Flux Compression Research Articles

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.

Seeding magnetic fields for laser-driven flux compression in high-energy-density plasmas

A compact, self-contained magnetic-seed-field generator (5 to 16 T) is the enabling technology for a novel laser-driven flux-compression scheme in laser-driven targets. A magnetized target is directly irradiated by a kilojoule or megajoule laser to compress the preseeded magnetic field to thousands of teslas. A fast (300 ns), 80 kA current pulse delivered by a portable pulsed-power system is discharged into a low-mass coil that surrounds the laser target. A >15 T target field has been demonstrated using a <100 J capacitor bank, a laser-triggered switch, and a low-impedance (<1 Omega) strip line. The device has been integrated into a series of magnetic-flux-compression experiments on the 60 beam, 30 kJ OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. The initial application is a novel magneto-inertial fusion approach [O. V. Gotchev et al., J. Fusion Energy 27, 25 (2008)] to inertial confinement fusion (ICF), where the amplified magnetic field can inhibit thermal conduction losses from the hot spot of a compressed target. This can lead to the ignition of massive shells imploded with low velocity-a way of reaching higher gains than is possible with conventional ICF.

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.

Magnetization process of geometrically frustrated spinel oxide up to an ultra-high magnetic field

We conducted precise magneto-optical Faraday rotation measurements in CdCr2O4and ZnCr2O4up to ultra-high magnetic fields using methods of the single turn coil(up to 190 T) and also the electro-magnetic flux compression(up to 360 T). In CdCr2O4, we have succeeded in observation of the full magnetization process. In ZnCr2O4, we observed magnetization process up to 2.7 μB(the full moment is 3.0 μB). The magnetization processes of both materials were well described by a 4 sub-lattice Heisenberg-spin model including spin-lattice interactions.

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.

Utilization of a Nonexplosive Test Bed for Flux-Compression-Generator Electroexplosive Opening Switches

Helical flux compression generators (HFCGs) of a 50-mm form factor have been shown to produce output energies on the order of ten times the seeded value and a typical deposited energy of 3 kJ into a 3-muH inductor. One way to drive a high-power microwave source with an HFCG is by power conditioning, such as an inductive energy storage system (IESS). The output performance of the IESS is contingent upon the opening switch scheme, usually an electroexplosive fuse. Our previous work involving fuse parameter characterization has established a baseline for potential fuse performance. In order to optimize the electroexplosive wire fuse, we have constructed a nonexplosive test bed which simulates the HFCG output with high accuracy. We have designed and implemented a capacitor-based magnetic switching scheme to generate the near-exponential rise of the HFCG. The use of the nonexplosive HFCG test bed will allow the verification of scalability of the fuse parameter model and also allow testing of exotic fuse materials. The nonexplosive test bed has provided a more efficient method for electroexplosive switch development and has allowed us to expand the study of opening switches. We will also discuss the a priori calculated baseline fuse design and compare the experimental results of the gold-wire-material with the silver-wire-material baseline design. With the results presented, an accurate PSpice model applicable to our 45-kA HFCG systems will be available.

Compact Electroexplosive Fuses for Explosively Driven Pulsed Power

Compact electroexplosive fuses (EEFs) as part of an explosively driven system are of interest for the one-time single-shot generation of high-power pulses. For instance, the transition from a very large driving current produced by an explosively driven flux compression generator (FCG), i.e., low impedance, to a large voltage spike delivered to the load, i.e., high impedance, can be done using an inductive storage system and an EEF. Typically, the EEF can be as large as, if not larger than, the current driver attached to it, thus making it one of the largest components in the system. Reduction in the size of the fuse will allow for size reductions of the entire high-power microwave (HPM) system. The goal of optimizing an EEF as an opening switch is to produce the greatest voltage multiplication possible to drive a load under physical size constraints. To optimize the fuse, several parameters are taken into account, including, but not limited to, fuse material, fuse length, fuse shape, and quenching medium. Individual optimization of these parameters will lead to complete optimization of an EEF, therefore resulting in a compact fuse capable of consistently producing maximum voltage multiplication for HPM systems.

