Excitation of helicons by current antennas
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LLATIONS AND WAVES IN PLASMA
Excitation of Helicons by Current Antennas E. D. Gospodchikova* and A. V. Timofeevb** a
Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, 603950 Russia b National Research Center “Kurchatov Institute,” Moscow, 123182 Russia *e-mail: [email protected] **e-mail: [email protected] Received October 18, 2016; in final form, December 20, 2016
Abstract—Depending on the angle θ between the wave vector and the magnetic field, helicons are conventionally divided into two branches: proper helicons (H mode), propagating at small θ , and Trivelpiece– Gould waves (TG mode), propagating at large θ . The latter are close to potential waves and have a significant electric component along the external magnetic field. It is believed that it is these waves that provide electron heating in helicon discharges. There is also commonly believed that current antennas, widely used to ignite helicon discharges, excite essentially nonpotential Н modes, which then transform into TG modes due to plasma inhomogeneity. In this work, it is demonstrated that electromagnetic energy can also be efficiently introduced in plasma by means of TG modes. DOI: 10.1134/S1063780X17060071
1. INTRODUCTION Helicon discharge is a unique method for plasma generation. The properties of helicon discharges have been studied in thousands of works (see, e.g., review [1]). However, some details of the process of helicon excitation by an RF antenna still remain unclear, because the helicon branch of plasma oscillations in a magnetic field possesses some specific features. The frequency of these oscillations is a nonmonotonic function of the wavenumber k ⊥ across the external magnetic field [1]. As a results, the same frequency may correspond to two values of k ⊥ . Oscillations with smaller k ⊥ (proper helicons, also called H mode) are essentially nonpotential and have a small longitudinal electric component. The shorter wavelength oscillations (the Trivelpiece–Gould (TG) mode) have a significant longitudinal electric field. Therefore, it is the TG mode that can efficiently heat electrons. Nevertheless, it is commonly believed that current antennas used to ignite helicon discharges excite only the H mode, while the TG mode arises as a secondary effect as a result of energy transfer from the H mode (see, e.g., [2–5]). However, as will be shown below, direct excitation of the TG mode can be an efficient channel for input of electromagnetic energy into plasma. Typically, due to their weak attenuation, H modes take the form of standing radial oscillations. In this case, it is difficult to determine what fraction of energy is accepted by TG modes directly from the RF antenna and what from H modes as a result of transformation
into TG modes at the plasma boundary.1 (Here, we mean H modes reflected from the plasma center and incident onto the plasma boundary from the interior of the plasma column.) In this work, in order to separate these processes, we consider a semi-infinite plasma the density of which tends to a
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