Magnetic Flux Compression Research Articles

Magnetic flux compression Generators

Magnetic flux compression generators offer the largest pulsed power output per unit size or weight when compared with other more conventional systems. They have found widespread use as pulsed power sources for hydrodynamics programs and high magnetic field research at national laboratories or in commercial applications, including exploration for oil and minerals and mine detection. Also, due to their nature as a true one-time-use device with superior energy density, a large portion of applications is defense related. A variety of basic magnetic flux compression generator designs have been developed and tested during the past four decades. All of them rely on the explosive-driven deformation of a system of conductors having an initial, preferably large, inductance. The most successful basic design is the helical flux compression generator, which is capable of producing a high-energy output into large impedance loads, just as it is needed for a practical pulsed power source. This paper will review the advances and state of the art of primarily helical magnetic flux compression generators mainly developed as pulsed power sources and will offer new insights gained as a result of a recently completed five-year AFOSR/DoD Multidisciplinary University Research Initiative program that studied the basic physics and engineering aspects of helical flux compression generators.

An Energy Source Based on a Spiral Magnetic-flux Compression Generator with Simultaneous Axial Initiation of an Explosive Charge

A spiral magnetic-flux compression generator with simultaneous axial initiation of an explosive charge is treated. An external solenoid is used to develop the initial field in the generator. A theoretical model of this generator operating into inductive storage and to break point of the current circuit is described. Experimental results are given obtained in testing a generator with an inside diameter of the spiral of 115 mm and a working volume length of 330 mm. A 100-GW current pulse is obtained at a break point of electrically exploding copper conductors. The experimental results are compared with the results of numerical simulation of the generator operation.

A Simple Model for Simulation of Armature Expanding Process

We propose a scheme to simulate the dynamic process of the armatureexpansion of explosive magnetic flux compression generators. Values ofthe deflective angle and the velocity of expanding the innermost radiusof copper cylinder (or armature) caused by explosives detonation arecalculated, which are in good agreement with the experimental data. Thecalculated results show that there may be some dynamic processes thatexist during explosive detonation.

Conceptual design for a periodic permanent magnetic ring magnetic flux compression generator

In a lot of single-shot applications and experiments at remote locations where helical magnetic flux compression generator (HMFCG) provides the most practicable power, it is impracticable or inconvenient to use a capacitor bank to be the initial energy source. Homogenous permanent magnet has been used to be the initial energy source to drive HMFCG, but the energy density stored in a single permanent magnet is very low. The possibility of using periodic permanent magnetic ring structure(PPMR) to be initial energy source of an MFCG was analyzed theoretically and numerically. It was concluded that 1kJ initial energy can be provided to an HMFCG by the PPMR which weights less than 30kg and the outside radius less than 30 cm. In this paper, an axially detonated PPMR HMFCG structure is put forward and studied numerically, and its current amplification expression is reduced. Meanwhile some numerical examples for such an HMFCG are also provided. It is concluded that an experimental investigation of such an HMFCG design is worthwhile.

Confinement and compression of magnetic flux by plasma shells

Confinement and compression of magnetic flux by plasma shells is of interest for a variety of applications associated with keV x-ray production in pulsed-power driven Z-pinch plasma radiation sources (PRS). Confinement of a Bz field with a Bθ field of the pinch current can help stabilize the implosion of a shell from a large initial radius. Compression of the azimuthal magnetic flux in the PRS (secondary) circuit with a plasma shell driven by Bθ field of the primary circuit may represent a new opportunity for using a low-cost, relatively slow pulsed power to generate large amounts of keV x rays. The magnetic field has to be compressed and/or confined by low-mass plasma shells emitting soft x-ray radiation that limits the temperature and conductivity of the shell plasma. The thickness of a plasma shell is established self-consistently during the implosion, and it is not obvious whether or not the shell becomes thick enough to confine or compress the magnetic flux. The results of analytical theory and numerical simulations demonstrate that the flux-compressing capability of a low-beta plasma shell is surprisingly good because the shell is shown to dynamically adjust its thickness so that it always remains of the order of its skin depth. The self-similar profiles of confined magnetic field predicted by the theory are consistently reproduced in the simulations.

Study on the coil design of the reusable linear magnetic flux compressor

The reusable linear magnetic flux compressor has been regarded as a preferred pulsed power supply for an electric gun. A coil with continuously varying pitch and area of the conductor cross section is the main part of the compressor and needs to be optimized in order to obtain a required shape of the current pulse. For this purpose, a two-dimensional finite-element model was constructed and the magnetic flux diffusion, the current, and temperature distributions in the coil were simulated and the inductance of the coil was calculated. In order to verify the model, a nonuniform spiral test coil was designed. The inductance of the coil was measured by a transient oscillation method at various frequencies. A comparison between the calculated and measured results shows that the model is an effective tool for optimal design of the compressor coil.