Improved Output Current Rise Time From Modified Helical Flux Compression Generators

Flux compression generators (FCGs) are widely used to generate extremely high power pulses. When a peak current from a seed source is flowing through the generator circuit, the chemical energy of high explosives is used to increase the amplitude of the output current pulse applied to the load. During the transfer of explosive energy to the output electrical pulse, the critical issue is achieving a change in the inductance in a fast and controlled way. There have been many significant experiments on different kinds of FCGs, and almost all of the theoretical models used to describe generator behavior are based on empirical equations. In this paper, simultaneous detonic, electromagnetic, and circuit simulations are used to study the characteristics of helical FCGs. For this purpose, the magnetic field diffusion and losses due to the induced currents in metallic components are considered in more detail. Based on the results of these theoretical considerations, new modifications for the FCG are proposed. It will be shown that the proposed modified FCG geometries yield output current rise times that are about 60% of those achieved with conventional helical FCGs generating the same current pulse amplitudes.

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.

Simultaneous Three-Dimensional Thermodynamic and Electromagnetic Simulation of Explosively Driven FCG Applied to Magnetic Coupled Plasma Modeling

In some plasma applications, such as confinement and pinches, an ultrahigh magnetic field (several teslas) is required. Such magnetic flux densities can be produced by using flux compression generators (FCGs). This paper has proposed a joint thermodynamic and electromagnetic simulation method for a common type of FCGs (i.e., helical type) with respect to the magnetic coupled plasma studies. The images of displacements, pressure, and magnetic flux density distribution in generator and plasma chamber are presented as simulation results.

Flux trapping and paramagnetic effects in superconducting thin films: The role of de Gennes boundary conditions

In the framework of the nonlinear time dependent Ginzburg–Landau (TDGL) equations we study the properties of a mesoscopic superconducting film with both surfaces in contact with different materials, whose properties are taken into account by de Gennes boundary conditions via the extrapolation length b. The vortex dynamics, the magnetization curves, vortex number and the modulus of the order parameter are studied as a function of the parameter b, which covers a broad range of positive and negative values (including the special cases b=0 and b→∞), the external magnetic field He and the temperature. We find that field cooled paramagnetic states, caused by flux compression, depends strongly on b and can be completely suppressed for low enough b values.

Sensitivity of $m = 0$ Instability to Different Resistivity Models

Instabilities can affect the quality of flux compression and other high-current experiments. The modeling of such experiments involves a number of numerical difficulties. One such difficulty is the selection of proper material property models. This paper investigates the effect of different electrical resistivity models on the development of the m = 0 instability in a single cylindrical conductor driven by a current typical of the Atlas generator. The nonlinear development of this magnetohydrodynamic (MHD) instability can lead to current disruption. An important challenge for modeling is to predict the maximum magnetic field on a rod surface that can be obtained prior to disruption. This problem can also be generalized to the more complicated moving liner problem. A series of 2-D MHD simulations was performed with the state-of-the-art MHRDR code to conduct a sensitivity study. Results indicate that the development and growth of m = 0 instability is sensitive to the resistivity models used. Furthermore, it was observed that models with lower values of resistivity near the metal-insulator transition produce higher growth-rate sausage instability.

A Zebra Experiment to Study Plasma Formation by Megagauss Fields

An alternative concept for fusion energy production is magnetized target fusion using metal liners to compress a mixture of magnetic flux and plasma fuel. In liner flux compression experiments, megagauss fields are produced at peak compression that heats the surfaces of aluminum walls of the liner cavity. Some radiation magnetohydrodynamic (MHD) modeling indicates that plasma formation should occur between 3 and 5 MG; however, such modeling depends on assumed material properties, which are a topic of ongoing research. Load hardware and diagnostics have been developed to study metal vapor and plasma formation on aluminum surfaces subjected to pulsed megagauss fields on the University of Nevada Zebra facility. The experiment is designed to study this interesting threshold for plasma formation. A current of 1 MA is pulsed along a stationary central rod to generate magnetic fields of 2-4 MG. The goal is to observe and diagnose the formation of metal vapor and plasma in the vicinity of the rod. The simple geometry enables easy access by diagnostics, which include magnetic sensors, filtered photodiode measurements, optical imaging, and laser schlieren, shadowgraphy, and interferometry. From these measurements, the magnetic field, the temperature of the surface metal plasma, the radiation field, and the growth of instabilities can be inferred. The diagnostics are time resolved to individually examine the distinct phases of heating, surface plasma formation predicted by radiation MHD modeling, and instability.

