Versuche zum toroidalen Einschluß von Hoch-Beta-Plasmen mit Hilfe von helischen Multipolfeldern

Helical multipole fields with multiplicities of 1 to 3 and up to 16 field cycles around the circumference of the torus are superposed on a toroidal theta pinch. The vacuum fields of the configurations are calculated and regions of closed flux surfaces are found. The experimental results are:

1. Equilibria of High-Beta plasmas also exist, if the vacuum field has no absolute minimum, provided that the region of closed flux surfaces in the vacuum field has a greater diameter than the plasma, and that the helical multipole fields do not periodically increase and decrease the gradient of the toroidal main field. These equilibria are possibly unstable.
2. If the helical multipole fields periodically increase and decrease the gradient of the theta pinch field along the circumference of the torus there is no equilibrium, even if the helical curvature of the field lines is great.
3. The main cause of the losses is the escape of particles along the field lines.

     

Elektronenbeschleunigung in helikalen Feldern

An electron accelerator with a betatronlike air core accelerating field and with helical stabilizing fields is described. It consists of a glass torus having a medium radius of 20 cm which is surrounded by three coils generating the following magnetic fields: a) a betatronlike accelerating field, b) a toroidal field (B _z-field) and c) a helical quadrupole field. Electrons are injected into the torus by a spark discharge at a gas pressure of less than 10^?5 mm Hg. In aB _z-field of 18000 gauss electrons have been accelerated up to 6 MeV, as measured with a 1,5 l bubble chamber, and up to 10 MeV, as measured with scintillation counters. In the energy range of 0,5 to 1,6 MeV, depending on experimental conditions, electron currents between 100 and 1000 amps have been observed, independently by Rogowsky-belt measurements and by determining the bremsstrahlung intensity. Considering the superposition of various drift velocities stability conditions are derived for the motion of the electrons.     

Observations of multiple magnetic islands in the core of a reversed field pinch

We describe in this Letter the first measurement of multiple islands in the core of a reversed field pinch (RFP). These islands appear with current profile modification leading to magnetic fluctuation reduction in the Madison symmetric torus RFP. Magnetic island widths decrease to an unprecedented level, reducing the overlap of adjacent islands and allowing distinct islands to appear. The structures are observed in multichord measurements of soft-x-ray emissivity. The soft-x-ray data is validated with Poincaré reconstructions of the magnetic field structure in the core.     

Magnetized hypermassive neutron-star collapse: a central engine for short gamma-ray bursts

A hypermassive neutron star (HMNS) is a possible transient formed after the merger of a neutron-star binary. In the latest axisymmetric magnetohydrodynamic simulations in full general relativity, we find that a magnetized HMNS undergoes "delayed" collapse to a rotating black hole (BH) as a result of angular momentum transport via magnetic braking and the magnetorotational instability. The outcome is a BH surrounded by a massive, hot torus with a collimated magnetic field. The torus accretes onto the BH at a quasisteady accretion rate [FORMULA: SEE TEXT]; the lifetime of the torus is approximately 10 ms. The torus has a temperature [FORMULA: SEE TEXT], leading to copious ([FORMULA: SEE TEXT]) thermal radiation that could trigger a fireball. Therefore, the collapse of a HMNS is a promising scenario for generating short-duration gamma-ray bursts and an accompanying burst of gravitational waves and neutrinos.     

Bifurcation to 3D helical magnetic equilibrium in an axisymmetric toroidal device

We report the first direct measurement of the internal magnetic field structure associated with a 3D helical equilibrium generated spontaneously in the core of an axisymmetric toroidal plasma containment device. Magnetohydrodynamic equilibrium bifurcation occurs in a reversed-field pinch when the innermost resonant magnetic perturbation grows to a large amplitude, reaching up to 8% of the mean field strength. Magnetic topology evolution is determined by measuring the Faraday effect, revealing that, as the perturbation grows, toroidal symmetry is broken and a helical equilibrium is established.     

Special correcting winding for a torsatron

The possibility of controlling in two dimensions the position of the region of closed magnetic surfaces in a helical magnetic confinement system is demonstrated by numerical simulations carried out using a two-pole model torsatron as an example. The displacement of this region in a direction perpendicular to the equatorial plane of the torus is provided by the magnetic field produced by a special correcting winding.     

