Theory of Electromagnetic Surface Waves in Plasma with Smooth Boundaries
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LLATIONS AND WAVES IN PLASMA
Theory of Electromagnetic Surface Waves in Plasma with Smooth Boundaries M. V. Kuzelev* Faculty of Physics, Moscow State University, Moscow, 119991 Russia *e-mail: [email protected] Received April 12, 2017; in final form, August 30, 2017
Abstract—A theory of nonpotential surface waves in plasma with smooth boundaries is developed. The complex frequencies of surface waves for plasma systems of different geometries and different profiles of the plasma density are calculated. Expressions for the rates of collisionless damping of surface waves due to their resonance interaction with local plasma waves of continuous spectrum are obtained. The influence of collisions in plasma is also considered. DOI: 10.1134/S1063780X18050094
1. The existing relativistic plasma microwave sources are based on Cherenkov excitation of a surface wave of annular plasma in a circular metal waveguide [1, 2]. Therefore, investigation of surface waves in plasma waveguides is an important problem of plasma microwave electronics. The dispersion and field structure of the fundamental surface wave (the E01 mode) of a thin-walled annular plasma in a waveguide in the presence of a strong longitudinal external magnetic field is determined by the plasma density per unit length and is practically independent of the transverse distribution of the plasma density inside the plasma wall [3]. The situation is different when an external magnetic field is either weak or absent at all. It was demonstrated in [4–7] that, in this case, the spatial distribution of the charged particle density in plasma plays an important role and the structure of the plasma boundary, which is not sharp under actual experimental conditions, becomes critically important. Since the possibility of implementation of plasma microwave sources operating without an external magnetic field or with a weak magnetic field is currently under consideration [8, 9], investigation of surface waves in systems with smooth (smeared) plasma boundaries becomes challenging. The above-cited works [4–7] are clearly insufficient, because the field in them was assumed to be potential. The surface plasma waves used in plasma microwave electronics are essentially nonpotential, which is of fundamental importance for efficient emission of such waves from a plasma waveguide, where they are driven by an electron beam. This work is aimed at developing a theory of nonpotential surface waves in plasma with smooth boundaries. The only significant simplifying assumption made in the developed theory is that the plasma inho-
mogeneity scale length Δ (the thickness of the boundary) is supposed to be much smaller than the surface wave wavelength. In addition, cold electron plasma is considered. Here, we mainly analyze a model problem on surface plasma waves in plane geometry. Although plane geometry is far from the configuration important for plasma microwave electronics, its analysis allows one to more clearly describe the essence of the proposed approach, the meaning of the adopted assumptions
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