On estimating the role of diffraction in electron cyclotron absorption at the periphery of the plasma column
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MA HEATING
On Estimating the Role of Diffraction in Electron Cyclotron Absorption at the Periphery of the Plasma Column A. A. Balakin, M. A. Balakina, and A. G. Shalashov Institute of Applied Physics, Russian Academy of Sciences, ul. Ul’yanova 46, Nizhni Novgorod, 603950 Russia Received June 29, 2006; in final form, October 20, 2006
Abstract—The propagation and absorption of microwave radiation at the periphery of a tokamak plasma under ECR conditions are considered. For microwaves propagating in a quasi-transverse direction, the range of plasma parameters is determined in which the effective plasma permittivity can be approximated by a piecewise linear function. With this approximation, it is possible to obtain analytic solutions to the wave equation and to use them to estimate the width of the power deposition region for different modes of microwave launching. A detailed analysis is given of tangential launching—the propagation of microwaves along a tangent to the resonance magnetic surface at which they begin to be absorbed under ECR conditions. Using the ITER tokamak as an example, it is shown that this launching method is most efficient in providing the narrowest power deposition profile at the plasma periphery. The results obtained are of interest for the problems of suppressing tearing-mode instabilities by localized ECR heating. PACS numbers: 52.25.Os, 52.35.Hr, 52.35.Py DOI: 10.1134/S1063780X07080077
1. INTRODUCTION The problem of stabilizing tearing modes with low numbers m is familiar in the theory of plasma confinement in tokamaks. It is successfully solved by the methods of local microwave heating or current drive under ECR conditions (see, e.g., [1–4] and references therein). In order for the instability to be efficiently suppressed, the power deposition region should be highly localized in the vicinity of a rational magnetic surface, so demanding requirements are imposed, among other things, on the width of this region. In [5], a method for launching a microwave beam was considered with which the power deposition region can be made substantially narrower. This method, which was called “tangential launching” in that paper, consists in choosing the launching parameters so that, in the ECR absorption region, the ray propagates exactly along a tangent to the magnetic surface, the point of tangency being the point where microwaves begin to be resonantly absorbed. In the case of tangential launching, the width of the heated region can be minimized because the wave front of the microwave beam is perpendicular to the magnetic surface (see the right part of Fig. 1), so the extent to which the power deposition region is localized is determined by the aperture width of the launched beam. In contrast, the finite beam width is unimportant when the beam crosses the resonant magnetic surfaces in the ECR interaction region (see the left part of Fig. 1); this launching method, which is the most widespread and simplest to implement, will be referred to as “intersecting launching.”
Tangential launching is optimal whe
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