Flux Compression Research Articles

Cyclotron resonance and Faraday rotation in n-type CdS at ultra-high magnetic fields

We have performed cyclotron resonance and Faraday rotation experiments in n-type CdS at ultra-high magnetic fields far above 100 T which is generated by either electromagnetic flux compression or single turn coil technique. The electron effective mass of CdS is investigated for both B || c -axis and B⊥ c-axis over a wide range of the photon energy up to 225 meV , and significant polaron effect is observed in the vicinity of the LO phonon energy. In Faraday rotation measurements, the nonlinearity of the Faraday effect is clearly observed above 50 T by employing a wavelength close to the absorption edge of CdS.

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.

Upper critical field for optimally-doped YBa 2Cu 3O 7− δ

We have observed clear indication of the transition from super- to normal-state conductivity in optimally doped YBa 2 Cu 3 O 7−δ (T c ∼90 K) in high magnetic fields up to 400 T applied parallel to the CuO 2 plane (B || CuO 2) , using the electromagnetic flux compression system. In order to perform high-sensitivity magnetoresistance measurements in short-pulsed ultra-high magnetic fields, we developed a contactless radio frequency transmission technique. Therefore, a complete phase diagram of the upper critical field ( H c2) for optimally-doped YBCO was constructed as a function of temperature on both cases of B⊥CuO 2 and B || CuO 2 . In the previous experiment, we found that the H c2 for B⊥CuO 2 would attain to 120 T at T=0 K , and follow the WHH phase boundary where quenching of the superconductivity was mainly governed by the orbital effect. On the other hand, it turned out that the observed H c2 for B||CuO 2 would reach around 250 T at T=0 K , which is consistent with the WHH theory that takes into account the spin Zeeman and spin–orbit effect.

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.

Magneto-transport in HgSe based solid solutions

Magneto-transport and transmission data on Hg(Mn)Se–Fe semiconductor structures in high magnetic fields up to the multi-megagauss range are presented. The magnetic fields were generated either by conventional DC-superconducting solenoids, nondestructive solenoids in pulsed operation up to 40T, single-turn coil technique up to 140T, or explosive flux compression up to 450T. The samples investigated range from 3D systems in bulk, 2D in quantum wells, 1D in self-organized quantum wires, and 0D in quantum dots. The semiconductor structures of low dimensionality were grown by MBE. The different experimental results are compared with theoretical concepts.

Transport measurements of high-Tc superconductors at megagauss magnetic fields by means of a radio frequency transmission technique

Abstract We measured the upper critical field ( B ∥CuO 2 plane) of optimally doped YBCO thin films ( T c ∼90 K) in very high magnetic fields up to 200 T generated in the single-turn coil and up to 400 T by electromagnetic flux compression. In order to overcome experimental difficulties such as high voltage induced in the measuring circuit and eddy current heating of the sample, we used a contactless radio frequency transmission technique. Clear evidence of the superconducting to normal (S–N) transition was observed for the magnetic field perpendicular to the c -axis at various temperatures. The results are consistent with our previous data obtained by means of the microwave strip-line technique. Including the data from higher fields up to 400 T, a complete magnetic phase diagram on the B – T plane has been constructed for YBCO. In addition to the S–N transition, the observed signal was found to reflect the vortex dynamics.

Flux Compression in HTS Films

We report on experimental investigation of the effect of flux compression in superconducting YBa 2 Cu 3 O x (YBCO) films and YBCO/CMR (Colossal Magnetoresistive) multilayers. The flux compression produces positive magnetic moment (m) upon the cooling in a field from above to below the critical temperature. We found effect of compression in all measured films and multilayers. In accordance with theoretical calculations, m is proportional to applied magnetic field. The amplitude of the effect depends on the cooling rate, which suggests the inhomogeneous cooling as its origin. The positive moment is always very small, a fraction of a percent of the ideal diamagnetic response. A CMR layer in contact with HTS decreases the amplitude of the effect. The flux compression weakly depends on sample size, but sensitive to its form and topology. The positive magnetic moment does not appear in bulk samples at low rates of the cooling. Our results show that the main features of the flux compression are very different from those in Paramagnetic Meissner effect observed in bulk high temperature superconductors and Nb disks.

Hypervelocity powder deposition by flux compression devices : M.D. Driga et al. (University of Texas at Austin, Austin, USA)

Hypervelocity powder deposition by flux compression devices : M.D. Driga et al. (University of Texas at Austin, Austin, USA)

THz Magneto-Spectroscopy in Megagauss Fields

Recent progress of megagauss field generation and its application to terahertz (THz) spectroscopy are reviewed. A new device feed gap compensator is introduced, which is found to improve the implosion symmetry of electromagnetic flux compression. 600-T fields can be generated reproducibly by this device. Cyclotron resonance (CR) experiments of p-In 1-x Mn x As, n-Cd 1-x Mn x Te and n-ZnO in the THz range have been performed. It has been found that the electron mass is strongly dependent on the magnetic ion concentration. The heavy hole CR is observed at around 460 T; its cyclotron mass (0.455m 0 ) is much larger than the calcuclted value (0.28m 0 ) or the literature value (0.35m 0 ). This fact may indicate the break down of the effective mass theory. The electron cyclotron mass obtained at 10.6 μm (28.3 THz) in n-ZnO is found to be explained by the resonant polaron effect. We also performed the low-temperature THz electron spin resonance experiment up to 150 T. We found the spin-phonon coupled mode in CuGeO 3 and a collective excitation in NaV 2 O 5 above the critical field where the spin gap closes.

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.