Adiabatic, shock, and plastic work heating of solids and exploding metal cylinders

Solids subjected to high pressures, shocks, and/or deformation experience an increase in internal energy density and temperature due to adiabatic compression, shock heating, and plastic work heating, respectively. Analytic approximations are derived here for the internal energy and temperature changes that result from these processes based on the analytic constitutive model and Gruneisen equation of state of Steinberg. Although of general use, the utility of the expressions is demonstrated by the detailed example of a cylindrical metal tube filled with high explosives, and detonated on axis at one end. This geometry is often used to determine the detonation properties of high explosives, where it is known as the cylinder test. The geometry is also of special interest for use as the armature of cylindrical magnetic flux compression pulsed current generators. The results are favorably compared with two dimension numerical simulations of the process using Lawrence Livermore National Laboratory's shock-hydro computer code CALE using the same model for the metal.

Thermodynamic state of the magnetic flux compression generator volume

The thermodynamic state of the gas trapped in the volume of helical magnetic flux compression generators was measured using optical emission spectroscopy and fast pressure probes. Three main stages of operation are discussed: (1) the initial stage, which can be represented by a freely expanding armature, that shows fairly low gas temperatures, as low as 2000 K; (2) the intermediate stage during 14-4 /spl mu/s before generator burnout that exhibits mainly an atomic copper line transition at about 0.8 eV; (3) the last few /spl mu/s that reveal a highly compressed gas with temperatures of about 5000 K and pressures of about 1500 bar. Most experiments were conducted in air, initially at STP, some results are given for argon and sulfur hexafluoride initially at one atmosphere. Additionally, the thermodynamic state is linked to the electrical volume breakdown threshold via simple resistance measurements that were conducted in current-free flux compression generators.

MEGAGAUSS CYCLOTRON RESONANCE IN SEMICONDUCTOR NANOSTRUCTURES AND DILUTED MAGNETIC SEMICONDUCTORS

We report the latest results of cyclotron resonance experiments on semiconductor nanostructures and diluted magnetic semiconductors (DMS) in very high magnetic fields up to 600 T produced by magnetic flux compression and the single turn coiled technique. Many new features were observed in the very high field range, such as characteristic behavior of low dimensional electrons, carrier dynamics or electron-electron interaction effects in quantum wells and quantum dot samples. In PbSe/PdEuTe quantum dots, which were regularly arranged to form an fcc superlattice, we observed an absorption peak with a splitting and a wavelength dependence of the absorption intensity. In DMS, such as CdMnTe and InMnAs, change of the carrier effective mass with Mn doping was studied in detail. We found anomalous mass increase with doping of magnetic ions. The amount of the observed mass increase cannot be explained by the k·p theory and suggests the importance of d-s or d-p hybridization.

The different dynamical modes of nested wire array Z pinches

Magnetohydrodynamic simulations are used to predict which modes of implosion are in operation in nested wire array experiments. Separate r–θ plane simulations of the flux of plasma imploding towards the axis from the outer array and the bombardment of the inner array by this flux are presented. The different implosion modes are distinguished by the level of momentum transfer and magnetic flux compression during collision. For typical configurations, large interwire gaps (>1.5 mm) are required in the inner array in order to access the advantageous mode where the outer array material passes through.

A mathematical model of the reusable linear magnetic flux compressor

The reusable linear magnetic flux compressor not only can offer fairly high energy density and work repeatedly, but also can be statically inert prior to firing. Therefore, it can be regarded as an ideal pulsed power supply for the electric gun. Based on theoretical studies of this magnetic flux compressor, a mathematical model is developed and described in this paper. This model includes a 2-D hydrodynamic treatment, armature motion, dynamic inductance, dynamic resistance, flux loss, circuit quantities and 1-D diffusion for the magnetic field, current and temperature inside the conductors. In this model, the gas pressure, which depends on the propellant burn law, propellant mass, armature position and armature velocity, is modeled by 2-D homogeneous phase fluid-dynamic equations. The electromagnetic force, which depends on the current and the inductance gradient of the helical winding, is described by electromagnetic parameter equations. The 2-D homogeneous phase fluid-dynamic equations are coupled with electromagnetic parameter equations through the armature motion equation. The simulation results show that the proposed model is helpful to the practical design.

Effect of mutual inductance on the pulsed current amplification of magnetic flux compressors

The reusable linear magnetic flux compressor is an ideal pulsed power supply for the electric gun. Based on the theoretical analysis of the model of the reusable linear magnetic flux compressor, which was completed by the authors, it is found that the pulse transformer not only can extend the application field of this flux compressor, but also can increase the current amplification. In order to verify this result, the authors used the computer code RMFC-I to perform some numerical experiments. The results of these numerical experiments show the same conclusion.

