Magnetic Flux Compression Research Articles

Crossover from paramagnetic compressed flux regime to diamagnetic pinned vortex lattice in a single crystal of cubic Ca3Rh4Sn13

We report the observation of positive magnetization on field cooling (PMFC) in low applied magnetic fields (H < 100 Oe) in a single crystal of Ca(3)Rh(4)Sn(13) near its superconducting transition temperature (T(c) approx 8.35 K). For 30 Oe < H < 100 Oe, the PMFC response crosses over to a diamagnetic response as the temperature is lowered below 8 K. For 100 Oe < H < 300 Oe, the diamagnetic response undergoes an unexpected reversal in its field dependence above a characteristic temperature (designated as T*(VL) = 7.9 K), where the field-cooled cool-down magnetization curves intersect. The in-phase and out-of-phase ac susceptibility data confirm the change in the superconducting state across T*(VL). We ascribe the PMFC response to a compression of magnetic flux caused by the nucleation of superconductivity at the surface of the sample. In very low fields (H < 20 Oe), the PMFC response has an interesting oscillatory behaviour which persists up to about 7 K. The oscillatory nature underlines the interplay between competing responses contributing to the magnetization signal in PMFC regime. We believe that the (i) counterintuitive field dependence of the diamagnetic response for H > 100 Oe and above T*(VL) (lasting up to T(c), (ii) the oscillatory character in PMFC response at low fields and (iii) the PMFC peaks near 8.2 K in 30 Oe <= H <= 100 Oe provide support in favour of a theoretical scenario based on the Ginzburg-Landau equations. The scenario predicts the possibility of complex magnetic fluctuations associated with transformation between different metastable giant vortex states prior to transforming into the conventional vortex state as the sample is cooled below T*(VL).

The Plasma Jet/Laser Driven Compression of Compact Plasmoids to Fusion Conditions

Magneto-inertial fusion (MIF) is based on both magnetic and inertial confinement. An embedded magnetic field is compressed along with the target plasma to achieve magnetic insulation and fusion condition. Several magnetic systems for plasma confinement may be used for laser-driven (LD) and plasma jet driven (PJ) magnetic flux compression. Estimations show the possibility in principle to realize regimes of PJMIF system with a plasma gain factor Q > 10.

Detonating Cord for Flux Compression Generation using Electrical Detonator No. 33

The paper highlights the use of electrical detonators for magnetic flux compression generator applications which requires synchronisation of two events with precise time delay of tens of ms and jitter within a few ms. These requirements are generally achieved by exploding bridge wire type detonators which are difficult to develop and are not commercially available. A technique has been developed using commercially available electrical detonator no. 33 to synchronise between peak of seed current in stator coil and detonation of explosive charge in armature. In present experiments, electrical signal generated by self-shorting pin due to bursting of electrical detonator has been used to trigger the capacitor discharge and the detonating cord of known length has been used to incorporate predetermined delay to synchronise the events. It has been demonstrated that using electrical detonator and known length of detonating cord, the two events can be synchronised with predetermined delay between 31 and 251 ms with variation of ± 0.5ms. The technique developed is suitable for defence applications like generation of high power microwaves using explosive driven magnetic flux compression generators. Defence Science Journal, 2011, 61(1), pp.19-24 , DOI:http://dx.doi.org/10.14429/dsj.61.30

螺线管型爆磁压缩发生器快速计算模型

螺线管型爆磁压缩发生器快速计算模型

Conversion of plasma energy into electrical pulse by magnetic flux compression

A conceptual study of magnetic flux compression inside a cylindrical coil by an expanding inertial fusion plasma sphere across the magnetic field produced by the coil itself have been performed numerically using a two-dimensional magnetohydrodynamic (MHD) simulations. The concept may find application in inertial fusion energy (IFE) system as a direct energy conversion scheme to convert a part of fusion plasma kinetic energy into pulsed electrical energy. Important theoretical and technical issues that has to be addressed are discussed. Preliminary theoretical analysis are given for the analysis of MHD interchange instabilities of expanding plasma across magnetic field. Overall efficiency of the system is determined numerically for a typical set of initial plasma and system parameters. Ultrahigh coil inter-turn voltages are predicted. Therefore, the application of magnetic self-insulation to avoid coil inter-turn break-down is considered. Also, we have analysed the system performance with different load conditions.

