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, or blobs, that are present in the edge plasma and the scrape-off layer. The difference in the permittivity between the surrounding plasma and interior of a filament leads to reflection, refraction, and diffraction of the waves. This, in turn, can affect the power flow into the core of the plasma and reduce the efficiency of heating and/or current generation. The scattering of RF waves -- lower hybrid, helicon, and ion cyclotron waves -- by a single cylindrical filament, embedded in a background plasma, is studied using a full-wave analytical theory developed previously [A. K. Ram and K. Hizanidis, Phys. Plasmas \textbf{23}, 022504-1--022504-17 (2016)]. The theory assumes that the plasma in and around a filament is homogeneous and cold. A detailed scattering analysis reveals a variety of common features that exist among the three distinctly different RF waves. These common attributes can be inferred intuitively based on an examination of the cold plasma dispersion relation. The physical intuition is a useful step to understanding experimental observations on scattering, as well as results from simulations that include general forms of edge plasma turbulence. 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 [J. H. Poynting, Phil. Mag. \textbf{9}, 393-406 (1905)]. 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 an RF antenna, a filament will couple power to the other mode -- a consequence of electromagnetic boundary conditions. This facet of scattering results in the radiation force having more diversified attributes than those in 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 the filament toward the RF source or push it away. For slow lower hybrid waves, filaments are pulled in regardless of whether they are more or less dense compared to the ambient plasma. For fast helicon and ion cyclotron waves, the direction of the force depends on the plasma and wave parameters; in particular, on the ambient density. For all three waves, the radiation force is large enough to impact the motion of a filament and could be measured experimentally. This suggests a possibility of modifying the edge turbulence using RF waves.