Stored Magnetic Field Energy Research Articles

Investigation Into Magnetic Reconnection Formation on Propellant Ignition in Electrical Explosion

In magnetic reconnection, magnetic lines break and reconnect to change their topology to a lower-energy state. This process can liberate stored magnetic field energy and accelerate particles during unsteady explosive events. Here, we report the observations of the magnetic reconnection and kink instability of plasma jet in single wire electrical explosion and their effect on propellant ignition. The results showed that the initial velocity of plasma was ∼2,000 m/s, and when the magnetic reconnection occurred, the velocity increased by ∼400–∼2,400 m/s. The evaluated Alfvén velocity was ∼500 m/s, the Alfvén time was ∼20 µs, and the Lundquist number S = 1.7 × 107. Based on these experimental results and model, the three-dimensional magnetic field topology and its evolution process was evaluated and presented. Furthermore, the magnetic reconnection occurred when its curvature reached a certain value due to the fact that the motion of the current sheet changes the topology of the magnetic field, and then, the plasma jet was accelerated and exhausted. The plasma jet angle was ∼50° in experiment 1, and it was consistent with the calculated results. The resulting magnetic reconnection plays an important role in propellant ignition, which enhances the ignition ability of wire electrical explosion. Furthermore, the results represent a key step towards resolving one of the most important problems of plasma physics and can be used to improve the understanding of wire array explosion and propellant ignition.

Pre-polarization fields for earth’s field NMR: Fast discharge for use with short T1 and large coils

The sensitivity of earth’s field NMR is greatly increased by the use of a pre-polarizing field Bp. When used with short T1 samples, the field must be decreased rapidly to avoid loss of the pre-polarized magnetization by relaxation. Such a rapid decrease in the field requires rapid discharge (∼10ms) of a large stored magnetic field energy (∼700J). In addition, in order that the full pre-polarized magnetization be available for the subsequent pulse sequence, the field discharge should be adiabatic. This requirement is difficult to fulfill in cases where Bp is not everywhere parallel to the earth’s field, such as with a large surface coil. Circuitry for rapid and controlled discharge is presented. Simulations and experiments confirm the importance of both of these conditions.

Multi-parameter magnetoelectric response modeling of magnetostrictive/piezoelectric laminate composites considering shear strain

A low-frequency magnetoelectric (ME) response model of magnetostrictive/piezoelectric laminate composites based on multi-parameter analysis is presented. This model, taken the shear effect as the basis, analyzes the stress and strain of the magnetostrictive and piezoelectric layers by the stress function. The interlayer shear stress and the low-frequency ME response characteristics of ME laminate composites are obtained by using Hamilton principle of minimum energy and in consideration of the influence of stored magnetic field energy of magnetostrictive layer. The theoretical results show that the ME voltage coefficient is related with the magnetic permeability, Poisson ratio, magnetomechanical coupling coefficient, thickness ratio of magnetostrictive material and the Poisson ratio and electromechanical coupling coefficient of piezoelectric material. The influence of these parameters is analyzed. The experimental results show that analytical results are also compared to experimental test data with excellent correlation and their errors are less than 6%.

MAGNETIC ENERGY BUILDUP FOR RELATIVISTIC MAGNETAR GIANT FLARES

Motivated by coronal mass ejection studies, we construct general relativistic models of a magnetar magnetosphere endowed with strong magnetic fields. The equilibrium states of the stationary, axisymmetric magnetic fields in the magnetar magnetosphere are obtained as solutions of the Grad-Shafranov equation in a Schwarzschild spacetime. To understand the magnetic energy buildup in the magnetar magnetosphere, a generalized magnetic virial theorem in the Schwarzschild metric is newly derived. We carefully address the question whether the magnetar magnetospheric magnetic field can build up sufficient magnetic energy to account for the work required to open up the magnetic field during magnetar giant flares. We point out the importance of the Aly-Sturrock constraint, which has been widely studied in solar corona mass ejections, as a reference state in understanding magnetar energy storage processes. We examine how the magnetic field can possess enough energy to overcome the Aly-Sturrock energy constraint and open up. In particular, general relativistic (GR) effects on the Aly-Sturrock energy constraint in the Schwarzschild spacetime are carefully investigated. It is found that, for magnetar outbursts, the Aly-Sturrock constraint is more stringent, i.e., the Aly-Sturrock energy threshold is enhanced due to the GR effects. In addition, neutron stars with greater mass have a higher Aly-Sturrock energy threshold and are more difficult to erupt. This indicates that magnetars are probably not neutron stars with extreme mass. For a typical neutron star with mass of $1-2 M_{\odot}$, we further explore the effects of cross-field current effects, caused by the mass loading, on the possibility of stored magnetic field energy exceeding the Aly-Sturrock threshold.

Quasi‐linear theory of anomalous resistivity

Magnetic reconnection is a favored mechanism for understanding charged‐particle acceleration phenomena in space and laboratory plasmas. A change in magnetic field line topology is envisioned in magnetic reconnection to release the stored magnetic field energy. In order for this to take place, some form of dissipation to break the frozen‐in condition is required. Since the classical resistivity is often inadequate for collisionless plasmas, anomalous resistivity via charged particles interacting with fluctuating electromagnetic fields is customarily invoked. However, anomalous resistivity is often modeled rather than computed from theory. In this article, we formulate the theory of anomalous transport from first principles. It is found that the effect of fluctuations can be defined through three anomalous transport terms governing momentum and energy transport and the resistivity. To illustrate the utility of these derived equations, examples that bear relevance to the consideration of breakdown in the frozen‐in condition in magnetic reconnection are discussed.

