify edge turbulence using RF waves.