Perturbational Approach to Force — Free Toroidal Equilibria

Solutions of the equation curl B = γ B inside a torus-shaped region with vanishing normal component on the torus surface are sought by the perturbation method. The perturbation parameter is proportional to the curvature of the torus, i.e zero-order solutions are exact helical solutions for a straight tube.     

Universal low-temperature properties of frustrated classical spin chain near the ferromagnet-helimagnet transition point

The thermodynamics of the classical frustrated spin chain near the transition point between the ferromagnetic and the helical phases is studied. The calculation of the partition and spin correlation functions at low temperature limit is reduced to the quantum mechanical problem of a particle in potential well. It is shown that the thermodynamic quantities are universal functions of the temperature normalized by the chiral domain wall energy. The obtained behavior of the static structure factor indicates that the short-range helical-type correlations existing at low temperatures on the helical side of the transition point disappear at some critical temperature, defining the Lifshitz point. It is also shown that the low-temperature susceptibility in the helical phase near the transition point has a maximum at some temperature. Such behavior is in agreement with that observed in several materials described by the quantum s = 1/2 version of this model.     

A class of time-machine solutions with a compact vacuum core

We present a class of curved-spacetime vacuum solutions which develop closed timelike curves at some particular moment. We then use these vacuum solutions to construct a time-machine model. The causality violation occurs inside an empty torus, which constitutes the time-machine core. The matter field surrounding this empty torus satisfies the weak, dominant, and strong energy conditions. The model is regular, asymptotically flat, and topologically trivial. Stability remains the main open question.     

Ring defects in a strongly confined chiral liquid crystal

We find numerically that a regular array of isolated ring defects can exist as a stable state in a highly chiral liquid crystal confined in a thin cell imposing fixed planar anchoring at the parallel confining surfaces. This peculiar defect structure can be stable when the cell thickness d is around 3/4 of the helical pitch p. A cell of thickness 3p/4 with parallel surface anchoring is incompatible with helical alignment that favors d=mp/2 (with m being an integer). Formation of ring defects can thus be regarded as a result of frustrations between the helical alignment with a specific pitch and the confining surfaces that prevent it.     

Numerical computation of the helical Chandrasekhar–Kendall modes

A new formulation is presented for numerically computing the helical Chandrasekhar–Kendall modes in an axisymmetric torus. It explicitly imposes ? · B = 0 and yields a standard matrix eigenvalue problem, which can then be solved by standard matrix eigenvalue techniques. Numerical implementation and computational results are shown for an axisymmetric torus typical of reversed field pinch and spherical tokamak.     

Superflow in a toroidal Bose-Einstein condensate: an atom circuit with a tunable weak link

We have created a long-lived (≈40 s) persistent current in a toroidal Bose-Einstein condensate held in an all-optical trap. A repulsive optical barrier across one side of the torus creates a tunable weak link in the condensate circuit, which can affect the current around the loop. Superflow stops abruptly at a barrier strength such that the local flow velocity at the barrier exceeds a critical velocity. The measured critical velocity is consistent with dissipation due to the creation of vortex-antivortex pairs. This system is the first realization of an elementary closed-loop atom circuit.     

MHD instabilities in wall-stabilized dc arcs

A helical structure of an arc at atmospheric pressure is observed in axial magnetic field. The helix moves along the magnetic field with Alfvén velocity so that 1 to 15 kHz oscillations are observed.     

EAST 装置固有杂质驱动蛇形振荡行为研究

利用多阵列高时空分辨的软X 射线阵列成像系统对EAST 芯部固有杂质驱动的蛇形振荡进行时间动态演化和空间结构反演研究,研究了实验上发现的两种时空行为明显不同的两类蛇形振荡,一类具有字母V 或W形状的动态频谱,空间上为理想近圆状热芯结构;另外一类蛇形振荡和锯齿崩塌共存,振荡的频谱呈现手掌状,空间上为很大的电阻性月牙形磁岛结构。结果表明,锯齿共存的蛇形振荡的饱和径向扰动位移通常比无锯齿的蛇形振荡大,并且会触发边界的新经典撕裂模。     

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利用多阵列高时空分辨的软X射线阵列成像系统对EAST芯部固有杂质驱动的蛇形振荡进行时间动态演化和空间结构反演研究,研究了实验上发现的两种时空行为明显不同的两类蛇形振荡,一类具有字母V或W形状的动态频谱,空间上为理想近圆状热芯结构;另外一类蛇形振荡和锯齿崩塌共存,振荡的频谱呈现手掌状,空间上为很大的电阻性月牙形磁岛结构。结果表明,锯齿共存的蛇形振荡的饱和径向扰动位移通常比无锯齿的蛇形振荡大,并且会触发边界的新经典撕裂模。     