Electrical behavior of a simple helical flux compression generator for code benchmarking

A variety of basic magnetic flux compression (MFC) generator geometries have been tested during the last three decades. Though size and operating regimes differ widely, it is apparent that the helical flux compression generator is the most promising concept with respect to current amplification and compactness. Though the geometry of the helical generator (dynamically expanding armature in the center of a current carrying helix) seems to be basic, it turns out that the understanding of all involved processes is rather difficult. This fact is apparent from the present lack of a computer model that is solely based on physical principles and manages without heuristic factors. A simple generator was designed to address flux and current losses of the helical generator. The generator's maximum current amplitude is given as a function of the seed current and the resulting seed-current spread is compared to the output of state-of-the-art computer models. Temporally resolved current and current time derivative signals are compared as well. The detailed generator geometry is introduced in order to facilitate future computer code bench marking or development. The impact of this research on the present understanding of magnetic flux losses in helical MFC generators is briefly discussed.

Field-induced valence transition of EuNi 2(Si 1− xGe x) 2 in ultra-high magnetic fields

The EuNi 2(Si 1− x Ge x ) 2 system exhibits a temperature- and field-induced valence transition from Eu 3+ to Eu 2+ in the concentration range 0.75⩽ x⩽0.85. In this study, we performed magnetization measurements on compounds with 0.5⩽ x⩽0.75 in ultra-high fields up to 300 T generated by explosive compression of magnetic flux. We observed a jump of the magnetization, indicating a field-induced valence change from Eu 3+ to Eu 2+ for each compound. The critical field increases linearly with decreasing x and reaches a value of 225 T for x=0.5.

Overview of the electric launch technology program in China

Since the 9th EML technology symposium, the electric launch technology program in China has been focused on the two aspects: basic and key issues in military applications of the EML technology and civilian applications of the EML technology. The first aspect involves the investigations of the erosion of the rail electrodes at the breech, the velocity skin effort, triggered vacuum switches, the influence of electrical energy input on the combustion behavior of propellant, the best wave form of the pulsed power supply for the ETC launcher and the mathematical model and a computer simulation code of the reusable linear magnetic flux compressor for electric launchers applications. The second aspect involves the investigations of the linear electromagnetic pumping unit, which is being developed based on the EML technology and experiments were conducted to test and verify the performance of the prototype model.

On one possibility of cascade MC-1 generator final-magnetic field increasing

The problem of magnetic flux compression by a thin-walled metal (tungsten) cylinder imploded with the cascade magnetocumulative MC-1 generator was numerically considered. The initial magnetic field diffuses into the cylinder during the MC-1 generator powering-time interval and the slow phase of magnetic flux compression by the MC-1 generator. Then the cylinder rapidly gathers a velocity approximately 1.5 times more than the velocity of the MC-1 generator liner and amplifies the magnetic field on the axis up to about 20 MG. The optimum ratio of cylinder parameters and MC-1 generator operation regime was determined using a numerical one-dimensional device model in order to obtain maximum final magnetic field in a small volume, but still suitable for applications with a diameter of about 3 mm.

Comparison of sulfur hexafluoride and synthetic air in a simulated flux compression generator environment

Expendable, explosively driven shock tubes are used to propel aluminum flyer plates toward dense stainless steel plugs to simulate gas compression in explosive magnetic flux compression generators (FCGs). Armature velocity in a typical FCG is calculated using the Gurney method and shock tubes are designed to drive the flyer plates at similar velocities. Experiments with sulfur hexafluoride and synthetic air are presented.

Two alternatives of magnetic cumulation

This paper deals with phenomena leading to a considerable increase in magnetic field and energy density during compression of a magnetic flux trapped by a conducting shell and joint deformation of a magnetic field and material. The main features and merits of these two alternative schemes of magnetic cumulation are discussed. A comparison is made between the classical and schock-wave schemes of magnetic compression in a material with a phase transition from a nonconducting to a conducting state. The possibility of magnetic-energy cumulation during stretching of magnetic field line by a transverse flow of a conducting material is considered.

Two different modes of nested wire array Z-pinch implosions

Two different modes of nested wire array implosion driven by a 1-MA, 240-ns current pulse were observed, determined by the fraction of total current induced in the inner array. Penetration by the outer array through the inner with switching of current occurred if current in the inner array was initially suppressed. Simultaneous implosion of arrays with apparent compression of magnetic flux between the arrays was observed if approximately 20% of the current was in the inner array. In both cases the x-ray pulse rise time of approximately 10 ns (for 260-ns implosion time) was considerably smaller than for a single array.

Energy conversion and high power pulse production using miniature magnetic flux compressors

The design, construction, and testing of miniature, high-power magnetic flux compressors are presented and discussed. The magnetic flux compressors are located inside high-speed, 30-mm projectiles that are launched with a high-pressure helium gun to velocities of approximately 300 m/s. The large deceleration force created when the projectile impacts the ground is used to power the magnetic flux compressor. Initial magnetic flux compressor currents of 200 mA are amplified up to 230 times to reach a peak current approaching 50 A. The output peak power is on the order of 3.5 kW into a 1-/spl Omega/ load with a pulse length of 100 to 150 /spl mu/s. Good agreement exists between the experimental results and the theoretical predictions for peak output current.