Magnetic Diffusion Processes Research Articles

Influence of the Wire Spatial Structure on the Distribution of Product and the Peak Overpressure of Shockwave Generated by the Electric Explosion

The deposition energy and the peak overpressure of shockwaves are the leading engineering parameters of wire electric explosion technology applied to enhance oil recovery. The thicker Cu wire deposits more energy, which transforms into the shockwave efficiently. Therefore, the effects of three diameters (0.3, 0.4, and 0.5 mm) and hollow ratios (0, 0.5, and 0.7) on the electric explosion efficiency were studied by collecting pulse current, explosion products, and shockwaves during the test. All spatial structure designs of the wire depend on the skin effect parameters of the pulse discharge current. The results found that the peak overpressure of the shockwave soars with the increase of the hollow ratios when the diameter is constant. The range of the peak overpressure is 25.2~47.7 MPa. However, the correlation between shockwave and wire diameter changes from negative to positive with the increase of the hollow ratio from 0 to 0.7. The phase distribution deduced by the particle morphology and quantity distribution indicates that it is going to be uniform gradually with the hollow ratio rising from 0 to 0.7. When the extreme simplification is carried out without considering the magnetic diffusion process, it is indicated that the distribution of temperature and phase states along the wire radial is a Bessel function depending on the skin effect of the current density when three times the theoretical enthalpy drives the Cu wire. It means that the desired shockwave could be obtained efficiently by increasing the diameter and the hollow ratio of wire during a wire electric explosion.

Experiments and Analysis of Downslope Low-Voltage Transition in C-Type Solid Armature Rail Gun

Transition in a rail gun system, namely, the phenomenon of solid contact turns into arcing contact, is undesirable because it is associated with severe damage to both armature and rail. The current downslope (− di/dt ) transition is one of the most significant transition mechanisms for C-type solid armatures. In this article, considering velocity skin effect (VSE) and magnetic diffusion processes in armature and rail, a simplified 1-D electromagnetic model is used to illustrate – di/dt effect and verify the occurrence of eddy current. However, further simulation with a 3-D model shows that the electromagnetic force caused by the negative net current in the armature’s tail is not the main cause of transition. In addition, a noncontact area between armature and rail is set up to enhance the eddy current, but the resulting reversal Lorentz force is not strong enough to lift off the armature tail from the rail. A set of launching experiments was also carried out using three types of armatures with different tail structures in a 20 mm $\times30$ mm rectangular caliber rail gun. The results show that low-voltage transitions occur when the driving current drops to approximately 90% of its peak value during the downslope.

Application of electrical action to design and analysis of magnetically driven solid liner implosion

As a typical cylindrical-convergent drive technique, magnetically driven solid liner implosion could compress interior substance with a shock or quasi-isentropic manner, which has been widely used to investigate the hydrodynamic behavior, the dynamic characteristics of material and fusion energy and so on. For aspecific facility, the implosion parameters depend on material, radius and thickness of the liner, and the ablation of liner restrict the optional parameters. The concept of electrical action is introduced via thin shell model, which not only is the representation of states for conductive metal, but also indicates the change of liner velocity under the condition of thin shell hypothesis. The result shows that the outer velocity of liner increases linearly with electrical action and is directly proportional to liner thickness but inversely proportional to liner density. The incompressible zero-dimensional model is used to calculate the dynamic parameters of thin shell liner, including the implosion time, the outer interface velocity, the implosion kinetic energy, and the electrical action under the condition of low linear current density. There exist optimal radius and thickness which can achieve the maximum velocity, momentum, and kinetic energy. The aluminum is suitable for reaching higher velocity and the copper can obtain higher pressure according to a proportionality coefficient <i>Q</i><sub>b</sub>/<i>ρ</i> which is an intrinsic quality of metal. A one-dimensional (1D) elastic plastic magnetic hydrodynamic code which is called SOL1D is developed to simulate liner implosion behavior. The modified relationship between resistivity and electrical action is introduced to SOL1D, which can adapt higher hydrodynamic pressure. According to current waves, the 1D code can be used to simulate liner implosion behavior for all kinds of current densities. The 1D simulation liner velocity is in agreement with both the experimental results and the electrical action model for liner implosion experiment on FP-1 facility. The simulation of isentropic compression experiment at ZR facility shows that the magnetic diffusion process is suppressed at extra high current density and hydrodynamic pressure, and the electrical action is larger than the experimental value of wire electrical explosion. The zero-dimensional (0D) and 1D simulation show that estimating the liner velocity and liner phase changing via the electrical action are suitable when thin shell hypothesis and low current density assumption are satisfied.

