On the Theory of ICR Heating by the Magnetic Beach Method
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ODS OF PLASMA HEATING
On the Theory of ICR Heating by the Magnetic Beach Method A. V. Timofeeva,* and E. D. Gospodchikovb,** a
b
National Research Center “Kurchatov Institute”, Moscow, 123098 Russia Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, 603950 Russia *e-mail: [email protected] **e-mail: [email protected] Received March 12, 2019; revised March 25, 2019; accepted March 25, 2019
Abstract—Current interest in ion cyclotron resonance (ICR) heating by the magnetic beach method is motivated by the experiments carried out with the VASIMR plasma thruster. The method implies excitation of Alfvén eigenmodes of a plasma column (or a plasma jet in the case of plasma thruster) outside the ICR zone. In the present work, it is shown that, under conditions typical of, e.g., plasma thrusters, most of the electromagnetic energy deposited in the plasma can be transferred to the so-called Alfvén continuum, rather than to the discrete spectrum of Alfvén eigenmodes. Due to the phenomenon of the Alfvén resonance, continuousspectrum oscillations transform into short-wavelength lower hybrid oscillations, which differ from the Alfvén oscillations in their polarization. These oscillations efficiently interact with electrons, thereby heating them. DOI: 10.1134/S1063780X19080105
1. INTRODUCTION Ion cyclotron resonance (ICR) heating by the socalled “magnetic beach” method applies for plasma in a magnetic field the magnitude of which varies along magnetic field lines [1]. The left-handed polarized Alfvén oscillations (AOs), the electric field of which rotates in the direction of ion gyration, should be driven in the region where the magnetic field exceeds the resonance value. As AOs approach the ICR region, their wavenumber rapidly increases, while the phase and group velocities tend to zero, i.e., the oscillations “stop.” Therefore, independently of the intensity of dissipative processes, the oscillations are eventually absorbed, because they are not reflected from the ICR zone. The magnetic configuration required for the magnetic beach method takes place in a number of space plasma thrusters. However, ICR heating is employed to create thrust only in the VASIMR thruster [2] and the method of plasma heating used in this device cannot be classified as the magnetic beach method. Indeed, electromagnetic oscillations in VASIMR are driven essentially in the ICR zone, whereas in the magnetic beach method, they should be driven far from this zone at distances substantially longer that oscillation wavelength. Although heating by the magnetic beach method has not find practical application as of yet, the possibility of efficient ion heating provided by this method justifies its investigation. In study [3], which, apparently, had a great impact on the concept of the VASIMR thruster, heating by the magnetic beach method was analyzed in the one-
dimensional approximation, namely, both the external magnetic field and the field of electromagnetic oscillations were assumed to vary only along magnetic field lines.
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