Control of Gradient-Driven Instabilities Using Shear Alfvén Beat Waves

Auerbach, Daniel J.; Carter, Troy; Vincena, S.; Popovich, P.

doi:10.1103/physrevlett.105.135005

Cited by 17 publications

(13 citation statements)

References 33 publications

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“…An experiment was performed in which two Alfvén waves were broadcast along a filamentary density depletion on which a coherent unstable mode was present. 119 The beating Alfvén waves were found to resonantly drive the instability when the beat frequency was tuned to match the unstable mode frequency. More interestingly, it was found that when the beat frequency was tuned to resonantly drive nearby (in frequency) damped modes, the original unstable mode was suppressed.…”

Section: -15mentioning

confidence: 99%

The many faces of shear Alfvén waves

et al. 2011

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One of the fundamental waves in magnetized plasmas is the shear Alfvén wave. This wave is responsible for rearranging current systems and, in fact all low frequency currents in magnetized plasmas are shear waves. It has become apparent that Alfvén waves are important in a wide variety of physical environments. Shear waves of various forms have been a topic of experimental research for more than fifteen years in the large plasma device (LAPD) at UCLA. The waves were first studied in both the kinetic and inertial regimes when excited by fluctuating currents with transverse dimension on the order of the collisionless skin depth. Theory and experiment on wave propagation in these regimes is presented, and the morphology of the wave is illustrated to be dependent on the generation mechanism. Three-dimensional currents associated with the waves have been mapped. The ion motion, which closes the current across the magnetic field, has been studied using laser induced fluorescence. The wave propagation in inhomogeneous magnetic fields and density gradients is presented as well as effects of collisions and reflections from boundaries. Reflections may result in Alfvénic field line resonances and in the right conditions maser action. The waves occur spontaneously on temperature and density gradients as hybrids with drift waves. These have been seen to affect cross-field heat and plasma transport. Although the waves are easily launched with antennas, they may also be generated by secondary processes, such as Cherenkov radiation. This is the case when intense shear Alfvén waves in a background magnetoplasma are produced by an exploding laser-produced plasma. Time varying magnetic flux ropes can be considered to be low frequency shear waves. Studies of the interaction of multiple ropes and the link between magnetic field line reconnection and rope dynamics are revealed. This manuscript gives us an overview of the major results from these experiments and provides a modern prospective for the earlier studies of shear Alfvén waves. V

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Section: -15mentioning

confidence: 99%

The many faces of shear Alfvén waves

et al. 2011

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“…Extensive prior work has focused on the properties of linear Alfvén waves [5,[28][29][30]. Studies of the nonlinear properties of Alfvén waves have also been performed on the LAPD; in these experiments, two launched Alfvén waves nonlinearly interact to drive a nonresonant mode [31], a drift wave [32], an acoustic mode [33,34], or an Alfvén wave [35].…”

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Observation of an Alfvén Wave Parametric Instability in a Laboratory Plasma

Dorfman

Carter

2016

Phys. Rev. Lett.