The second type of sharp-front wave mechanism of strong magnetic field diffusion in solid metal

When a strong magnetic field diffuses into a metal, the metal is ablated by Joule heating accompanying the magnetic diffusion process, and the metal’s resistance changes violently with the fast-growing temperature. This results in the formation of a so-called “nonlinear diffusion wave” characterized by a sharp “wave-front” where the magnetic field abruptly decays. A metal has its own threshold magnetic field value, which is determined by the critical temperature of the metal. If the constant vacuum magnetic field B0 is above the threshold value Bc, the magnetic diffusion process can be approximately described by sharp-front diffusion wave theory [B. Xiao et al., Physics of Plasmas 23, 082104 (2016)], which gives a simple formula to describe the velocity of the diffusion process. However, if B0 is below Bc, the sharp-front diffusion wave theory is no longer applicable. In this situation, one would need another type of sharp-front diffusion wave theory (type II theory) to describe the magnetic diffusion behaviors. In type II theory, the sharp-front diffusion wave velocity depends on three parameters, i.e., the magnetic boundary condition B0, the critical temperature Tc, and the cold metal resistance ηs. The dependence of the velocity on these three parameters is analyzed in detail in this paper.

Simulations on Current Distribution in Railgun Under Imperfect Contact Conditions

In the process of electromagnetic launch, the initial contact pressure distribution of armature/rail (A/R) contacts interface is uneven, and the actual contact area is obviously different from the perfect contact. The uneven contact pressure leads to uneven contact resistance distribution, affecting the magnetic diffusion process and changing the A/R interface current distribution. Therefore, it is necessary to consider the influence of contact pressure on the current distribution. In the past, the simulation studies usually considered the contact between rails and armature as the perfect contact, and the result obtained may be different from the actual situation. This paper uses the finite element software COMSOL to perform coupling simulation analysis on the electromagnetic module and solid mechanics module in the initial stage of electromagnetic launch. At the initial stage of electromagnetic launch, the contact pressure is mainly provided by the prepressure generated by the interference fit. In this paper, the influence of contact pressure on current distribution and the influence of magnitude of interference on contact position, contact pressure, and current distribution are analyzed.

On the Submergence and Reemergence of Magnetic Fields in Young Supernovae

In this work I present numerical magnetohydrodynamic (MHD) simulations of the early dynamics around newly born neutrons stars using the AMR Flash method. When the core-collapse supernovae occurs a reverse shock is formed allowing strong accretion onto the neutron star surface (hypercritical phase). In such regime large amounts of matter are deposited on the neutron star surface, submerging the magnetic field in the new crust. When the hypercritical regime is over, the magnetic field can suffer a reemergence episode due to magnetic diffusion processes, allowing the delayed switch-on of pulsars.

Optimal Tracking for a Divergent-Type Parabolic PDE System in Current Profile Control

We consider the optimal tracking problem for a divergent-type parabolic PDE system, which can be used to model the spatial-temporal evolution of the magnetic diffusion process in a tokamak plasma. With the feedforward trajectories generated by numerical optimization, we derive a linear system to describe the error evolution. The weak variation approach is then extended to cylindrical coordinates to obtain a Riccati-type PDE for feedback kernel synthesis. Then, a finite difference method is used to give numerical solutions for the Riccati-type PDE. The obtained feedback kernel is finally used to simulate the closed-loop system. The simulation results demonstrate that the proposed tracking method can attenuate the tracking error effectively.

