On kinetic Alfvén waves in a dusty plasma

The dispersion and damping of a dust-modified kinetic Alfvén wave, and a lower frequency dust kinetic Alfvén wave, is investigated in a dusty plasma comprising electrons, ions, and negative dust. It is found that, when dust dynamics is neglected, the presence of cold dust modifies the usual kinetic Alfvén wave dispersion and damping owing to the inequality of the electron and ion densities. The dispersion relation of the dust kinetic Alfvén wave, with frequency below the dust cyclotron frequency, depends on the density and temperature parameters of all three species, and the wave damping is due to both electrons and ions.     

Laboratory observation of a nonlinear interaction between shear Alfvén waves

An experimental investigation of nonlinear interactions between shear Alfvén waves in a laboratory plasma is presented. Two Alfvén waves, generated by a resonant cavity, are observed to beat together, driving a pseudomode at the beat frequency. The pseudomode then scatters the Alfvén waves, generating a series of sidebands. The observed interaction is very strong, with the normalized amplitude of the driven pseudomode comparable to the normalized magnetic field amplitude (deltaB/B) of the interacting Alfvén waves.     

Production of Alfvén waves by a rapidly expanding dense plasma

The expansion of a dense (initially, n(lpp)/n(0)>1) laser-produced plasma into an ambient magnetized plasma ( n(0) = 2 x 10(12) cm(-3)) capable of supporting Alfvén waves has been studied. The interaction results in the production of shear Alfvén waves as well as large density perturbations (Delta n/n(0) approximately 0.3) associated with the moving dense plasma. The waves propagate away from the target and are observed to become plasma-column resonances. Spatial patterns of the wave magnetic fields are measured and are used to estimate the coupling efficiency of the laser energy and the kinetic energy of the dense plasma into wave energy.     

Alfvén waves in temperature anisotropic Cairns distributed plasma

The dispersion relations and Landau damping of Alfven waves in kinetic and inertial limits are studied in temperature anisotropic Cairns distributed plasma.In the case of kinetic Alfven waves(KAWs),it is found that the real frequency is enhanced when either the electron perpendicular temperature or the non-thermal parameter A increases.For inertial Alfven waves(IAWs),the real frequency is slightly affected by the electron temperature anisotropy and A.Besides the real frequency,the damping rate of KAWs is reduced when the electron perpendicular temperature or A increases.In the case of IAWs,the temperature anisotropy and A either enhance or reduce the damping rate depending upon the perpendicular wavelength.These results may be helpful to understand the dynamics of KAWs and IAWs in space plasmas where the non-Maxwellian distribution of particles are routinely observed.     

Modulational instability of ultra-low-frequency shear dust Alfvén waves in a plasma medium of positive and negatively charged dust fluids

The propagation of finite amplitude ultra-low-frequency shear dust Alfvén (SDA) waves, and their modulational instability in a magnetized plasma medium of positive and negatively charged dust fluids have been theoretically investigated by using the reductive perturbation method. The derivative nonlinear Schrödinger equation is derived to examine the stability analysis of such SDA waves. It is found that the SDA waves propagating in such an opposite polarity dust plasma medium are modulationally unstable, and that the instability criterion and the growth rate of these unstable SDA waves in such a novel opposite polarity dust plasma medium are found to be significantly different from those in electron–ion or electron–positron plasma media. The implications of the present investigation in different space environments and laboratory devices are briefly discussed.     

High-n ideal and resistive shear Alfvén waves in tokamaks

Ideal and resistive MHD equations for the shear Alfvén waves are studied in a low-β toroidal model by employing the high-n ballooning formalism. The ion sound effects are neglected. For an infinite shear slab, the ideal MHD model gives rise to acontinuous spectrum of real frequencies and discrete eigenmodes (Alfvén-Landau modes) with complex frequencies. With toroidal coupling effects due to nonuniform toroidal magnetic field, the continuum is broken up into small continuum bands and new discrete toroidal eigenmodes can exist inside the continuum gaps. Unstable ballooning eigenmodes are also introduced by the bad curvature when β > β_c. The resistivity (η) can be considered perturbatively for the ideal modes. In addition, four branches of resistive modes are induced by the resistivity: (1) resistive entropy modes which are stable with frequencies going to zero with resistivity as $\text{η}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$; (2) tearing modes which are stable (Δ′ < 0) with frequencies approaching zero as $\text{η}{}^{\mathrm{<mtext>3</mtext><mtext>5</mtext>}}$; (3) resistive periodic shear Alfvén waves which approach the finite frequency end points of the continuum bands as $\text{η}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$; and (4) resistive ballooning modes which are purely growing with growth rate proportional to $\text{η}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{β}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ as η → 0 and β → 0.     

Control of gradient-driven instabilities using shear Alfvén beat waves

A new technique for manipulation and control of gradient-driven instabilities through nonlinear interaction with Alfvén waves in a laboratory plasma is presented. A narrow, field-aligned density depletion is created in the Large Plasma Device, resulting in coherent, unstable fluctuations on the periphery of the depletion. Two independent shear Alfvén waves are launched along the depletion at separate frequencies, creating a nonlinear beat-wave response at or near the frequency of the original instability. When the beat wave has sufficient amplitude, the original unstable mode is suppressed, leaving only the beat-wave response, generally at lower amplitude.     