Research in Super-High Pulsed Magnetic Fields at the Megagauss Laboratory of the University of Tokyo

We present in this paper a brief review of the facilities of the Megagauss Laboratory (MGL) of the University of Tokyo, and an overview of the highlights of solid state physics in megagauss fields at MGL. The Megagauss Laboratory has a long history since the first prototype was established in 1972 in order to pursue new phenomena which may occur in the extreme high magnetic field limit. Since then, it has achieved successful development in the last three decades, and very recently in 1999, it was enlarged and renewed at the Kashiwa new campus. At the MGL, pulsed high magnetic fields are produced by three different techniques: electromagnetic flux compression (EMFC), the single-turn coil (STC) technique, and non-destructive long pulse magnets. The highest magnetic fields of 622 T has been obtained by EMFC. By the STC technique, very high fields exceeding 300 T have been obtained depending on the coil bore, in two systems with horizontal and vertical coil axes. These fields are applied in many different kinds of experiments, such as optical spectroscopy, cyclotron and electron spin resonance, magnetization and transport measurements. Non-destructive long pulse fields up to nearly 70 T from wire-wound coils are also available.

Generation of 600 T by electromagnetic flux compression with improved implosion symmetry

We have developed a device for generating ultrahigh magnetic fields by means of electromagnetic flux compression. The device that we call “feed gap compensator” is a flux concentrator that consists of a thick-walled copper cylinder with a number of thin radial slits. This is inserted between the primary coil and the liner; it has resulted in a substantial improvement of the implosion symmetry that is normally disturbed by the feed gap of the primary coil. A maximum field exceeding 600 T can be generated reproducibly by using this device. Because of the improved symmetry of the liner motion, we observed turnaround phenomena not only in the wave form of the magnetic field but also in the photographs of the liner that are taken by a high-speed image converter camera. The final implosion speed and the turnaround radius have been determined by calculating the magnetic flux from a combination of framing camera data and the measured field. It is confirmed that the turnaround field is a function of the implosion speed; this function is given by the particle speed of the shock wave associated with the pressure pulse induced by the magnetic field. Further optimization of the flux compression system is discussed with a view to obtaining higher fields that are suitable for application in experiments.

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.

ADVANCES IN MEGAGAUSS FIELD GENERATION AND APPLICATION AT ISSP

We report recent progress in the generation and application of megagauss fields at the new Megagauss Laboratory of ISSP in Kashiwa. New facilities for studying physics in ultra-high magnetic fields were built at the new campus of the University of Tokyo at Kashiwa, where pulsed high magnetic fields are produced by three different techniques: electromagnetic flux compression, the single-turn coil technique and non-destructive long pulse magnets. Considerable progress has been made in each of these techniques, as compared with the old facilities at Roppongi. For electromagnetic flux compression, the rise time of the primary current was reduced by decreasing the residual inductance of the capacitor bank. The cylindrical symmetry of the liner implosion was remarkably improved by inserting a "feed gap compensator" between the primary coil and the liner. So far, a maximum field of 622T has been achieved. By the single-turn coil technique, very high fields exceeding 300T have been obtained depending on the coil bore, in two systems with horizontal and vertical coil axes. These fields are successfully applied in many different kinds of experiments. Non-destructive long pulse fields up to 60T are available from wire-wound coils.

Experimental measurements of a converging flux conserver suitable for compressing a field reversed configuration for magnetized target fusion

Data are presented that are part of a first step in establishing the scientific basis of magnetized target fusion (MTF) as a cost effective approach to fusion energy. A radially converging flux compressor shell with characteristics suitable for MTF is demonstrated to be feasible. The key scientific and engineering question for this experiment is whether the large radial force density required to uniformly pinch this cylindrical shell would do so without buckling or kinking its shape. The time evolution of the shell has been measured with several independent diagnostic methods. The uniformity, height to diameter ratio and radial convergence are all better than required to compress a high density field reversed configuration to fusion relevant temperature and density.

High-speed optical diagnostic of an exploding wire fuse

Explosive flux compression generators generate hundreds of kiloamperes and voltages of a few kilovolts. A power conditioning stage is required since typically voltages in the hundreds of kilovolts range are needed. Inductive energy storage systems with an opening switch provide the necessary voltage gain. In our application, the opening switch has been implemented as an exploding wire fuse. The voltage gain, and hence the performance of the system, is greatly dependent on the opening switch. We utilized high-speed optical imaging (up to 10/sup 7/ pictures/s) to assess the performance of the exploding wire fuse.

Desenvolvimento de comprimidos de AAS 500 mg: influência do Amido 1500® na compressão direta

The present work aims to show the technologic viability in the development of AAS 500 mg tablets using direct compression (CD) process. We employed Starch 1500 ® as excipient to develop the formulations, trying to investigate its compression, disintegration, aggregation and lubrication properties. Technologic, physical and chemical essays were performed using methodologies anticipated in pharmacology. Results demonstrated that the powder mix showed characteristics of free flux and good compression qualities very important for the excipients of direct compression in the market. Starch 1500 ® interfered in the disintegration time. The formulation with 6,35% of Starch 1500 ® proposed in this work obtained a highest proximity between the products in the Brazilian market

Excitons and magneto-absorption in Cd1−xMnxTe at 500 T-fields

The magneto-absorption spectra in ultra-high magnetic fields have been measured continuously as a function of magnetic field up to 500T using a combination of electromagnetic flux compression and the streak spectrometer. The absorption bands due to the exciton ground state and the inter-Landau level transitions have been observed. No significant anomaly in the field dependence of the peak energies of these absorption bands has been observed up to 500T. Two series of new absorption lines have been observed at the lower energy side of the exciton ground state in high fields over 150 and 350T. The field dependence of these new absorption peaks is considerably smaller than that of the exciton ground states. It is likely that the new peaks originate from some localized states related to the d states of the Mn ions that become stable in such high fields.