Magnetic Cumulation Research Articles

A Metropolis study of the isothermal magnetic entropy change in spin-1/2 Ising model

Magnetic refrigeration is a sound, environment friendly alternative to conventional refrigeration techniques. Isothermal magnetic entropy change, for a magnetic substance, is an important quantity to be investigated for determining its potential for Magnetic refrigeration. We have performed extensive Monte Carlo simulations with Metropolis single spin-flip algorithm on spin-1/2 Ising model on a square Bravais lattice to study the change in the magnetic entropy, ΔSm for five values of the external magnetic field, where spin-field energies are either equal to or greater than the in-plane ferromagnetic coupling strength. Binder’s fourth order magnetization cumulant is employed for the determination of the critical point. We observe the peak of ΔSm to coincide with the critical point or the Curie point. With a gradual increase in the magnitude of the field, the peak value of ΔSm increases. The behaviour of isothermal magnetic entropy change is systematic over the entire range of temperature, under observation. This implies such materials, which obey nearest neighbour Ising interactions, are good candidates for Magnetocaloric applications.

Andrei Sakharov’s research work and modern physics

We follow the work carried out by Andrei Sakharov in both research and engineering, from the first domestic thermonuclear charges to fundamental physics. We emphasize the current status of the research fields recognized to have originated in Sakharov’s work: controlled fusion, magnetic cumulation and magnetic explosion generators, induced gravity, cosmological ‘Sakharov’ (baryonic acoustic) oscillations, and baryon asymmetry of the Universe. Another subject that unexpectedly gained momentum in the 21st century is the model of a pulsating universe, which was among Sakharov’s ideas. Other subjects that were dear to him, such as quantum cosmology and the anthropic principle, are also currently at the forefront of science.

Specific Features of the Effect of Structural Defects on the Correlation and Interaction of Vortex Excitations in the Nonequilibrium BKT Dynamics of the Two-Dimensional XY Model

A Monte Carlo simulation of the nonequilibrium critical behavior is carried out for the two-dimensional structurally disordered XY model during its evolution from a high-temperature initial state. The features of the effect of structural disorder on the time dependence of the correlation length ξ(t) of the system and the dynamical scaling of the spin-spin autocorrelation function CSS(t, tw) are revealed. A direct calculation of the dynamic behavior of the correlation length ξ(t) of the two-dimensional structurally disordered XY model is carried out, and it is shown that this model, just as the pure model, exhibits a time dependence with a logarithmic correction ξ(t) ∝ (t/lnt)1/2, which is associated with the nonequilibrium annihilation of vortices and antivortices in the forming vortex pairs. Based on the analysis of the time dependence of the correlation length ξ(t) and the magnetization cumulant g2(t), it is shown that the two-dimensional XY model with the spin concentration p = 0.7 is so close to the spin percolation threshold pc that the influence of the attraction of the percolation fixed point becomes crucial for the relaxation properties of the system. However, the features of the critical dynamics of systems with spin concentrations of p = 0.9 and p = 0.8 are determined by the attraction of the pure fixed point. Temperature and concentration dependences of the Fisher critical exponent η(p, T) are determined using the scaling properties of the calculated two-time dependence of the spin-spin autocorrelation function CSS(t, tw). The scaling functions of the two-time dependence of the spin-spin autocorrelation function CSS(t, tw) are calculated using the dynamic dependence of the correlation length ξ(t) obtained as a result of simulation, and the values of the decay exponents λC(p, T) of the scaling functions in the long-time regime are determined, which are in good agreement with the relation λC(p, T) = 1 + η(p, T) and prove that the dynamical scaling is realized for the nonequilibrium characteristics of structurally disordered systems.

