Scattering of ECRF waves by edge density fluctuations and blobs

Abhay K Ram,Kyriakos Hizanidis

doi:10.1051/epjconf/20158701004

Abstract

The scattering of electron cyclotron waves by density blobs embedded in the edge region of a fusion plasma is studied using a full-wave model. The full-wave theory is a generalization of the usual approach of geometric optics ray scattering by blobs. While the latter allows for only refraction of waves, the former, more general formulation, includes refraction, reflection, and diffraction of waves. Furthermore, the geometric optics, ray tracing, model is limited to blob densities that are slightly different from the background plasma density. Observations in tokamak experiments show that the fluctuating density differs from the background plasma density by 20% or more. Thus, the geometric optics model is not a physically realistic model of scattering of electron cyclotron waves by plasma blobs. The differences between the ray tracing approach and the full-wave approach to scattering are illustrated in this paper.

Highlights

Previous studies on the scattering of Radio frequency (RF) waves by blobs have been based on the geometric optics ray tracing approximation [7, 8]
Experimental observations point to a much larger range of density fluctuations [9], so that the full-wave model is the most appropriate paradigm for wave scattering off blobs
The anisotropy induced by the magnetic field is such that the propagation characteristics and the polarization of the RF waves depend on the polar angle with respect to the direction of the magnetic field

Summary

Introduction

Previous studies on the scattering of RF waves by blobs have been based on the geometric optics ray tracing approximation [7, 8] These studies are of limited applicability as effects like reflection and diffraction are ignored. In these models, the density inside the blob has to be within a few percent of the background density. The axis of the cylindrical blobs is aligned along the magnetic field line The plasma, both inside and outside the blobs, is assumed to be cold and homogeneous with arbitrary densities in either region; we are not limited to small density fluctuations. A combination of the incident wave and the scattered fields has to match with the electromagnetic fields inside the blob so as to properly satisfy the boundary conditions. Observations show that the scale lengths of the blobs typically range from a fraction of a centimeter to several centimeters [9]

Spherical coordinate system

Cylindrical coordinate system

Electromagnetic fields inside and outside the blob

Results and discussion