scholarly journals Distinctive features of the structure of current sheets formed in plasma in three-dimensional magnetic configurations with an X line (a review)

2021 ◽  
Vol 9 (6) ◽  
pp. 464-478
Author(s):  
Anna Frank
Keyword(s):  
Longitudinal Magnetic Field ◽  
Initial Conditions ◽  
Three Dimensional ◽  
Limited Range ◽  
Transverse Plane ◽  
Plasma Current ◽  
Initial Value ◽  
Distinctive Features ◽  
Current Sheets ◽  
Guide Field

A review is presented on experimental results related to investigation of distinctive features of the structure and evolution of plasma current sheets formed in three dimensional (3D) magnetic configurations with an X line, in the presence of a longitudinal magnetic field component (guide field) directed along the X line. It is shown that formation of a plasma current sheet results in enhancement of the guide field within the sheet. The excessive guide field is maintained by plasma currents that flow in the transverse plane relative to the main current in the sheet. As a result, the structure of the currents becomes three-dimensional. Increasing the initial value of the guide field brings about a decrease of compression into the sheet of both the electric current and plasma. This effect is caused by changing the pres- sure balance in the sheet when an excessive guide field appears in it. Deformation of plasma current sheets in 3D magnetic configurations, namely, an appearance of asymmetric and tilted sheets, results from excitation of the Hall currents and their interaction with the guide field. It is shown that the formation of current sheets in 3D magnetic configurations with an X line is possible in a relatively wide, but limited range of initial conditions

Classical harmonic oscillator approach of a helical-wiggler free-electron laserwith axial guide field

2000 ◽  
Vol 78 (12) ◽  
pp. 1069-1085 ◽  
Author(s):  
M N Rhimi ◽  
R El-Bahi ◽  
A W Cheikhrouhou
Keyword(s):  
Fixed Point ◽  
Free Electron ◽  
Initial Conditions ◽  
Three Dimensional ◽  
Basic Feature ◽  
Operating Conditions ◽  
The Other ◽  
Guide Field ◽  
Constants Of Motion ◽  
Definition Of

Electron beam dynamics in a helical-wiggler free-electron laser (FEL) with a uniform axial guide magnetic field are studied using a three-dimensional Hamiltonian approach. The basic feature of the analysis is the definition of a rotational variable, [Formula: see text], that plays the primordial role in lowering to the half the dimension of the quadratic Hamiltonian as a system of two uncoupled oscillators with definite frequencies and amplitudes. It is through applying this variable in the vicinity of a fixed point that the Heisenberg picture of the dynamics of the particles comes to light, leading thus to the association of the steady-state ideal helical trajectories with arbitrary trajectories. The approach recognized the usual two constants of motion, one being the total energy while the other is the canonical axial angular momentum, Pz'. If the value of the latter is such that a fixed point exists, the Hamiltonian is expanded about the fixed point up to second order. The so-obtained oscillator characteristic frequencies allowed one to study the different modes of propagation and to identify, and then avoid the problematic operating conditions of the FEL concerned. On the other hand, the amplitudes of the oscillations, which do depend on the frequencies, are fortunately found to be constants of motion and then controlled by the boundary conditions (initial conditions). PACS Nos.: 52.40-w, 52.60+h, 42.55.Tb, 52.75Ms

Energy growth of three-dimensional disturbances in plane Poiseuille flow

1991 ◽  
Vol 224 ◽  
pp. 241-260 ◽  
Author(s):  
L. Hårkan Gustavsson
Keyword(s):  
Energy Density ◽  
Poiseuille Flow ◽  
Initial Conditions ◽  
Formal Solution ◽  
Three Dimensional ◽  
Propagation Speed ◽  
Inhomogeneous Equation ◽  
Normal Velocity ◽  
Plane Poiseuille Flow ◽  
Initial Value

