Investigation of an ion-ion hybrid Alfvén wave resonator

Abstract

A theoretical and experimental investigation is made of a wave resonator based on the concept of wave reflection along the confinement magnetic field at a spatial location where the wave frequency matches the local value of the ion-ion hybrid frequency. Such a situation can be realized by shear Alfv en waves in a magnetized plasma with two ion species because this mode has zero parallel group velocity and experiences a cut-off at the ion-ion hybrid frequency. Since the ion-ion hybrid frequency is proportional to the magnetic field, it is expected that a magnetic well configuration in a two-ion plasma can result in an Alfv en wave resonator. Such a concept has been proposed in various space plasma studies and could have relevance to mirror and tokamak fusion devices. This study demonstrates such a resonator in a controlled laboratory experiment using a H þ -He þ mixture. The resonator response is investigated by launching monochromatic waves and impulses from a magnetic loop antenna. The observed frequency spectra are found to agree with predictions of a theoretical model of trapped eigenmodes. V C 2013 American Institute of Physics.

Propagation of shear Alvén waves in two-ion species plasmas confined by a nonuniform magnetic field

“…The parameters correspond to those of the recent resonator experiment in LAPD. 13,14 A hydrogen-helium plasma is considered with concentration ratio n H =n e ¼ 0:45 and satisfying charge quasi-neutrality. The electron density is n e ¼ 1:3 Â 10 12 cm À3 , the magnetic field, B ¼ 750 G, giving an Alfv en speed of v A ¼ 8:85 Â 10 7 cm=s.…”

“…15 The section also presents expressions for the group velocity that are used in analyzing the results for different plasma devices. Section III examines group velocity contours and ray trajectories relevant to a recent experiment performed in LAPD 13,14 and estimates the quality factors expected for the resonator due to radial spreading of the wave energy. Section IV examines the ray trajectories of waves launched in the Enormous Toroidal Plasma Device (ETPD) 21 and discusses the effects of field-line curvature on the ray.…”

“…Specifically, ray tracing studies are performed in a geometry relevant to LAPD to assess the importance of radial spreading in determining the low quality-factors (Q) measured. 13,14 Ray tracing studies are also performed in toroidal geometries to understand more clearly whether a resonator could exist within a burning plasma. Because of the large ion temperatures that would exist in such a plasma, finite Larmor radius (FLR) effects for the ions must be included in the dispersion relation.…”

“…The quality factors (Q) of the resonances were observed to be between 11 and 18, a value much smaller than expected from cold plasma theory considerations. 14 For a specific magnetic well geometry, ray tracing studies are useful in determining the degree to which the wave energy can be trapped. Rauch and Roux performed ray tracing studies in a dipole magnetic geometry for a cold plasma 15 in an effort to explain GEOS 1 and 2 spacecraft measurements.…”

Localization of Ultra-Low Frequency Waves in Multi-Ion Plasmas of the Planetary Magnetosphere

“…Because the IIH waves are partially reflected at the Buchsbaum resonance location where ω = ω bb , the waves are possibly localized near the magnetic equator between two Buchsbaum resonance locations (Klimushkin et al 2010;Vincena et al 2011). The localization of modeconverted IIH waves is referred to as IIH Alfven resonance, and is experimentally detected in laboratory plasmas (Vincena et al 2011(Vincena et al , 2013Farmer & Morales 2014).…”