Results and Prospects of Material Strength Studies on Electrophysical Facilities Based on Perturbation Growth in Liner Systems

Data are presented describing refined simulations of perturbation growth in a three-layer cylindrical liner system tested with disk explosive magnetic flux compression generators (DEMGs) to study the strength properties of copper and polyethylene at shocklessly applied pressures of up to ~15 GPa. The calculated performance for the same liner system in the experiments to study the strength properties of copper at shockless loading of up to ~40-GPa pressures is presented. The feasibility of similar strength experiments with quasi-isentropic material compression to ~2000 GPa using DEMGs is demonstrated.

The Inductance Computation for the Passive Compulsator

In order to decrease the internal impedance of the passively compensated pulsed generator (or passive compulsator) and enhance its power density, air gap windings are generally selected as the armature winding of the passive compulsator, for its inductance is much lower than that of the general embedded windings in the machine slot. Due to that the peak power of an electromagnetic launcher system driven by a compulsator is simply determined by system impedance and the output voltage of the compulsator, it becomes necessary to gauge the total inductance of the air gap windings accurately with straight forward calculation, while neglecting the easily obtained resistance parameters, particularly in its design period for electromagnetic launchers. By taking into account the flux compression effects generated by the eddy current of the continuous conductive shield in the compulsator, the magnetic field of the armature coils in the air gap of generator is analyzed by Ampere's law. The analytical solution to the inductance of armature air gap coil is deduced. This one equation solution is used for re-designing a 25 MW compulsator with the final calculation result: 4.04 muH. The measurement on the compulsator (4.05 muH) verified the accuracy of the analysis equation.

CYCLOTRON RESONANCE IN MEGAGAUSS FIELDS

A review of magneto-optical experiments on semiconductor structures in megagauss fields is presented, featuring both the single-turn coil-generator and the explosive flux compression. Special emphasis is laid on the transient character of the fields resulting in unexpected and sophisticated response of the materials investigated.

Measurement of the upper critical field of optimally-doped YBa2Cu3O7-δ in megagauss magnetic fields

We have observed crossover from superconducting state to normal state in optimally doped YBa2Cu3O7−δ () in very high magnetic fields (B) up to 600 T applied parallel to CuO2 planes (). This is the first systematic critical field measurement at low temperatures in the geometry of the B ∥CuO2-planes using superconductors with Tc greater than 90 K. In order to perform high-sensitivity magneto-conductivity measurements in short-pulsed high magnetic fields, we developed a contactless radio frequency transmission technique. Ultrahigh magnetic fields higher than 200 T are generated by using electromagnetic flux compression, while a single-turn coil system is used for generating magnetic fields up to 200 T. We observed clearly the crossover from the superconducting to normal state conductivity at 250 T at a temperature of 5 K, and a systematic decrease of the critical field with increasing temperature. The field–temperature phase diagram was constructed over a wide temperature range between 5 K and 300 K.

Magnetic Field and Inductance Calculations in Theta-Pinch and Z-Pinch Geometries

Two codes have been developed to model solid metal or wire-wound conductors. The calculations are based on the decomposition of the conductors into arrays of thin wires. The first code, EDDY, models cylindrically symmetric conductors with currents in the theta direction. This code accurately models eddy current induction and magnetic diffusion. It was created in order to aid the design of magnetic-field shields in the FRX-L experiment for Magnetized Target Fusion (MTF). EDDY uses fast, accurate elliptic integral subroutines from MATLAB to solve for the time-dependent current flowing through each wire loop and the resultant magnetic field configuration. The second code, INDIV, models arbitrarily shaped conductors with current flow in the z direction. It was designed to model current division in an inductive divider that would inject current into a liner cavity, for magnetic flux and magnetized-plasma compression experiments. An experiment has been performed to test the INDIV code and the inductive division concept. The numerical results compare well with those of the experiment.