Generation of ion-acoustic and magnetoacoustic waves in an RF helicon discharge
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MA OSCILLATIONS AND WAVES
Generation of Ion-Acoustic and Magnetoacoustic Waves in an RF Helicon Discharge A. S. Belov and G. A. Markov Lobachevsky State University, pr. Gagarina 23, Nizhni Novgorod, 630950 Russia Received October 17, 2005; in final form, December 7, 2005
Abstract—A study is made of the generation of ion-acoustic and magnetoacoustic waves in a discharge excited in an external magnetic field by an electromagnetic wave in the whistler frequency range (ωLH ω ωHe, where ωLH = ω He ω Hi and ωHe and ωHi are the electron and ion gyrofrequencies, respectively). The excitation of acoustic waves is attributed to the decay of a high-frequency hybrid mode forming a plasma waveguide into low-frequency acoustic waves and new high-frequency waves that satisfy both the decay conditions and the waveguide dispersion relations. The excitation of acoustic waves is resonant in character because the conditions for the generation of waveguide modes and for the occurrence of the corresponding nonlinear wave processes should be satisfied simultaneously. An unexpected effect is the generation of magnetoacoustic waves by whistlers. A diagnostic technique is proposed that allows one to determine the thermal electron velocity by analyzing decay conditions and dispersion relations for waves in the discharge channel. PACS numbers: 52.35.Mw, 52.35.Fp, 52.80.Pi DOI: 10.1134/S1063780X06090066
1. INTRODUCTION Complicated nonlinear processes in the magnetized plasma of a discharge excited by high-frequency (HF) waves in the whistler frequency range have attracted considerable interest over more than two decades [1]. Such a discharge is initiated by the near field of an antenna. However, when the plasma density becomes higher than the critical one (ωpe > ω), the structure of both the near and far antenna fields in the whistler frequency range changes radically and conical resonance surfaces emerge from the antenna rings. The resonant increase in the HF field on these surfaces is caused by the excitation of quasistatic plasma waves of different wavelength (from wavelengths as short as the diameter of the ring wire to those exceeding the source dimensions). Ionization occurs mainly near the resonance surface, where the amplitude of the HF field is maximum. Therefore, the energy of the HF source is primarily spent on the generation of plasma waves. Near the focus of the resonance cone, a plasma column (waveguide) forms that guides the source radiation along the external magnetic field [2–4]. The column is detached from the wall of the discharge tube and is localized on the source axis. The near (quasistatic) field of the quadrupole source rapidly decreases with increasing distance from the antenna rings, so the wave field beyond the focus of the resonance cone is considerably higher than the near field of the antenna. This is evidenced by the emergence of a narrow plasma column from the focal region. This column serves as a waveguide along which the energy needed to create and
sustain it is transferred from the HF source. Shortwavel
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