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A shear Alfvén wave parametric instability is observed for the first time in the laboratory. When a single finite ω/Ωi kinetic Alfvén wave (KAW) is launched in the Large Plasma Device above a threshold amplitude, three daughter modes are produced. These daughter modes have frequencies and parallel wave numbers that are consistent with copropagating KAW sidebands and a low frequency nonresonant mode. The observed process is parametric in nature, with the frequency of the daughter modes varying as a function of pump wave amplitude. The daughter modes are spatially localized on a gradient of the pump wave magnetic field amplitude in the plane perpendicular to the background field, suggesting that perpendicular nonlinear forces (and therefore k ⊥ of the pump wave) play an important role in the instability process. Despite this, modulational instability theory with k ⊥ = 0 has several features in common with the observed nonresonant mode and Alfvén wave sidebands.PACS numbers: 52.35. Mw, 52.35.Bj Alfvén waves, a fundamental mode of magnetized plasmas, are ubiquitous in space, astrophysical, and laboratory plasmas. While the linear behavior of these waves has been extensively studied [1][2][3][4][5], nonlinear effects are important in many real systems, including the solar wind and solar corona. Theoretical predictions show that these Alfvén waves may be unstable to various parametric instabilities (e.g., Refs. [6-8]) even at very low amplitudes (δB/B < 10 −3 ). Parametric instabilities could contribute to coronal heating [9], the observed spectrum and cross-helicity of solar wind turbulence [10][11][12], and damping of fast magnetosonic waves in fusion plasmas [13,14].An abundance of theoretical work [6,7,[15][16][17][18][19] has found three types of parametric instabilities for a k ⊥ = 0 Alfvén wave: decay, modulational, and beat. The decay instability is the most widely known and involves the decay of a forward propagating Alfvén wave into a backward propagating Alfvén wave and a forward propagating sound wave. By contrast, the modulational instability results in forward propagating upper and lower Alfvénic sidebands as well as well as a nonresonant acoustic mode at the sideband separation frequency. To allow the forward propagating waves to interact, the pump wave must be dispersive-therefore the modulational instability at k ⊥ = 0 requires finite ω/Ω i through inclusion of Hall effects [7]. Ponderomotive coupling between the pump and sideband Alfvén modes self-consistently drives the nonresonant density perturbation parallel to the background magnetic field. In this context, "nonresonant" means that the mode does not satisfy a dispersion relation in the absence of the instability drive; this is also called a quasimode in the fusion community [20,21].Both shear Alfvén wave decay and modulational instabilities have been produced in numerical simulations [11,[22][23][24][25], but observational evidence is limited. Observations in the ion foreshock region upstream of the bow shock in the Earth's magnetosphere have...

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“…24 In a second set of experiments, the copropagating beat mode is driven near the frequency of existing drift wave turbulence; the beat mode is then found to couple to the drift modes, altering the drift wave frequency spectrum. 25 In this paper, the first laboratory observations of the Alfv en-acoustic mode coupling at the heart of the parametric decay instability described in Dorfman and Carter 26 are presented in detail. Counter-propagating shear Alfv en waves are launched from antennas and allowed to interact nonlinearly.…”

Section: Introductionmentioning

confidence: 97%

“…5,[21][22][23] Studies of the nonlinear properties of Alfv en waves have also been performed on LAPD. 24,25 For instance, the interaction between two co-propagating Alfv en waves may nonresonantly drive a quasimode in the plasma; this quasimode then couples to the original waves to produce a spectrum of Alfv enic sidebands. 24 In a second set of experiments, the copropagating beat mode is driven near the frequency of existing drift wave turbulence; the beat mode is then found to couple to the drift modes, altering the drift wave frequency spectrum.…”

Section: Introductionmentioning

confidence: 99%

Non-linear Alfvén wave interaction leading to resonant excitation of an acoustic mode in the laboratorya)

Dorfman

Carter

2015

Phys. Plasmas

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The nonlinear three-wave interaction process at the heart of the parametric decay process is studied by launching counter-propagating Alfv en waves from antennas placed at either end of the Large Plasma Device [W. Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)]. A resonance in the beat wave response produced by the two launched Alfv en waves is observed and is identified as a damped ion acoustic mode based on the measured dispersion relation. Other properties of the interaction including the spatial profile of the beat mode and response amplitude are also consistent with theoretical predictions for a three-wave interaction driven by a nonlinear ponderomotive force. A simple damped, driven oscillator model making use of the MHD equations well-predicts most of the observations, but the width of the resonance curve is still under investigation.

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Control of Gradient-Driven Instabilities Using Shear Alfvén Beat Waves

Cited by 17 publications

References 33 publications

The many faces of shear Alfvén waves

The many faces of shear Alfvén waves

Observation of an Alfvén Wave Parametric Instability in a Laboratory Plasma

Non-linear Alfvén wave interaction leading to resonant excitation of an acoustic mode in the laboratorya)

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