Magnetorotational instability in a nonuniform magnetic field

It is shown that the characteristics of the magnetorotational instability noticeably change in the presence of the inhomogeneity of a magnetic field in which rotation occurs. A decrease in the magnetic field with an increase in the distance from the rotation center, which is typical for astrophysical objects, can lead to a significant decrease in the threshold velocity of the object medium rotation, as well as to mitigation of the requirements on a rotation velocity profile that are necessary for the development of instability. Other examples that demonstrate the indicated effects are given. Original Russian Text ? V.I. Ilgisonis, I.V. Khalzov, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 11, pp. 815–818.     

Concept of multipole magnetic field rotation in ECRIS

The conventional type of magnetic well is formed by superposition of two types of magnetic field, axial bumpy field and radial multipole field. It is used to contain plasma that consists of neutrals, ions and electrons. These particles are in constant motion in the well and energetic electrons create plasma by violent collisions with neutrals and ions. The confined electrons are constantly heated by ECR technique in the presence of magnetic field. In this paper it has been shown theoretically that how the electron motion is influenced in terms of heating, containment and azimuthal uniformity of plasma, by the axial rotation of the multipole magnetic field [1,2]. Afterwards, the feasibility of achieving a rotating magnetic multipole field is discussed to some extent. And it is seen that it is not beyond the capability of the scientific community in the present scenario of the advanced technology. Presently, it can be achieved for lesser field and slightly larger size of the multipole electromagnet and can be used for improvement of the ECR ion source (ECRIS).     

Forced magnetic reconnection and field penetration of an externally applied rotating helical magnetic field in the TEXTOR tokamak

The magnetic field penetration process into a magnetized plasma is of basic interest both for plasma physics and astrophysics. In this context special measurements on the field penetration and field amplification are performed by a Hall probe on the dynamic ergodic divertor (DED) on the TEXTOR tokamak and the data are interpreted by a two-fluid plasma model. It is observed that the growth of the forced magnetic reconnection by the rotating DED field is accompanied by a change of the plasma fluid rotation. The differential rotation frequency between the DED field and the plasma plays an important role in the process of the excitation of tearing modes. The momentum input from the rotating DED field to the plasma is interpreted by both a ponderomotive force at the rational surface and a radial electric field modified by an edge ergodization.     

The Origin of Short-Time Variations in Cosmic-Ray Intensity

It is well known today that a continuous stream of highly ionized plasma is emitted from the Sun’s surface. This plasma is called the solar wind and consists of protons, electrons, and light nuclei. The solar wind pushes the solar magnetic field into interplanetary space to form the interplanetary magnetic field. The interplanetary magnetic field is a dynamical system that depends on the solar cycle and the Sun’s rotation phase. Thus, the Solar System is a natural plasma physics laboratory with an enormous multitude of different effects showing the current state of the system. By recording cosmic-ray fluxes, one can study the behavior of the interplanetary magnetic field and obtain information about processes that occur both on the Sun’s surface and throughout the Solar System. The main short-time variations in cosmic-ray intensity include the 27-day variations and the Forbush decreases. These variations are caused by complex solar plasma structures, which are generated by different processes on the Sun’s surface and propagate through space in a wide range of velocities. Cosmic-ray fluxes recorded with the PAMELA magnetic spectrometer on board the Resurs DK1 satellite in 2006–2016 are used to show some examples of cosmic-ray variations.     

Absolute negative resistance in a nonequilibrium two-dimensional electron system in a strong magnetic field

The effect of microwave electromagnetic radiation on the resistance of the 2D electron gas in a GaAs/AlAs heterostructure in a strong magnetic field is investigated. It is shown that, under the nonequilibrium conditions caused by microwave radiation, the aforementioned 2D system exhibits giant oscillations of its resistance with varying magnetic field. When the measuring current density is small, an increase in the microwave power leads to the appearance of an absolute negative resistance at the main minimum of these oscillations, which lies near the cyclotron resonance. The experimental data are found to be in qualitative agreement with the theory of multiphoton photoinduced impurity scattering [J. Inarrea and G. Platero, Appl. Phys. Lett. 89, 052109 (2006)]. Original Russian Text ? A.A. Bykov, D.R. Islamov, D.V. Nomokonov, A.K. Bakarov, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 9, pp. 695–698.     