Phase diagram of the helical structure of a two-subsystem frustrated antiferromagnet

The expansion of the thermodynamic potential for the two-subsystem antiferromagnet with frustrated intersubsystem isotropic exchange is obtained. It is demonstrated that this expansion contains the first derivatives with respect to the antiferromagnetic vectors of the subsystems, i.e., the Lifshitz invariant. The equation for the temperature-field boundary of the helical phase for the two-subsystem frustrated antiferromagnet is derived by linearizing the variational equations for the minimum free energy within the mean-field approximation. Relationships are obtained for the critical field at T = 0, the angle of canting of moments of the antiferromagnetic sublattices, and the temperature of spontaneous appearance of helical ordering in the absence of an external field. It is revealed that there is a second higher temperature of formation of the helical magnetic structure induced by the magnetic field with the wave vector of the helix nonmonotonically depending on the external field. The phase boundary of the helical phase and the temperature dependence of the orientation of moments of the magnetic subsystem with weak exchange interaction are determined using numerical minimization of the free energy. It is shown that the transition to the commensurate phase is a first-order transition with a small magnetization jump. A comparative analysis of models with different spatial displacements of ions in the subsystems along the direction of the vector of the helical structure is performed. A criterion is proposed for the choice of the direction of the vector of the incommensurate magnetic structure.     

Quasi-Stable Configurations of Torus Vortex Knots and Links

The dynamics of torus vortex configurations V_{n, p, q} in a superfluid liquid at zero temperature (n is the number of quantum vortices, p is the number of turns of each filament around the symmetry axis of the torus, and q is the number of turns of the filament around its central circle; radii R₀ and r₀ of the torus at the initial instant are much larger than vortex core width ξ) has been simulated numerically based on the regularized Biot–Savart law. The lifetime of vortex systems till the instant of their substantial deformation has been calculated with a small step in parameter B₀ = r₀/R₀ for various values of parameter Λ = ln(R₀/ξ). It turns out that for certain values of n, p, and q, there exist quasi-stability regions in the plane of parameters (B₀, Λ), in which the vortices remain almost invariable during dozens and even hundreds of characteristic lengths.

     

Torus factor-The relationship between radiofrequency field and radial position in toroid cavity probes

Toroid detectors are resonators for high-pressure in situ NMR spectroscopy or one-dimensional rotating-frame imaging. One of their unique qualities is a mathematically well-defined nonuniform radiofrequency field confined to the inside of the detector. A single parameter (i.e., the torus factor) is sufficient to describe the relationship between this radiofrequency field and the radial distance from the center axis of the torus. Because accurate determination of the torus factor is essential to optimize toroid cavity NMR experiments or conduct toroid cavity imaging, a fast numerical algorithm for accurate, precise, and convenient determination of torus factors from pulse width-dependent signal intensities is introduced. In addition, the new algorithm provides for 99% confidence intervals around the refined torus factors. A computer program in which the optimization progress is visualized during the torus factor refinement is presented. Upon completion of the program, the best-fit simulated data and the residuals between best fit and experimental data are provided. Copyright 2000 Academic Press.     

A swirling jet with vortex breakdown: three-dimensional coherent structures

The paper reports on shape of a three-dimensional coherent structure in a velocity field of a high-swirl turbulent jet with the bubble-type vortex breakdown. A set of the 3D instantenous velocity fields was measured by using the tomographic particle image velocimetry (tomographic PIV) technique and processed by the proper orthogonal decomposition (POD) method. The detected intensive coherent velocity component corresponded to a helical vortex core of the swirling jet and two secondary spiral vortices. The entire coherent structure was rotating around the jet axis in compliance with the direction of the flow swirl. From the 3D data it is concluded that the dynamics of the strsucture can be described by a traveling wave equation: Re[A(y, r)·e ^{i(mθ + ky - ωt)}] with the number of the spiral mode m = +1 for positively defined k and ω.     

Cholesteric liquid crystals in oscillating magnetic fields

Using the hydrodynamical equations, it is shown that an oscillating magnetic field applied perpendicular to the helical axis of a cholesteric liquid crystal causes the helical structure to oscillate at even harmonics of the field frequency. The feasibility of doing light scattering from these driven modes is also discussed.