Effect of flux surface curvature on the linear coupling of electron cyclotron waves in tokamak plasmas

Abstract

Specific features of the linear interaction of ordinary and extraordinary electromagnetic waves in the electron cyclotron frequency range in a nonuniform plasma confined in a toroidal magnetic trap are con sidered. Reduced wave equations taking into account the curvature of the cut off surfaces in toroidal geom etry are formulated. Using these equations, the distributions of the wave fields in the coupling region are ana lyzed. A method for calculating quasi optical beams passed through the region of linear wave interaction is proposed.

Impact of poloidal curvature on linear mode conversion of quasi-optical wave beams in tokamak plasmas

“…The necessary condition for the applicability of the approximation of plane flat cutoff surfaces is the condition that the beam is insignificantly contributed by the modes with λ n λ cr that do not correspond to the plane cutoff approximation. There are several ways to satisfy this condition; a detailed study may be found in [17]. Here we consider the case most relevant for fusion applications in which the original incident beam consists of localized modes with λ n < λ cr that fit the 'parabolic' well, and contribution of other modes to the spectrum is negligibly small.…”

The effect of spectrum broadening on the O–X mode coupling due scattering of a microwave beam on plasma density fluctuations

“…As noted in [21][22][23], the curvatures of the magnetic field lines and toroidal magnetic surfaces can contribute to the coupling efficiency in the same way as the fluctuations. Their influence can also be roughly estimated through the redefined parameter K y in (18) as…”

“…On the other hand, such an assumption is certainly not satisfied for toroidal devices except for the special case of the equatorial launch of microwave radiation. For a regular toroidal plasma, it is known that the non-one-dimensional geometry of the coupling region leads to qualitatively new physical effects [16][17][18], which under certain conditions can substantially influence the O-X mode coupling efficiency [19][20][21][22][23].…”

“…(21) induced by the density fluctuations with the effective phase modulation (25) associated with the flux surface curvature, and take into account the condition(22), we obtain a modified criteria for the length of the beam trace, in which fluctuations become the dominant effect…”