Observation of centrifugally driven interchange instabilities in a plasma confined by a magnetic dipole

Centrifugally driven interchange instabilities are observed in a laboratory plasma confined by a dipole magnetic field. The instabilities appear when an equatorial mesh is biased to drive a radial current that causes rapid axisymmetric plasma rotation. The observed instabilities are quasicoherent in the laboratory frame of reference; they have global radial mode structures and low azimuthal mode numbers, and they are modified by the presence of energetic, magnetically confined electrons. Results from a self-consistent nonlinear simulation reproduce the measured mode structures.     

Amplification of mean magnetic field and magnetic helicity production in hot lepton plasma of early universe

We argue that the magnetic helicity conservation is violated at the lepton stage in the evolution of early Universe owing to the parity violation in the Standard Model of electroweak interactions. As a result, a cosmological magnetic field which can be a seed for the galactic dynamo obtains from the beginning a substantial magnetic helicity which has to be taken into account in the magnetic helicity balance at the later stage of galactic dynamo. The particle physics mechanism suggested in our works depends neither on helicity of matter turbulence with plasma vortices resulting in the standard α effect in dynamo theory nor on general rotation. The mechanism can result in a self-exitation of an (almost) uniform cosmological magnetic field. The text was submitted by the authors in English.     

Rotational properties of annulus dusty plasma in a strong magnetic field

The collective dynamics of an annulus dusty plasma formed between a co-centric conducting (non-conducting) disk and ring configuration is studied in a strongly magnetized radiofrequency (rf) discharge. A superconducting electromagnet is used to introduce a homogeneous magnetic field to the dusty plasma medium. In the absence of the magnetic field, the dust grains exhibit thermal motion around their equilibrium position. The dust grains start to rotate in the anticlockwise direction with increasing magnetic field (B > 0.02 T ), and the constant value of the angular frequency at various strengths of the magnetic field confirms the rigid body rotation. The angular frequency of dust grains linearly increases up to a threshold magnetic field (B > 0.6 T ) and after that its value remains nearly constant in a certain range of magnetic field. Further increase in magnetic field (B > 1 T ) lowers the angular frequency. Low value of the angular frequency is expected by reducing the width of the annulus dusty plasma or the input rf power. The azimuthal ion drag force due to the magnetic field is assumed to be the energy source which drives the rotational motion. The resultant radial electric field in the presence of a magnetic field determines the direction of rotation. The variation of floating (plasma) potential across the annular region at given magnetic field explains the rotational properties of the annulus dusty plasma in the presence of a magnetic field.     

The structure of chaotic magnetic field lines in a tokamak withexternal nonsymmetric magnetic perturbations

We consider the effects of external nonsymmetric magnetostatic perturbations caused by resonant helical windings and a chaotic magnetic limiter on the plasma confined in a tokamak. The main purpose of both types of perturbation is to create a region in which field lines are chaotic in the Lagrangian sense: two initially nearby field lines diverge exponentially through many turns around the tokamak. The equilibrium field is obtained from the equations of magneto-hydrodynamic equilibrium written down in a polar toroidal coordinate system. The magnetic fields generated by the resonant helical windings and the chaotic magnetic limiter are obtained through an analytical solution of Laplace equation. The magnetic field line equations are integrated to give a Hamiltonian mapping of field lines that we use to characterize the structure of chaotic field lines. In the case of resonant windings, we obtained the map by both numerical integration and a Hamiltonian formulation. For a chaotic limiter, we analytically derived a symplectic map by using a Hamiltonian formulation     

Features of the dispersion characteristics of nonuniform plasma waveguides in a longitudinal magnetic field

We study the features of the dispersion curves and field structures of the fundamental axisymmetric mode of nonuniform layered plasma waveguides in a longitudinal magnetic field. It is shown that the presence of sharp boundaries between layers leads to the appearance of additional branches of the dispersion curves in the frequency range ω _Be < ω < ω_UH(0), where ω_Be is the electron gyrofrequency and ω_UH(0) is the upper-hybrid resonance frequency for the near-axis region of a nonuniform waveguide. The fields of eigenmodes corresponding to these branches comprise resonance structures near the sharp plasma-density variation at which the upper-hybrid resonance conditions are satisfied and plasma waves are excited. The frequency interval of such a branch is limited by the resonant frequencies of the neighboring uniform layers. It turns out that in the case of a strong magnetic field ({ie392-01}, where ω_p is the plasma frequency having the value {ie392-02} in the near-axis region of a nonuniform waveguide), the fundamental-mode field is localized in the near-axis region of a nonuniform waveguide, whereas in the opposite case {ie392-03}, the maximum wave fields are localized in either the upper-hybrid resonance region or the outer (near-boundary) layer of the waveguide if there is no resonance region. It is found that the whistler (helicon) contribution to the field structure of the fundamental axisymmetric mode is very small for narrow nonuniform waveguides (b < λ₀, where b is the waveguide radius and λ₀ is the wavelength in free space) if the plasma density on the axis is high compared with the cutoff density {ie392-04}. We present one of the possible explanations for the effect of narrowing of the plasma channel of a high-frequency whistler-range discharge with distance from a source in the increasing magnetic field. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 5, pp. 434–446, May 2008.     