Asymmetry in the bipolar signatures of flux transfer events

Several conceptual models have been proposed for the formation of flux transfer events (FTEs), including models based on reconnection at a single reconnection line (X line) and at multiple X lines. Two‐dimensional magnetohydrodynamic models have previously been used to simulate both scenarios and have found a tendency for FTEs generated by single X line reconnection to exhibit an asymmetry in the bipolar BN signature that is the major in situ signature of FTE structures, with the leading peak being substantially smaller than the trailing peak. On the other hand, simulated FTEs generated by multiple X line reconnection led to more symmetric signatures. We present a comparison of these simulation results with observations made at the Earth's magnetopause by the Cluster spacecraft, using a data set of 213 FTEs which were observed by all four spacecraft in 2002/2003 at the high‐latitude magnetopause near local noon and at low latitudes on the flanks, and 36 FTEs which were observed by one or more Cluster spacecraft near the subsolar point in 2007 and 2008. A tendency is found for the BN signatures to be asymmetric but with the leading peak larger in amplitude than the trailing peak, opposite to the prediction made by the 2‐D single X line simulations. This tendency is weaker in the subsolar FTEs. Therefore, the observations are not consistent with 2‐D MHD simulations of single X line reconnection. The signatures observed near the subsolar point are more consistent with those predicted by 2‐D simulations of multiple X line reconnection, although the multiple X line simulation studies did not report any net asymmetry. We propose that the observed asymmetry can be explained by a compression of magnetic flux ahead of the propagating FTE structure and a rarefaction behind it. The weaker tendency nearer the subsolar point is consistent with a weaker compression and rarefaction due to lower FTE velocities.

Potential micrometeoroid and orbital debris protection system using a gradient magnetic field and magnetic flux compression

A system for using a magnetic field in conjunction with conventional shielding configurations to protect against micrometeoroid and orbital debris is presented. Analytical, numerical, and experimental studies of a conductor moving through a gradient magnetic field have been performed. The results show that in the high magnetic Reynolds number regime a conducting object will experience large forces that tend to deform it while moving through the gradient field. Additionally a configuration using magnetic flux compression is introduced to act as a magnetic shock absorber. Separately or together, this technology may augment or replace current protection designs for space systems.

High-Magnetic-Field Generation by Magnetic Flux Compression in Imploding Plasma Liners

The MIG generator was used in a magnetic flux compression experiment to produce a high magnetic field inside a dielectric tube located along the axis of an imploding plasma liner. With a liner current of ~ 1 MA and a tube diameter of 2-5 mm, magnetic fields of up to 60 T with a quasi-steady pulse duration of 100 ns and more were produced. The high magnetic field region is protected from the liner plasma and radiation by the tube wall, and it can be used to study the physical phenomena that occur in high and ultrahigh magnetic fields. The physical processes responsible for the evolution of the magnetic field in a dielectric tube are discussed.

Miniature Explosively Driven High-Current Transverse-Shock-Wave Ferromagnetic Generators

Comprehensive studies of explosively driven ferromagnetic generators (FMGs) have been performed. It has been experimentally established that the initial magnetostatic energy stored in the magnetic element of the FMG (determined by the magnet's maximum energy product (BH)max and its volume), not the residual magnetic flux density Br, is the principal parameter that determines the output energy and the amplitude of the signals produced by the generator. Systematic studies were carried out with miniature high-current FMGs containing Nd2Fe14B high-energy hard ferromagnetic elements having a wide range of sizes. Utilization of transverse-shock demagnetization of Nd2Fe14B magnets (shock wave propagates across the magnetization vector \mmb M) instead of longitudinal-shock demagnetization dramatically changed the design of the FMGs and reduced the mass of the explosives used by two orders of magnitude in comparison with that used in longitudinal FMGs. Data for the initial magnetic flux and shock-induced magnetic flux change in Nd2 Fe14B magnets are presented. It was shown that FMGs with a volume of 25 cm3 are capable of producing pulsed currents with amplitudes of up to 4.4 kA and rise times of 23 μs in the seed coil of a magnetic flux compression generator.