Torsional Alfvén and slow magnetoacoustic waves generated by a plasma in a magnetic field

An intense electron-antineutrino source with a hard spectrum (\(E_{{{\tilde v}_e}}^{\max }\) = 13 MeV and \(\left\langle {{E_{{{\tilde v}_e}}}} \right\rangle \) = 6.5MeV) can be created on the basis of the short-lived isotope ⁸Li (β^?-decay, T_1/2 = 0.84 s) formed via the (n, γ) activation of ⁷Li. In contrast to a reactor antineutrino spectrum whose uncertainty is large, particularly in the high-energy region \({E_{{{\tilde v}_e}}}\) > 6 MeV, which is experimentally relevant, the lithium \({\tilde v_e}\) spectrum is accurately determined. The proposed accelerator-driven experimental scheme with a neutron-producing target and a lithium converter as an intense \({\tilde v_e}\) source is an alternative to a nuclear reactor. The required amount of high-purity ⁷Li will be reduced in many times by using the suggested heavy-water LiOD solutions. A possible experiment involving the lithium source on search for sterile neutrinos in the mass region Δm² ≥ 0.2 eV² with a very high sensitivity to mixing-angle values down to sin²(2Θ) ≈ (7–10) × 10^–4 at the 95% C.L. has been considered.     

Preliminary computation of the gap eigenmode of shear Alfvén waves on the LAPD

Characterizing the gap eigenmode of shear Alfv′en waves(SAWs) and its interaction with energetic ions is important to the success of magnetically confined fusion. Previous studies have reported an experimental observation of the spectral gap of SAW on the on Large Plasma Device(LAPD)(Zhang et al. 2008 Phys. Plasmas 15 012103), a linear large plasma device(Gekelman et al. 1991 Rev. Sci. Instrum. 62 2875) possessing easier diagnostic access and lower cost compared with traditional fusion devices, and analytical theory and numerical gap eigenmode using ideal conditions(Chang 2014 Ph.D Thesis at Australian National University). To guide experimental implementation, the present work models the gap eigenmode of SAWs using exact LAPD parameters. A full picture of the wave field for previous experiment reveals that the previously observed spectral gap is not global but an axially local result. To form a global spectral gap, the number of magnetic mirrors has to be increased and stronger static magnetic field makes it clearer. Such a spectral gap is obtained for the magnetic field of B_0(z) = 1.2 + 0.6 cos[2π(z-33.68)/3.63] with 7.74-m magnetic beach. By introducing two types of local defects(corresponding to E_θ(z_0) = 0 and E'_θ(z_0) = 0 respectively), odd-parity and even-parity discrete eigenmodes are formed clearly inside the gap. The strength of these gap eigenmodes decreases significantly with collision frequency, which is consistent with previous studies. Parameter scans show that these gap eigenmodes can be even formed successfully for the field strength of B_0(z) = 0.2 + 0.1 cos[2π(z-33.68)/3.63] and with only four magnetic mirrors, which are achievable by the LAPD at its present status. This work can serve as a strong motivation and direct reference for the experimental implementation of the gap eigenmode of SAWs on the LAPD and other linear plasma devices.     

Cross-scale nonlinear coupling and plasma energization by Alfvén waves

We present a new channel for the nonlocal transport of wave energy from the large (MHD) scales to the small (kinetic) scales generated by the resonant decay of MHD Alfvén waves into kinetic Alfvén waves. This process does not impose any restriction on the wave numbers or frequencies of initial MHD waves, which makes it superior compared to the mechanisms of spectral transport studied before. Because of dissipative properties of the nonlinearly driven kinetic Alfvén waves, the decay leads to plasma heating and particle acceleration, which is observed in a variety of space and astrophysical plasmas. Two examples in the solar corona and the terrestrial magnetosphere are briefly discussed.     

Characteristic parameters of drift vortices coupled to Alfvén waves in an inhomogeneous space plasma

We present detailed measurements of ion scale vortices of drift type coupled to Alfvén waves in an inhomogeneous and collisionless space magnetoplasma. The two free parameters of a dipolar vortex, intensity and spatial radius, are measured. The vortices are driven by a strong density gradient on a boundary layer with scale size of the same order as the vortex diameter. Observations of vortices off the gradient show that symmetry-breaking conditions in a real inhomogeneous plasma can lead not only to cross-field but also to cross-boundary anomalous transport of particles and energy.     

On the properties of Alfvén waves in relativistic magnetohydrodynamics

Simple Alfvén waves and Alfven shocks are considered in the framework of relativistic magnetohydrodynamics. It is found that the tangential components of vector fields trace ellipses instead of the circles of Newtonian MHD. Their properties are studied in the general wave frame, Hoffmann-Teller wave frame, and the general laboratory frame.     

Topological constraints on solar loop plasma instabilities and Alfvén waves

Inhomogeneous plasmas-solar instabilities-are investigated by using the techniques of classical differential geometry for curves, where the Frenet torsion and curvature describe completely the motion of a curve. In our case, the Frenet frame changes in time and also depends upon the other coordinates, taking into account the inhomogeneity of the plasma. The exponential perturbation method, so commonly used to describe cosmological perturbations, is applied to the magnetohydrodynamic (MHD) plasma equations to find modes describing Alfvén wave propagation in the medium of planar loops. Stability is investigated in the imaginary axis of the spectra of complex frequencies ω, i.e. m (ω) ≠ 0. A pratical guide for experimental solar physicists is given by computing the twist of force-free solar loops, which generalizes the Parker formula relating the twist to the Frenet torsion. In our expression the twist of the solar loops also depends on the abnormality of the normal vector of the frame.      

Observation of reversed-shear Alfvén eigenmodes excited by energetic ions in a helical plasma

Reversed-shear Alfvén eigenmodes were observed for the first time in a helical plasma having negative q?' (the curvature of the safety factor q at the zero shear layer). The frequency is swept downward and upward sequentially via the time variation in the maximum of q. The eigenmodes calculated by ideal MHD theory are consistent with the experimental data. The frequency sweeping is mainly determined by the effects of energetic ions and the bulk pressure gradient. Coupling of reversed-shear Alfvén eigenmodes with energetic ion driven geodesic acoustic modes generates a multitude of frequency-sweeping modes.