Near field of a loop antenna operating in plasma in the whistler frequency range
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A OSCILLATIONS AND WAVES
Near Field of a Loop Antenna Operating in Plasma in the Whistler Frequency Range S. V. Korobkova, M. E. Gushchina, A. V. Kostrova, A. V. Strikovskiœa, and C. Krafftb a Institute
of Applied Physics, Russian Academy of Sciences, ul. Ul’yanova 46, Nizhni Novgorod, 603950 Russia de Physique des Gaz et des Plasmas, Université Paris-Sud, 91405, Orsay, France
b Laboratoire
Received January 11, 2006; in final form, June 8, 2006
Abstract—The structure of the RF magnetic field in the vicinity of a loop antenna operating in the whistler frequency range has been studied experimentally and theoretically. The experiments were performed over a wide frequency range at different values of the plasma density, electron temperature, and ambient magnetic field strength. It is shown that, when a loop antenna is smaller than the wavelength of a quasi-longitudinal whistler, the structure of the magnetic field of such an antenna is nearly the same as that of the field of a currentcarrying loop in vacuum; otherwise, the RF field is localized near the antenna wire. The results of numerical calculations agree with the measured field distributions. The antenna field is calculated by expanding it in the eigenmodes of a magnetized plasma with allowance for not only propagating but also nonpropagating (exponentially decaying) waves, which make the main contribution to the near field. An analytic estimate of the depth to which the RF magnetic field of a loop antenna penetrates into the plasma is obtained. PACS numbers: 52.35.Hr DOI: 10.1134/S1063780X07020031
1. INTRODUCTION Interest in studying the fields and impedance characteristics of loop antennas in plasma stems from their widespread use in space and laboratory experiments. Loop antennas (or magnetic loops) onboard spacecraft are used to transmit and receive radio waves in various frequency ranges and to detect natural emissions in the ionosphere and magnetosphere of the Earth [1, 2]. In some ionospheric experiments, satellite-borne loop antennas as large as 20 m in diameter were used [3, 4]. Magnetic loops are also employed as emitting and diagnostic antennas in studying wave processes in laboratory plasmas [5]. Of particular interest are loop antennas operating in the whistler frequency range. The properties of such antennas were investigated in many theoretical works: their impedance characteristics were considered in [6, 7], the radiation fields were studied in [8], and the parameters of antennas to be used in space experiments were computed in [9]. The structure of the magnetic field near a small (smaller than the wavelength of the excited whistlers) loop antenna was studied in a quasistatic approximation in [7, 10]. Model experiments and theoretical calculations showed that the impedance of a small loop antenna placed in a magnetized plasma was nearly the same as that of a current-carrying loop in vacuum [11]. Meanwhile, it is of considerable interest to study the distribution of the RF field near a loop antenna that cannot be regarded as small. The structur
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