The development of a small three-dimensional disturbance in plane Poiseuille flow is considered. Its kinetic energy is expressed in terms of the velocity and vorticity components normal to the wall. The normal vorticity develops according to the mechanism of vortex stretching and is described by an inhomogeneous equation, where the spanwise variation of the normal velocity acts as forcing. To study specifically the effect of the forcing, the initial normal vorticity is set to zero and the energy density in the wavenumber plane, induced by the normal velocity, is determined. In particular, the response from individual (and damped) Orr–Sommerfeld modes is calculated, on the basis of a formal solution to the initial-value problem. The relevant timescale for the development of the perturbation is identified as a viscous one. Even so, the induced energy density can greatly exceed that associated with the initial normal velocity, before decay sets in. Initial conditions corresponding to the least-damped Orr–Sommerfeld mode induce the largest energy density and a maximum is obtained for structures infinitely elongated in the streamwise direction. In this limit, the asymptotic solution is derived and it shows that the spanwise wavenumbers at which the largest amplification occurs are 2.60 and 1.98, for symmetric and antisymmetric normal vorticity, respectively. The asymptotic analysis also shows that the propagation speed for induced symmetric vorticity is confined to a narrower range than that for antisymmetric vorticity. From a consideration of the neglected nonlinear terms it is found that the normal velocity component cannot be nonlinearly affected by the normal vorticity growth for structures with no streamwise dependence.

scholarly journals Effect of guide field on three-dimensional electron shear flow instabilities in electron current sheets

2015 ◽  
Vol 81 (6) ◽  
Author(s):  
Neeraj Jain ◽  
Jörg Büchner
Keyword(s):  
Shear Flow ◽  
Current Sheet ◽  
Three Dimensional ◽  
Sheet Thickness ◽  
Electron Current ◽  
Tearing Mode ◽  
Current Sheets ◽  
Tearing Modes ◽  
Guide Field ◽  
Half Thickness

We examine, in the limit of electron plasma ${\it\beta}_{e}\ll 1$, the effect of an external guide field and current sheet thickness on the growth rates and nature of three-dimensional (3-D) unstable modes of an electron current sheet driven by electron shear flow. The growth rate of the fastest growing mode drops rapidly with current sheet thickness but increases slowly with the strength of the guide field. The fastest growing mode is tearing type only for thin current sheets (half-thickness ${\approx}d_{e}$, where $d_{e}=c/{\it\omega}_{pe}$ is the electron inertial length) and zero guide field. For finite guide field or thicker current sheets, the fastest growing mode is a non-tearing type. However, growth rates of the fastest 2-D tearing and 3-D non-tearing modes are comparable for thin current sheets ($d_{e}<\text{half thickness}<2\,d_{e}$) and small guide field (of the order of the asymptotic value of the component of magnetic field supporting the electron current sheet). It is shown that the general mode resonance conditions for tearing modes depend on the effective dissipation mechanism. The usual tearing mode resonance condition ($\boldsymbol{k}\boldsymbol{\cdot }\boldsymbol{B}_{0}=0$, $\boldsymbol{k}$ is the wavevector and $\boldsymbol{B}_{0}$ is the equilibrium magnetic field) can be recovered from the general resonance conditions in the limit of weak dissipation. The conditions (relating current sheet thickness, strength of the guide field and wavenumbers) for the non-existence of tearing mode are obtained from the general mode resonance conditions. We discuss the role of electron shear flow instabilities in magnetic reconnection.

Toward laboratory torsional spine magnetic reconnection

2017 ◽  
Vol 83 (6) ◽  
Author(s):  
David L. Chesny ◽  
N. Brice Orange ◽  
Hakeem M. Oluseyi ◽  
David R. Valletta
Keyword(s):  
Magnetic Reconnection ◽  
Kinetic Parameters ◽  
Three Dimensional ◽  
Limited Range ◽  
Null Point ◽  
Plasma Parameters ◽  
Astrophysical Plasmas ◽  
Current Sheets ◽  
Practical Applications ◽  
Field Lines