Cascade Magnetic Cumulation Generator on the Basis of Inductively Coupled Circuits with A Variable Coupling Coefficient

A scheme of a multicascade magnetic cumulation generator based on a dynamic variation of the coupling coefficient of inductively coupled circuits is proposed. Each cascade contains two circuits including two pairs of inductively coupled coils. One pair of coils is subjected to simultaneous deformation, and one of the coils in the other pair is arranged with the opposite connection. It is shown that the energy in the load can be gradually increased (from one cascade to another) by using additional cascades. By an example of a two-cascade system, the proposed circuit is compared to the known circuits of the cascade system design based on the magnetic cumulation principle (generator with a step-up transformer and dynamic transformer). Within the framework of the model that ignores the ohmic resistance of conductors, it is demonstrated that the proposed scheme allows one to obtain a greater energy in the high-inductance load than the schemes with a step-up or dynamic transformer owing to a change in the sign of the magnetic flux in the secondary circuit. It is found that the increase in energy in the new scheme is independent of the coupling coefficient (at high values of this coefficient) and becomes greater as the number of cascades is increased.

Pushing the limits of Monte Carlo simulations for the three-dimensional Ising model.

While the three-dimensional Ising model has defied analytic solution, various numerical methods like Monte Carlo, Monte Carlo renormalization group, and series expansion have provided precise information about the phase transition. Using Monte Carlo simulation that employs the Wolff cluster flipping algorithm with both 32-bit and 53-bit random number generators and data analysis with histogram reweighting and quadruple precision arithmetic, we have investigated the critical behavior of the simple cubic Ising Model, with lattice sizes ranging from 16^{3} to 1024^{3}. By analyzing data with cross correlations between various thermodynamic quantities obtained from the same data pool, e.g., logarithmic derivatives of magnetization and derivatives of magnetization cumulants, we have obtained the critical inverse temperature K_{c}=0.221654626(5) and the critical exponent of the correlation length ν=0.629912(86) with precision that exceeds all previous Monte Carlo estimates.

Generation of electromagnetic energy in a magnetic cumulation generator with the use of inductively coupled circuits with a variable coupling coefficient

A method of generation of electromagnetic energy and magnetic flux in a magnetic cumulation generator is proposed. The method is based on dynamic variation of the circuit coupling coefficient. This circuit is compared with other available circuits of magnetic energy generation with the help of magnetic cumulation (classical magnetic cumulation generator, generator with transformer coupling, and generator with a dynamic transformer). It is demonstrated that the proposed method allows obtaining high values of magnetic energy. The proposed circuit is found to be more effective than the known transformer circuit. Experiments on electromagnetic energy generation are performed, which demonstrate the efficiency of the proposed method.

Generation of Electromagnetic Energy in a Magnetic Cumulation Generator with the use of Inductively Coupled Circuits with a Variable Coupling Coefficient

Generation of Electromagnetic Energy in a Magnetic Cumulation Generator with the use of Inductively Coupled Circuits with a Variable Coupling Coefficient

An MC-1 cascade magnetocumulative generator of multimegagauss magnetic fields—ideas and their realization

The transformation of a brilliant idea of A.D. Sakharov about the magnetic cumulation, viz., an explosive conversion of the chemical energy of an explosive into the energy of a magnetic field, into a method for obtaining ultrastrong magnetic fields and the development of a physical instrument—a generator of reproducible multimegagauss fields—on the basis of this method resulted in the advancement of several ideas. The long-term work on the practical realization of these ideas for several years resulted in the development of a family of MC-1 cascade generators of magnetic fields of the 10- and 20-megagauss ranges, which are successfully used in systematic investigations of properties of substances under extreme conditions.

Magnetic cummulation generators applied for rapid charging of forming lines

We describe an off-line nanosecond charging device for a short forming line; the device has been created on the basis of an inductive energy storage unit. Energy storage is carried out by the current of an explosive magnetic cumulation generator, and the energy output to the load, by means of an electroexplosive current interrupter. The use of a two-stage, instead of a one-stage, magnetic cumulation generator, consisting of a preamplifier and a dynamic transformer, and two smaller-size generators with sequential connection of secondary windings of dynamic transformers, one of which is connected to the inductive storage unit, and the other, to the current interrupter—has made it possible to substantially increase the line charge voltage. As a result, within a time on the order of 100 ns, it was possible to charge a forming line with an electric length of 5 ns to a voltage of ≥1 MV.