Performance analysis of magnetic flux compression by plasma liner

The paper presents the results of the theoretical and numerical performance analysis of the experimental scheme for amplification of magnetic flux intensity via its compression by plasma liner. 0D estimations and 2D computations results are compared. The simulations were carried out with the use of RMHD code MARPLE (IMM RAS). The scheme performance affected by the Rayleigh-Taylor instability, developed in the case of initially disturbed plasma shell density, is studied. The possible penetration of the compressor shell plasma from the discharge chamber into the load area results in the nonuniformity of magnetic pressure in it. The simulation proves the possibility of elimination of this unwanted effect by proper selection of the experiment’s parameters. The correlation of the numerical results for this kind of problems using a simplified 0D model and 2D RMHD simulation with the MARPLE code are demonstrated. The prospects of the plasma magnetic flux compression scheme are discussed.

Effect of Wire Diameter and Addition of an Axial Magnetic Field on the Dynamics of Radial Wire Array $Z$-Pinches

The operation of radial wire array Z-pinches driven by a 1-MA 250-ns current pulse was studied. Variation in the cathode diameter and wire diameter does not affect the overall plasma dynamics but controls the time of wire breakage and the time of pinch formation. The measured times of full wire ablation at the cathode were used to determine the ablation velocity (V abl), and the results give a scaling V abl ~ (wire diameter)-0.46. Experiments with added axial magnetic field show an increase in the pinched plasma diameter, possibly due to the compression of the axial magnetic flux by the imploding plasma.

螺旋型爆磁压缩脉冲发生器电阻及磁通损失的数值计算

螺旋型爆磁压缩脉冲发生器电阻及磁通损失的数值计算

Generation of a strong magnetic field inside a dielectric tube

Experiments on compression of an axial magnetic flux by a plasma liner are conducted with the MIG high-current generator. It is shown that, when the current passing through the liner is about 1 MA, a magnetic field with a strength of up to 50 T and a duration of 300 ns (in the steady state) can be achieved in a dielectric tube 3–4 mm in diameter that is placed on the axis. The physics of magnetic field evolution in the tube is discussed.

Electric discharge caused by expanding armatures in flux compression generators

In this letter, we experimentally demonstrate that explosively driven expansion of metallic armature of the magnetic flux compression generator (FCG) plays a dominant role in the formation of plasma and electric discharge initiation inside the FCG.

Nonphysical Reverse Currents in Transient Finite-Element Magnetics Simulation

We are concerned with the simulation of low-frequency transient magnetic fields and currents in conductors as modeled by the vector diffusion equation. In particular, the key topic of this paper is the diffusion of fields and currents into a conductor due to a rapidly varying magnetic field in the surrounding air. As an example, an explosive magnetic flux compression generator may be designed to produce an exponentially increasing magnetic field in the in the air gap between armature and stator. As a second example, in a standard railgun the motion of the armature causes a rapidly increasing magnetic field in the air ahead of the armature, inducing currents in the rails ahead of the armature. It has been observed that for these types of transient magnetics problems finite-element simulations can produce nonphysical ldquoreverse currentsrdquo in conductors, currents that are flowing in the wrong direction. In this paper the root cause of these nonphysical reverse currents is explained, and several mitigation strategies are discussed.

Magnetic Flux Compression Generator as Future Military Pulsed Power Supply

As a kind of maneuverable pulsed power supply (PPS), the Magnetic Flux Compression Generator (MFCG) can be used in many military fields, but the MFCG concept has not been explored systematically. The definitions of MFCG were given in two ways: the electric circuit model and the magnetic field model, and the latter was indispensable to resolve many technical difficulties. According to the field model, the working principles of the MFCG with perfect solenoid were shown as the magnetic flux conservation or the magnetic flux linkage conservation. The inherent coherence of the two definitions was also outlined. The names of MFCGs were discussed, and the distinguishing features of MFCG from other pulsed generators were summarized. According to the different forms of the windings and the different moving modes of the armatures, 14 kinds of MFCGs were classified systematically, and their characteristics were generalized respectively. The most useful kinds of MFCGs were the piston type, the explosive one, and the rotary one. The key technology of the piston type MFCG was analyzed and the conclusion is that the conical armature without any longitudinal slits on its surface is necessary. The working process of the two stages explosive MFCG for the microwave bomb was reviewed: the first stage as a magnetic energy accumulator, and the second as an ordinary pulsed generator. The CPA, as a kind of rotary MFCG, was developed as a kind of drum alternator in the big ship to power the EM launcher. The essential work was important for MFCG concept, theory research, technology development, and military applications.