Rigid and differential plasma crystal rotation induced by magnetic fields

Observations show that plasma crystals, suspended in the sheath of a radio-frequency discharge, rotate under the influence of a vertical magnetic field. Depending on the discharge conditions, two different cases are observed: a rigid-body rotation (all the particles move with a constant angular velocity) and sheared rotation (the angular velocity of particles has a radial distribution). When the discharge voltage is increased sufficiently, the particles may even reverse their direction of motion. A simple analytical model is used to explain qualitatively the mechanism of the observed particle motion and its dependence on the confining potential and discharge conditions. The model takes into account electrostatic, ion drag, neutral drag, and effective interparticle interaction forces. For the special case of rigid-body rotation, the confining potential is reconstructed. Using data for the radial dependence of particle rotation velocity, the shear stresses are estimated. The critical shear stress at which shear-induced melting occurs is used to roughly estimate the shear elastic modulus of the plasma crystal. The latter is also used to estimate the viscosity contribution due to elasticity in the plasma liquid. Further development is suggested in order to quantitatively implement these ideas.     

Effect of resonant magnetic perturbations on edge plasma parameters in the STOR-M tokamak

Edge plasma properties have been modified in the Saskatchewan Torus-Modified tokamak by means of resonant magnetic perturbations (RMP). It has been found that the radial profiles of ion saturation current and floating potential in the edge region can be modified by an externally applied static (m?=?2, n?=?1) RMP field. An increase in the pedestal plasma density (n) and more negative electric field (E_r) have been observed in the plasma edge region. It is believed that the RMP field altered the plasma transport in the edge and scrape-off layer regions, leading to a higher density pedestal and a potential drop in some cases. During the enhanced confinement phase, it is possible to identify a region where intermittent transport events, the so-called blobs, are created and the holes of lower density left behind.     

Suppression of the equilibrium tunneling current between slightly disordered two-dimensional electron systems with different electron concentrations in a high magnetic field

Tunneling between parallel two-dimensional electron gases (2DEG) in accumulation layers formed on both sides of the single doped AlGaAs barrier are examined in both zero and high magnetic field. Accumulation layers are separated from highly n-doped contact regions which freely supply electrons to the 2DEGs via 80 nm thick lightly n-doped spacer layers. Strongly oscillating current with magnetic field along the 2DEGs is absent in this arrangement. Without magnetic field resonant tunneling between 2DEGs with different as grown electron concentration could be settle by application of external voltage bias. High magnetic fields (ν<1) shift resonant tunneling to zero external bias and suppresses tunneling current, creating wide gap in the tunneling density of states at the Fermi level arisen from the in-plane Coulomb interaction in the 2DEGs. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 236–241 (10 February 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Observations of an ion-driven instability in non-neutral plasmas confined on magnetic surfaces

The first detailed experimental study of an instability driven by the presence of a finite ion fraction in an electron-rich non-neutral plasma confined on magnetic surfaces is presented. The instability has a poloidal mode number m=1, implying that the parallel force balance of the electron fluid is broken and that the instability involves rotation of the entire plasma, equivalent to ion-resonant instabilities in Penning traps and toroidal field traps. The mode appears when the ion density exceeds approximately 10% of the electron density. The measured frequency decreases with increasing magnetic field strength, and increases with increasing radial electric field, showing that the instability is linked to the E x B flow of the electron plasma. The frequency does not, however, scale exactly with E/B, and it depends on the ion species that is introduced, implying that the instability consists of interacting perturbations of ions and electrons.     

