Scattering of electromagnetic waves by a plasma sphere embedded in a magnetized plasma

Abstract

The scattering of electromagnetic waves by a spherical plasma blob, surrounded by a uniform magnetized plasma, is formulated using a full-wave theory. The theoretical approach followed in this formulation is similar to that for Mie scattering of electromagnetic waves by dielectric spheres. The plasma, both inside and outside the sphere, is assumed to be homogeneous and cold so that there are only two independent wave modes. The anisotropy induced by the magnetic field is such that the propagation characteristics and the polarization of the wave modes depend on the polar angle with respect to the direction of the magnetic field. Consequently, an incident plasma wave not only is scattered by the plasma sphere, but also couples power to a different plasma mode. The formulation is applied to the scattering of radio frequency (RF) waves by spherical blobs in toroidal fusion plasmas. In the edge region of such plasmas, density fluctuations and blob structures are commonly observed. These can affect the spectrum and direction of propagation of RF waves used to deliver energy and momentum to the core of the plasma. The scattering of the RF wave depends on the frequency of the wave and its angle of propagation with respect to the direction of the magnetic field. The effect of blobs on short wavelength electron cyclotron waves and long wavelength ion cyclotron waves are discussed for some relevant experimental parameters. The short wavelengths are comparable to or smaller than the radius of the blob, while the longer wavelengths are large compared to the blob radius.

Keywords:

• scattering of electromagnetic waves in magnetized plasmas
• Mie scattering in magnetized plasmas
• radio frequency plasma waves
• coherent edge density fluctuations

Acknowledgements

We are grateful to Dr Richard Garner for numerous informative and suggestive discussions. This work was supported by US DOE Grant number DE-FG02-91ER-54109, the European Fusion Program (Association EURATOM Hellenic Republic), and the Hellenic General Secretariat of Research and Technology.