Magnetic reconnection is a fundamental energy conversion mechanism in nature. Major attempts to study this process in controlled settings on Earth have largely been limited to reproducing approximately two-dimensional (2-D) reconnection dynamics. Other experiments describing reconnection near three-dimensional null points are non-driven, and do not induce any of the 3-D modes of spine fan, torsional fan or torsional spine reconnection. In order to study these important 3-D modes observed in astrophysical plasmas (e.g. the solar atmosphere), laboratory set-ups must be designed to induce driven reconnection about an isolated magnetic null point. As such, we consider the limited range of fundamental resistive magnetohydrodynamic (MHD) and kinetic parameters of dynamic laboratory plasmas that are necessary to induce the torsional spine reconnection (TSR) mode characterized by a driven rotational slippage of field lines – a feature that has yet to be achieved in operational laboratory magnetic reconnection experiments. Leveraging existing reconnection models, we show that within a${\lesssim}1~\text{m}^{3}$apparatus, TSR can be achieved in dense plasma regimes (${\sim}10^{24}~\text{m}^{-3}$) in magnetic fields of${\sim}10^{-1}~\text{T}$. We find that MHD and kinetic parameters predict reconnection in thin${\lesssim}20~\unicode[STIX]{x03BC}\text{m}$current sheets on time scales of${\lesssim}10~\text{ns}$. While these plasma regimes may not explicitly replicate the plasma parameters of observed astrophysical phenomena, studying the dynamics of the TSR mode within achievable set-ups signifies an important step in understanding the fundamentals of driven 3-D magnetic reconnection and the self-organization of current sheets. Explicit control of this reconnection mode may have implications for understanding particle acceleration in astrophysical environments, and may even have practical applications to fields such as spacecraft propulsion.

scholarly journals A faster escape does not enhance survival in zebrafish larvae

2017 ◽  
Vol 284 (1852) ◽  
pp. 20170359 ◽  
Author(s):  
Arjun Nair ◽  
Christy Nguyen ◽  
Matthew J. McHenry
Keyword(s):  
High Speed ◽  
Larval Fish ◽  
Body Position ◽  
Three Dimensional ◽  
Limited Range ◽  
The Body ◽  
Model Parameters ◽  
Fish Prey ◽  
Zebrafish Larvae ◽  
Fish Predators

An escape response is a rapid manoeuvre used by prey to evade predators. Performing this manoeuvre at greater speed, in a favourable direction, or from a longer distance have been hypothesized to enhance the survival of prey, but these ideas are difficult to test experimentally. We examined how prey survival depends on escape kinematics through a novel combination of experimentation and mathematical modelling. This approach focused on zebrafish ( Danio rerio ) larvae under predation by adults and juveniles of the same species. High-speed three-dimensional kinematics were used to track the body position of prey and predator and to determine the probability of behavioural actions by both fish. These measurements provided the basis for an agent-based probabilistic model that simulated the trajectories of the animals. Predictions of survivorship by this model were found by Monte Carlo simulations to agree with our observations and we examined how these predictions varied by changing individual model parameters. Contrary to expectation, we found that survival may not be improved by increasing the speed or altering the direction of the escape. Rather, zebrafish larvae operate with sufficiently high locomotor performance due to the relatively slow approach and limited range of suction feeding by fish predators. We did find that survival was enhanced when prey responded from a greater distance. This is an ability that depends on the capacity of the visual and lateral line systems to detect a looming threat. Therefore, performance in sensing, and not locomotion, is decisive for improving the survival of larval fish prey. These results offer a framework for understanding the evolution of predator–prey strategy that may inform prey survival in a broad diversity of animals.