Modeling of the magnetic field diffusion in the high- Tc superconducting tube for fault current limitation

Operation of the superconducting fault current limiter of the inductive type is analyzed together with the magnetic diffusion process that occurs in the high- T c superconducting tube. Due to the highly non-linear voltage–current relation of the superconductor, a critical state model based on the power law is used to compute resistivity changes during transition from the superconducting state to the resistive state. The influence of the penetration depth, for various operating modes, on the magnetic flux density distribution across the SC tube wall is investigated using the slab geometry and a linear approximation of the magnetic diffusion process.

고온 초전도체 관에서의 과도 자기확산 해석

Transient magnetic diffusion process in a melt-cast BSCCO-2212 tube is analyzed by an analytical method. The transient diffusion partial differential equation is transformed into an ordinary differential equation by integral method. The penetration depth of magnetic field into a superconducting tube is obtained by solving the differential equation numerically. The results show that the penetration depth as a function of time which is somewhat different from the results by Bean's critical state model. The reason of the difference between the present results and that of Bean's model is discussed and compared in this paper.

Analysis on the cryogenic stability and mechanical properties of the LHD helical coils

Transient normal-transitions have been observed in the superconducting helical coils of LHD. Propagation of a normal-zone is analyzed with a numerical simulation code that deals with the magnetic diffusion process in a pure aluminum stabilizer. During excitation tests, a number of spike signals are observed in the balance voltage of the helical coils, which seem to be caused by mechanical disturbances. The spike signals are analyzed by applying pulse height analysis and the mechanical properties of the coil windings are investigated.

Electron heating process of the lower hybrid drift instability

In order to clarify the macroscopic magnetic field diffusion process associated with the lower hybrid drift and the drift kink instabilities, a large-scale two-dimensional electromagnetic full particle simulation has been performed. We find a new magnetic diffusion process which is caused by coupling of the lower hybrid and drift kink instabilities. This process can dissipate the cross-tail current much more efficiently than without coupling. Furthermore, we find that electrons are accelerated along the ambient magnetic field and form flat-top distributions at the outside edge of the plasma sheet in the case of oblique propagation. This electron acceleration may result from the wave-particle interaction between the obliquely propagating lower hybrid waves and electrons.

Propagation of neutralized plasma beams

Beams of neutralized plasma will cross a transverse magnetic field, magnetized plasma, and neutral gas, by a combination of single particle and collective-plasma processes. At low density the beam will deflect; at higher density the beam will exhibit undeflected motion by the E×B drift or diamagnetic (high-beta) exclusion of the magnetic field. In the high-beta regime, where β>1 (β is the ratio of plasma-beam energy density to magnetic field energy density), beam magnetization occurs on a time scale orders of magnitude faster than the classical Spitzer time scale. Recent studies of fast magnetization suggest that transport processes in large ion gyroradius beams, y/ρi<1, are dominated by ion motion rather than electron motion as previously understood for small gyroradius beams, y/ρi>1, where y and ρi are the beam transverse width and ion gyroradius, respectively. Although quantitative aspects of the small gyroradius propagation physics theory are still valid, most notably the E×B drift, much work remains to fully understand fast magnetic diffusion processes, beam-trapping mechanisms, and long-range propagation limits.

Physics of rail gun plasma armatures

High current rail gun plasma armatures are appropriately described by the MHD equations of motion, supplemented by radiation transport and Saha equilibrium. Procedures which allow accurate 3-D solutions to the governing equations are presented, as well as investigations of plasma-only projectile characteristics. Current research areas of concern, including turbulence effects, wall thermal loading, and magnetic diffusion processes are also discussed.