Protection system for the superconducting coils in WENDELSTEIN 7-X

Apart from scientific aims, an important technical goal of WENDELSTEIN 7-X is to demonstrate stationary operation. This is a fundamental fusion-reactor requirement and a characteristic advantage of stellarators over tokamak fusion reactors. Magnetic confinement is achieved by means of 70 coils with a diameter of up to 5 m and a magnetic induction of up to 3 T on the axis. To allow continuous operation, the coil windings are made of superconducting material. Because of the great amount of stored magnetic field energy of up to 1 GJ, the protection system is an essential part of the power supply system. This text describes the protection system for the superconducting coils as well as key preliminary experiments which demonstrate the suitability of selected components of the protection system for the WENDELSTEIN 7-X experiment.

A magnetic force in dry-reed relays

A method for calculating the magnetic force between reeds is presented. The method is based on the premise that an energizing coil creates two magnetic bodies inside itself that are similar to two permanent magnets. The magnetic field is generated outside the bodies. Magnetic reeds can be replaced by an equivalent set of current-carrying solenoids that produce exactly the same magnetic field distribution as the bodies. The demagnetizing effects in reeds was measured and incorporated. Having found the component of the magnetic field with the same direction as the magnetic force used and the motion of the reeds, the author calculates the stored magnetic field energy in an overlapping space between the reeds. The inhomogeneity and high concentration of the magnetic field at the end of the reeds are taken into consideration. The magnetic force between the reeds depends on the change of the stored magnetic field energy during the motion of the reeds.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Superconducting cyclotrons

Superconducting cyclotrons with compact geometry and solenoidal coils are operating since several years, indicating the successful introduction of superconductivity in cyclotron design. Some of these facilities use or will use tandems as injectors, some operate as stand-alone machines. The technical problems of this concept are caused mainly by the compactness, which is the attractive feature of the design of course. In order to achieve specific energies above ∼ 300 MeV/u separated sector cyclotrons with superconducting magnets are still under consideration. Technical difficulties are related to the large stored magnetic field energy, to the strong electromagnetic forces and to the non-circular shape of the coils. A rather small separated orbit cyclotron with superconducting channel magnets and superconducting cavities is under construction in Munich. This type of cyclotron offers unique focusing properties at relaxed injection and extraction conditions. The large number of individual controllable channel magnets gives some flexibility. On the other hand it demands a high degree of reliability of many complex components, requiering simple solutions wherever possible. The present status of the Tritron-project is given in detail.

Particle acceleration during reconnection in laboratory plasmas

The basic physics of magnetic field line reconnection near magnetic null points is investigated in a large collisionless laboratory plasma. The observations focus on the close vicinity of the neutral sheet (Δ z ⋍ ϱ ci ) , a region which is difficult to resolve in space. Thus, the high resolution laboratory data are complimentary to the more global space observations. The neutral sheet is generated by applying a pulsed magnetic field with X-type null point to the plasma. The reconnection of field lines and the acceleration of particles are established from time resolved three-dimensional field and flow measurements. Both steady and impulsive reconnection events are observed. The latter are characterized by spontaneous current disruptions and tearing of the current sheet, formation of a double layer at the region of current disruption, and particle acceleration to large inductive voltages due to the impulsive release of stored magnetic field energy. Detailed observations of the particle distribution functions f( v , r , t) have been performed. Suprathermal electron tails are observed in the neutral sheet caused by particle runaway in the electric field along the separator. Beam-like distributions exist adjacent to double layers. While the reconnection electric fields are the source for the particle acceleration the plasma turbulence associated with velocity space instabilities causes enhanced resistivity, heating and dissipation of magnetic energy. This is evident when the current sheet is switched off; it breaks up into intense internally closed current filaments which, via instabilities, enhance the dissipation and explain the anomalously fast loss of internally stored magnetic energy and the rapid electron heating.

Double layer formation during current sheet disruptions in a reconnection experiment

When the current density in the center of a neutral sheet is increased to a critical value spontaneous current disruptions are observed. The release of stored magnetic field energy results in a large inductive voltage pulse which drops off inside the plasma in the form of a potential double layer. Particles are energized, microinstabilities are generated, the plasma is thinned, and the current flow is redirected. These laboratory observations qualitatively support recent models of magnetic substorms and solar flares.

Magnetic field line reconnection experiments, 4. Resistivity, heating, and energy flow

Detailed spatial and temporal measurements of the total vector electric field E and current density J in a plasma with dynamic magnetic field line reconnection have been made. The resistivity calculated via the generalized Ohm's law is found to be spatially inhomogeneous with values exceeding the classical resistivity by 1 to 2 orders of magnitude. Resistivity and current density do not maximize in the same locations. The dissipation E · J is determined and analyzed in terms of particle heating and fluid acceleration. Electron heating is found to be the dominant dissipation process in the diffusion region. Independent measurements of the divergence of the Poynting vector ▽ · (E × H), the change in stored magnetic field energy ∂/∂t(B²/2µo), and the dissipation give a consistent, detailed picture of the energy flow. Efficient conversion of electromagnetic energy into particle heating is observed.

An Air-Cored Synchro-Cyclotron for 400 mev. Protons

It is considered that the most reasonable proposition for accelerating protons to very high energies, using an instrument which employs a magnetic field produced by an air-cored coil, is to operate the machine as a synchro-cyclotron. An investigation of the energies available from such accelerators for given amounts of stored magnetic-field energy is carried out on the basis that the limit is set by the mechanical forces exerted on the coil. It is shown that the power requirements are comparable with those of large iron-cored accelerators at present under construction In a machine with a useful orbit of radius 11.8 cm., it is possible to provide a field of 2.7 × 105 gauss over the circle enclosed by that orbit, so that protons would be accelerated to an energy of 400 MeV.