Research in ultrahigh magnetic field physics

The history of the achievements of the All-Russian Research Institute of Experimental Physics (VNIIEF in Russ. abbr. )i n the field of ultrahigh magnetic field (UHMF) generation and applications in fundamental physical studies begins in 1952, when Andrey D Sakharov put forward the idea of magnetic cumulation as one of the possible methods for achieving a controlled thermonuclear reaction [1]. He also proposed two types of magnetocumulative generators of UHMFs (MC-1) and energy (MC-2) [1, 2]. In the first of them, a special device produces the initial axial magnetic field flux in the cavity of a cylindrical metal shell (liner). A converging detonation wave is initiated in a circular explosive charge surrounding the liner so that it arrives at the external boundary of the liner at the instant of time when the initial magnetic field in the liner achieves a maximum. Under the action of pressure of the detonation products, the liner collapses to the center, compressing the initial magnetic flux. If the compression is rapid enough,the magnetic flux inthe cavity is preserved, and themagnetic fieldstrengthonthelineraxisincreasesinversely proportionally to the squared radius of the liner, achieving a few megagausses. The chemical energy of the explosive is transformed into the magnetic field energy through the kinetic energy of the liner. Extensive attempts made in many countries to reproduce UHMFs by the explosive compression of a magnetic flux revealed unexplainable difficulties in obtaining magnetic fields exceeding 3 MG, which resulted in the termination of work in this field. 2. MC-1 cascade generator A group of researchers at VNIIEF headed by A I Pavlovskii proposed and realized a number of concepts supplementing and developing the magnetic cumulation idea and solved the problem of the reproducible generation of UHMFs. First, it was proposed to make the shells of the MC-1 generator from a material with a controllable electrical conduction. Such a material in the initial state is either completely nonconducting or conducts current only in one direction. At the required instant, a shock wave is passed through the material, making it conducting in all directions. For example, such a material can be produced from closely packed parallel isolated copper wires glued with an epoxy compound. Second, unique solenoids of the initial magnetic field in

Studying a spark discharge in the ground during reproduction of a negative lightning-current pulse using magnetic cumulation generators

Using an energy source based on a single BMΓ-160 generator, a current pulse with an amplitude of 50 kA and a rise time of 2 μs has been formed in a grounding rod electrode with a length of 1 m in the ground with a resistivity of 160 Ω m. The maximum length of spark channels on the ground surface from the point of current injection is 3–4 m. An energy source based on two BMΓ-160 generators connected in series allows an increase in the current pulse amplitude to 90 kA, the rise-time duration being the same, 2 μs. In this experiment, the maximum length of spark channels on the ground surface from the point of current injection exceeds 8 m.

A helical-radial magnetic cumulation fast-growing current pulse generator

A design-experimental study of a magnetic cumulation generator (further referred to as generator) with flat helical is performed. This generator combines advantages of both spiral-cylindrical and disk generators. As to speed of operation, the spiral-radial generator is equivalent to the disk generator, but can operate in high-impedance loads. To calculate characteristics of spiral-radial generators, including external-excitation generators, a semiempirical procedure for determining laws of variations of the inductances and mutual inductance of helical of the generator is developed. The procedure is based on measuring the current oscillation frequency in the course of the magnetic-field compression, while the helix is loaded into a small capacitance. A satisfactory compliance of the experimental data and semiempirical calculations is obtained. The experimental effective time values of the current buildup of the spiral-radial generators do not exceed the same characteristic for spiral-cylindrical generators with an axial charge initiation of the explosive material. A possibility of further decreasing the effective time is shown for the plane-parallel throwing of liner plates.

Model of shock-wave magnetic cumulation

A shock-wave magnetic cumulation technique is used to generate a megagauss magnetic field in an aluminium powder under quasi-cylindrical geometry. A magnetohydrodynamic (MHD) model of shock-wave magnetic cumulation is proposed. The model is based on the Oh–Persson equation of state and experimental data on electrical conductivity of powders. The 1D simulation reveals the features of the MHD flow under converging of the cylindrical shock wave. The experimental record of the magnetic field in the generator is in reasonable agreement with simulation results. This fact validates the model and allows one to optimize the cumulation system for producing a higher magnetic field.

