Radiation pressure by electron cyclotron waves on density fluctuations

Abstract

The presence of turbulence in the form of large density fluctuations and coherent filamentary structures in the edge region of fusion plasmas has been well documented. Radio frequency waves, launched from structures near the wall of a tokamak, have to propagate through this turbulent plasma before reaching the core. These density fluctuations can reflect, refract, and diffract the electromagnetic waves, thereby modifying the flow of energy and momentum to the core plasma. Conversely, the radiation pressure of the radio frequency waves can modify the turbulence, whether it is in the edge region or in the core. This article examines some consequences of the radiation force induced by electron cyclotron waves in plasmas. The effect of waves on two different representations of density fluctuations are studied. In the first representation, suitable for both edge and core plasmas, it is assumed that a planar interface separates two different density regimes. The physics basis for the radiation force on an interface separating two different scalar dielectric media was first elucidated by Poynting in 1905 [J. H. Poynting, The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science9, 393-406 (1905)]. Poynting’s results are explained within the context of Snell’s law and Fresnel equations, and, subsequently, extended to magnetized plasmas. The analysis shows that electron cyclotron waves lead to peaking of the density profile – the interface is pushed towards the region of higher density. The planar interface approximation is the basis of Kirchhoff theory [P. Beckmann and A. Spizzichino,The Scattering of Electromagnetic Waves from Rough Surfaces(Artech, Massachusetts, 1987) Chapter 3] used to study wave scattering by turbulent media. In the second representation, appropriate for coherent structures in edge plasmas, the radiation force on a cylindrical filament embedded in a background plasma is determined using the Maxwell stress tensor. A detailed study reveals that the radiation force has a different effect on filaments – those with densities higher than the background density are pulled in towards the source launching the waves, while the lower density filaments are pushed away. The reaction on a filament is large enough to be observed experimentally.