Scattering of ECRF waves by edge density blobs and fluctuations in tokamak plasmas

¹ Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, USA
² National Technical University of Athens (part of HELLAS), School of Electrical and Computer Engineering, Association EURATOM-Hellenic Republic, GR-15773, Greece

^a e-mail: abhay@mit.edu

Abstract

There are two basic approaches to studying the effects of density blobs and edge fluctuations on the coupling of electron cyclotron (EC) radio frequency waves to the core of tokamak plasmas. The first is the geometric optics approach in which the effect of fluctuations is to change the refractive properties of the EC beam or rays. There are two consequences of refractive scattering – diffusion in real space leading to a spatial deflection of the rays and diffusion in wave vector space leading to the broadening of the launched spectrum. The geometric optics approach is limited to small density fluctuations of 10% or less. The second approach to studying the effect of blobs on EC fields is using the full wave approach. This approach extends the range of validity well beyond that of geometric optics; however, it is theoretically and computationally much more challenging. In this paper a full wave model for scattering of radio frequency waves is developed. Results from the model demonstrate diffractive scattering of EC waves by density blobs and the enhancement of the electric fields near the surface of the blob.