Finite difference time domain algorithm for electromagnetic problems involving material movement

The finite difference time domain (FDTD) method, adapted for magnetic field diffusion problems, is used to study the electromagnetic induction in moving materials by including motional emf in standard FDTD electromagnetic equations. The material movement is implemented by continuously changing material properties in each computational cell consistent to material advection. The flux corrected transport (FCT) algorithm is used to transport magnetic field in a fixed eulerian cell. A higher time-step is achieved by artificially increasing permittivity of the medium. This new approach is validated with standard analytical solutions for planar magnetic flux compression system and magnetic field diffusion in moving conductors with a non-relativistic velocity. To our knowledge, this is the first approach to use FDTD method for electromagnetic problems involving material motion.

Fast-charging compact seed source for magnetic flux compression generators

Flux compression generators (FCGs) are some of the most attractive sources of single-use compact pulsed power available today due to their high energy density output and mobility. Driving FCGs requires some seed energy, which is typically provided by applying a high seed current, usually in the kiloampere range for midsized helical FCGs. This initial current is supplied by a high-current seed source that is capable of driving an inductive load. High-current seed sources have typically been comprised of discharging large capacitors using spark gaps and overvoltage triggering mechanisms to provide the prime power for FCGs. This paper will discuss a recent design of a self-contained (battery powered with full charge time less than 40 s), single-use compact seed source (CSS) using solid-state components for the switching scheme. The CSS developed is a system (0.005 m(3) volume and weighing 3.9 kg) capable of delivering over 360 J ( approximately 12 kA) into a 5.20 muH load with a trigger energy of microjoules at the TTL triggering level. The newly designed solid-state switching scheme of the CSS incorporates off-the-shelf high-voltage semiconductor components that minimize system cost and size as necessary for a single-use application. A detailed evaluation of the CSS is presented primarily focusing on the switching mechanics and experimental characterization of the solid-state components used in the system.

Zero-dimensional simulations of wire array experiments and Z-pinch flux compression using the SPEED2 generator

Zero-dimensional simulations are used to gain an insight into the optimal setup for which the best energy transfer efficiency between the generator and the load system, and high currents at the moment of pinch are expected in wire array experiments using the SPEED2 generator. The generator is characterized by 4.1 μF equivalent Marx generator capacity, 300 kV maximum charge voltage, 187 kJ and 4 MA maximum current in short circuit at maximum charge voltage, 400 ns first quarter of period and dI/dt∼1013 A s−1. The model used in the simulations describes the dynamics of the current sheet coupled to the dynamics of the electric circuit. In order to have a better understanding of the generator behaviour when magnetic flux compression experiments using wire arrays are driven in SPEED2, the presence of an external axial magnetic field has also been considered. Based on the simulations results, a load design criteria for wire arrays experiments in SPEED2 is presented . The procedure to find a load design criterion presented here, is quite general and could be applied to experiments such as wire arrays or gas puff Z-pinches when these are driven by using low impedance capacitive generators.

The Challenge of Wall–Plasma Interaction with Pulsed Megagauss Magnetic Fields

A method is described for choosing experimental parameters in studies of high-energy-density (HED) physics relevant to fusion energy, as well as other applications. An important HED issue for magneto-inertial fusion (MIF) is the interaction of metal pusher materials with megagauss (MG) magnetic fields during liner compression of magnetic flux and fusion fuel. The experimental approach described here is to study a stationary conductor when a pulsed current generates MG fields at the surface, instead of studying the inner surface of a moving liner. This places less demand upon the pulsed power system, and significantly improves diagnostic access. Thus the deceptively simple geometry chosen for this work is that of a z pinch composed of a metal cylinder carrying large current. Consideration of well known stability issues for the z pinch shows that for given peak current and rise time from a particular power supply, there is a minimum radius and thus maximum B field that can be created without disruption of the conductor before peak current. The reasons are reviewed why MG levels of magnetic field, as required for MIF, result in high temperatures and plasma formation at the surface of the metal in response to Ohmic heating. The distinction is noted between the liner regime obtained with cylindrical rods, which have a skin depth small compared to the conductor radius, and the exploding thin-wire regime, which has skin depth larger than the wire radius. A means of diagnostic development is described using a small facility (DPM15) built at the University of Nevada, Reno. It is argued that surface plasma temperature measurements in the 10-eV range are feasible based on the intensity of visible light emission.