Macroscopic magnetic fields of antiferromagnets

The macroscopic magnetic fields arising in the space outside or in the bulk of an antiferromagnet in the absence of external currents are a result of surface magnetization. The general problem of determining these fields is formulated. It is shown that the field distribution near special lines on the surface is monopolar. An experimental study of the field makes it possible to determine the surface magnetization on the faces of an antiferromagnetic crystal. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 9, 724–728 (10 May 1996)     

Resonant dislocation motions in NaCl crystals under EPR conditions in the Earth’s magnetic field with a radio-frequency pump field

Three resonant peaks l(ν) of dislocation paths, appearing in NaCl crystals under simultaneous exposure to the Earth’s magnetic field B Earth and an ac pump field $$\tilde B$$ have been observed in the radio-frequency range (ν ∼ 1 MHz). Each peak corresponds to a specific experimental geometry. Resonance frequencies correspond to ordinary EPR conditions in which, however, instead of the total Earth’s field, its projections onto particular crystallographic directions play part. These directions are probably related to orientations of magnetic moments at impurity centers in dislocation cores. A physical model describing well the positions all three EPR peaks l(ν) is proposed. The peak height l m is proportional to the squared amplitude of the pump field and the time t of sample exposure to crossed fields: $$l_m \propto \tilde B_m^2 t$$ . The threshold amplitude $$\tilde B_m^{th} \sim 1 \mu {\rm T}$$ of the pump field is detected, below which the effect rapidly disappears. The time of spin-lattice relaxation of radical pairs on dislocations is estimated by the threshold field $$\tilde B_m^{th}$$ level.     

表面等离子共振现象的新应用——微弱磁场的测量

提出了一种新的表面等离子体共振传感器, 它包含三层结构: 棱镜、金属薄膜及二能级介质. 通过理论分析发现, 与通常表面等离子体共振系统不同, 这一物理系统中同时存在两种共振效应 (表面等离子体共振和能级间量子跃迁的共振效应), 它们共同作用的结果导致一系列新的物理现象, 其中一个令人感兴趣的现象是入射光的反射率对外场导致的微小能级移动十分敏感 (这一现象是通常的表面等离子体共振系统所不具有的). 由于能级移动依赖于外场, 所以最终入射光的反射率对外场具有灵敏的响应. 本文以外磁场导致能级移动的情况进行了理论计算, 结果表明, 这种表面等离子体共振系统的入射光的反射率对外加磁场极其敏感. 这一特性可以用来测量物质表面附近的微弱磁场, 有可能发展成为一种新型检测技术.     

Thermoelectric rotating torus for fusion

A plasma toroid is rotated toroidally to supersonic speeds by external means. The input power maintains the rotation and also heats the plasma. The thermoelectric effect from the resulting temperature gradient creates and maintains a poloidal magnetic field against resistive decay, confining the plasma in steady state. The shear in the rotation keeps the plasma stable to MHD kinks and interchanges. Such a system has two novel advantages as a fusion device: there are no strong electromagnets needed to create the confining magnetic field, and there is effectively no limit on the field strength and, hence, no limit on the plasma pressure contained. The system has to be of a large aspect ratio, to minimize centrifugal effects, and a weak, external vertical magnetic field is needed to balance the radial hoop force.     

Magnetic breakdown and quantum magnetotransport with a constant pseudospin under tilted magnetic fields in an n -In x Ga1? x As/GaAs double quantum well

A procedure is proposed for precise scanning of the (B _⊥, B _‖) plane between the magnetic field projections that are perpendicular and parallel to (quasi-)two-dimensional layers when measuring their longitudinal and Hall magnetoresistances. Investigations of a n-In _x Ga_1−x As/GaAs double quantum well (x ≈ 0.2) performed using this procedure make it possible to reveal a number of the features of the magnetoresistance, which appear due to a complex energy spectrum of the double quantum well in a parallel field, and to separate them from the structures associated with the magnetic breakdown. The trajectories representing the features of the magnetoresistance in the (B _⊥, B _‖) plane are described by the semiclassical calculations of the quantization of the energy spectrum of the double quantum well under the action of the perpendicular field component. The structures appearing due to the magnetic breakdown are amplified with increasing the total magnetic field magnitude and, in the samples with low mobility, completely suppress the features caused by the motion of an electron with a constant pseudospin component. The peaks corresponding to the magnetic breakdown are split in a strong parallel field due to the spin splitting of the Landau levels. These splittings correspond to the effective Landé factor |g*| ≈ 3. Original Russian Text ? M.V. Yakunin, S.M. Podgornykh, V.N. Neverov, 2007, published in Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 132, No. 1, pp. 241–249.