Experimental study of shock-wave magnetic cumulation

Experiments with aluminum, copper, and silicon powders are performed to study the mechanism of shock-wave magnetic cumulation. For all substances examined, the magnetic field as a function of the cavity area is described by a power dependence with a constant exponent α. The value of α depends substantially on the substance porosity and particle size. For copper and silicon powders and for small-size aluminum powder, the value of α is consistent with the ratio of the particle velocity u to the wave velocity D, as is predicted by a simple model of magnetic cumulation. For the fine and coarse aluminum powders, the value of α is noticeably smaller than the ratio u/D. The lower effectiveness of magnetic compression can be attributed to insufficiently high electrical conductivity (fine powder) and the emergence of conductivity with incomplete compression of the substance (coarse powder). In the first case, diffusion losses of the magnetic flux in the compressed substance are fairly noticeable. In the second case, the work against the magnetic forces is performed by a layer in the shock-transition region, which has a lower particle velocity. The mechanism of magnetic cumulation involves substance metallization under shock compression and expelling of some portion of the magnetic flux to the non-conducting region ahead of the shock front. The two-stage mechanism of cumulation known in the literature (metallization in the elastic precursor and subsequent compression of the field in the main wave) is not validated by experiments with measurements of the particle and wave velocities and electrical conductivity and by experiments on magnetic cumulation.

Vladimir Konstantinovich Chernyshev

Vladimir Konstantinovich Chernyshev

THE SELF-ORGANIZED MULTI-LATTICE MONTE CARLO SIMULATION

Self-organized Monte Carlo simulations of 2D Ising ferromagnet on the square lattice are performed. The essence of the suggested simulation method is an artificial dynamics consisting of the well-known single-spin-flip Metropolis algorithm supplemented by a random walk on the temperature axis. The walk is biased towards the critical region through a feedback based on instantaneous energy and magnetization cumulants, which are updated at every Monte Carlo step and filtered through a special recursion algorithm. The simulations revealed the invariance of the temperature probability distribution function, once some self-organized critical steady regime is reached, which is called here noncanonical equilibrium. The mean value of this distribution approximates the pseudocritical temperature of canonical equilibrium. In order to suppress finite-size effects, the self-organized approach is extended to multi-lattice systems, where the feedback basis on pairs of instantaneous estimates of the fourth-order magnetization cumulant on two systems of different size. These replica-based simulations resemble, in Monte Carlo lattice systems, some of the invariant statistical distributions of standard self-organized critical systems.

Completely explosive pulsed power minisystem

It is demonstrated that it is feasible to produce pulsed power using an autonomous completely explosive system that harnesses two physical phenomena successively: the transverse shock wave demagnetization of Nd2Fe14B high-energy hard ferromagnets and magnetic cumulation.

Generation of magnetic field by detonation waves

The generation of a magnetic field by a system of detonation waves in a condensed explosive is reported. The convergence of the detonation waves, which exhibit a high conductivity in the chemical reaction zone, increases the magnetic field at the axis of the system. The fact of magnetic field generation is demonstrated experimentally. Features of the new method of magnetic cumulation are discussed. A simple compression model that qualitatively agrees with experimental data is proposed.

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.

Large-q expansion of the energy and magnetization cumulants for the two-dimensional q-state Potts model

We have calculated the large- q expansion for the energy cumulants and the magnetization cumulants at the phase transition point in the two-dimensional q-state Potts model to the 21st or 23rd order in 1 q using the finite lattice method. The obtained series allow us to give very precise estimates of the cumulants for q > 4 on the first-order transition point. The result confirms us the correctness of the conjecture by Bhattacharya et al. on the asymptotic behavior not only of the energy cumulants but also the magnetization cumulants for q → 4 +.