A Quasi-three-dimensional Finite-volume Shallow Water Model for Green Water on Deck

2013 ◽  
Vol 57 (03) ◽  
pp. 125-140
Author(s):  
Daniel A. Liut ◽  
Kenneth M. Weems ◽  
Tin-Guen Yen
Keyword(s):  
Shallow Water ◽  
Finite Volume ◽  
Time Domain ◽  
Degrees Of Freedom ◽  
Initial Conditions ◽  
Three Dimensional ◽  
Green Water ◽  
Water Model ◽  
Ship Motions ◽  
Shallow Water Model

A quasi-three-dimensional hydrodynamic model is presented to simulate shallow water phenomena. The method is based on a finite-volume approach designed to solve shallow water equations in the time domain. The nonlinearities of the governing equations are considered. The methodology can be used to compute green water effects on a variety of platforms with six-degrees-of-freedom motions. Different boundary and initial conditions can be applied for multiple types of moving platforms, like a ship's deck, tanks, etc. Comparisons with experimental data are discussed. The shallow water model has been integrated with the Large Amplitude Motions Program to compute the effects of green water flow over decks within a time-domain simulation of ship motions in waves. Results associated to this implementation are presented.

Compact Storage Ring with Longitudinal Magnetic Field and Electron Cooling

2010 ◽  
Vol 5 (3) ◽  
pp. 79-89
Author(s):  
Maxim I. Bryzgunov ◽  
Aleksandr V. Bublej ◽  
Vladimir A. Vostrikov ◽  
Vitaliy M. Panasyuk ◽  
Vasiliy V. Parkhomchuk ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Storage Ring ◽  
Longitudinal Magnetic Field ◽  
High Energy ◽  
Electron Cooling ◽  
Distinctive Features ◽  
Cooling Method ◽  
Energy Proton ◽  
Proton Current ◽  
Main Effects

Project of compact low energy proton storage ring (1.75 MeV) for production of high energy photons in reaction 13C(p,γ) 14N on internal target is presented. With the help of the photons it is possible to realize the scheme for detection of substances with heightened content of nitrogen using Nuclear Resonance Absorption method. Distinctive features of the construction are use of electron cooling method for beam heating compensation, and strong longitudinal magnetic field along full perimeter of the ring. For optimal work of electron cooling, compensation of space charge of the beam circulating in the ring is necessary. Results of simulation of main effects in the storage ring are shown. The results allow us to expect that proton current up to 100 mA can be stored and luminosity up to 3·1033 cm2 s -1 can be achieved

A Combined Eulerian and Lagrangian Method for Prediction of Evaporating Sprays

2002 ◽  
Vol 124 (3) ◽  
pp. 481-488 ◽  
Author(s):  
M. Burger ◽  
G. Klose ◽  
G. Rottenkolber ◽  
R. Schmehl ◽  
D. Giebert ◽  
...  
Keyword(s):  
Two Phase Flow ◽  
Initial Conditions ◽  
Three Dimensional ◽  
Lagrangian Method ◽  
Phase Flow ◽  
Two Phase Flows ◽  
Two Phase ◽  
Lagrangian Methods ◽  
Eulerian Method ◽  
Lagrangian Modeling

Polydisperse sprays in complex three-dimensional flow systems are important in many technical applications. Numerical descriptions of sprays are used to achieve a fast and accurate prediction of complex two-phase flows. The Eulerian and Lagrangian methods are two essentially different approaches for the modeling of disperse two-phase flows. Both methods have been implemented into the same computational fluid dynamics package which is based on a three-dimensional body-fitted finite volume method. Considering sprays represented by a small number of droplet starting conditions, the Eulerian method is clearly superior in terms of computational efficiency. However, with respect to complex polydisperse sprays, the Lagrangian technique gives a higher accuracy. In addition, Lagrangian modeling of secondary effects such as spray-wall interaction enhances the physical description of the two-phase flow. Therefore, in the present approach the Eulerian and the Lagrangian methods have been combined in a hybrid method. The Eulerian method is used to determine a preliminary solution of the two-phase flow field. Subsequently, the Lagrangian method is employed to improve the accuracy of the first solution using detailed sets of initial conditions. Consequently, this combined approach improves the overall convergence behavior of the simulation. In the final section, the advantages of each method are discussed when predicting an evaporating spray in an intake manifold of an internal combustion engine.

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