Self-generated Magnetic Field Research Articles

Modeling magnetic field amplification in supernova remnants driven by laser

Abstract The origin of magnetic fields and their amplification have always been hot topics in fields such as astrophysics and high-energy-density physics. Among them, the turbulent dynamo effect is an important candidate mechanism, and the interaction between supernova remnants (SNRs) is an important carrier for studying the amplification effect of turbulent magnetic fields. In this paper, we use the radiation magnetohydrodynamic simulation program to carry out a scaling simulation study on the amplification effect of turbulent magnetic fields in the interaction of SNRs driven by powerful lasers. We investigate and compare the evolution of turbulence under different laser driving methods, different directions, and different intensities of initial external environmental magnetic fields. Here, we carefully identify the contributions of Biermann self-generated magnetic fields and environmental magnetic fields in the process of magnetic field amplification, present magnetic energy spectra, and magnetic field amplification factors, and analyze the influence of radiative cooling effect on turbulence and magnetic field evolution. The results show that the collision direction component of the environmental magnetic field dominates the process of magnetic field amplification, and the frequency spectrum of turbulence is consistent with Kolmogorov’s law. The research results are necessary for sorting out and elucidating the physical mechanism of magnetic field amplification in SNRs, and have reference significance for regulating turbulence in strong magnetic fields in the future.

Karpman–Washimi ponderomotive force and self-generated magnetic field in nonextensive plasmas

The non-stationary Karpman–Washimi ponderomotive force and self-generated magnetic field in an unmagnetized system are investigated in the context of nonextensive distribution based on the kinetic theory. The ponderomotive force, magnetization, and radiation power are obtained as functions of the nonextensive parameter q, wave frequency, and wave number. It is shown that the presence of high-velocity electrons leads to an increase in temporal and spatial variation parts of ponderomotive force, magnetization, and radiation power. Furthermore, the results indicate that the self-generated magnetic field driven by the Karpman–Washimi ponderomotive force primarily manifests as small-scale and low-frequency magnetic field.

Self-generated magnetic field in laser plasma with super-Gaussian distributed electrons

Considering the effects of non-Maxwellian distributed electrons and the generation of magnetic field on the inertial confinement fusion driven by laser, the quasi-static magnetic field generated by nonlinear magnetization current in laser plasma with super-Gaussian distributed electrons is studied. Based on the kinetic theory, an analytical expression for the spontaneous magnetic field in Fourier space, valid for arbitrary frequencies, has been obtained. According to the existing experimental data, the effects of the frequency, plasma temperature, and super-Gaussian index on the spontaneous magnetic field are analyzed through numerical calculations. It is shown that the strength of the spontaneous magnetic field first decreases and then increases with the increase in the super-Gaussian index when the laser intensity is ∼1012W/cm2. It is about the order of 100 G, which is much smaller than the experimental observation. When the laser intensity is ∼1016W/cm2, the strength of the spontaneous magnetic field increases monotonically with the super-Gaussian index in the low-frequency region, and behaves similarly to the one of the laser intensity ∼1012W/cm2 in the high-frequency region. It will be as large as megagauss, which is consistent with the experimental observation results. Moreover, the strength of the spontaneous magnetic field increases with the increase in frequency and the decrease in the electron temperature.

Self-generated magnetic fields in the hot spot of direct-drive cryogenic implosions at Omega

Self-generated magnetic fields in the hot spot of direct-drive cryogenic implosions at Omega

The electron jet in relativistic laser-plasma with circular magnetic fields

Self-generated magnetic field and electron jet are observed when ultra-intense lasers (>1×1018W/cm2) interact with plasma. It is found that the self-generated magnetic field plays a significant role in the generation of electron jets. The generation mechanism of electron jets under the influence of a self-generated circular magnetic field is examined. It is revealed that magnetic modulation of self-generated magnetic fields can result in the collapse of these fields, consequently leading to the production of electron jets. Furthermore, it has been discovered that the velocity of the electron jets is associated with the maximum growth rate of the modulational instability. As the maximum growth rate of the modulational instability decreases, the velocity of the electron jets is reduced. The present work aids in getting a deeper understanding of the generation of electron jets in relativistic laser plasmas.

Optimal collimation of proton beams accelerated by a Laguerre–Gaussian laser with a proper pulse duration

In this study, three-dimensional particle-in-cell (PIC) simulations were conducted to evaluate a collimated proton beam accelerated by an intense Laguerre–Gaussian (LG) laser with different pulse durations. The flux and energy of the collimated proton beam could be simultaneously enhanced by selecting an optimal pulse duration. This phenomenon can be primarily attributed to the correlation between the LG laser driven self-generated magnetic field and pulse duration, and this correlation enables the collimation of protons during their interaction and transport. The results obtained in this study elucidate the formation mechanism of different collimated proton patterns, driven by femtosecond and picosecond LG lasers, observed in previous experiments. In addition, based on these results, an optimum pulse duration for high-quality proton beams is proposed for various future applications.

Charged particle transport coefficient challenges in high energy density plasmas

High energy density physics (HEDP) and inertial confinement fusion (ICF) research typically relies on computational modeling using radiation-hydrodynamics codes in order to design experiments and understand their results. These tools, in turn, rely on numerous charged particle transport and relaxation coefficients to account for laser energy absorption, viscous dissipation, mass transport, thermal conduction, electrical conduction, non-local ion (including charged fusion product) transport, non-local electron transport, magnetohydrodynamics, multi-ion-species thermalization, and electron-ion equilibration. In many situations, these coefficients couple to other physics, such as imposed or self-generated magnetic fields. Furthermore, how these coefficients combine are sensitive to plasma conditions as well as how materials are distributed within a computational cell. Uncertainties in these coefficients and how they couple to other physics could explain many of the discrepancies between simulation predictions and experimental results that persist in even the most detailed calculations. This paper reviews the challenges faced by radiation-hydrodynamics in predicting the results of HEDP and ICF experiments with regard to these and other physics models typically included in simulation codes.

Two-dimensional investigation of characteristic parameters and their gradients for the self-generated electric and magnetic fields of laser-induced zirconium plasma

Two-dimensional diagnosis of laser-induced zirconium (Zr) plasma has been experimentally performed using the time-of-flight method by employing Faraday cups in addition to electric and magnetic probes. The characteristic parameters of laser-induced Zr plasma have been evaluated as a function of different laser irradiances ranging from 4.5 to 11.7 GW cm−2 at different axial positions of 1–4 cm with a fixed radial distance of 2 cm. A well-supporting correlation between the plume parameters and the laser-plasma-produced spontaneous electric and magnetic (E and B) fields was established. The measurements of the characteristic parameters and spontaneously induced fields were observed to have an increasing trend with the increasing laser irradiance. However, when increasing the spatial distance in both the axial and radial directions, the plasma parameters (electron/ion number density, temperature and kinetic energy) did not show either continuously increasing or decreasing trends due to various kinetic and dynamic processes during the spatial evolution of the plume. However, the E and B fields were observed to be always diffusing away from the target. The radial component of electron number densities remained higher than the axial number density component, whereas the axial ion number density at all laser irradiances and axial distances remained higher than the radial ion number density. The higher axial self-generated electric field (SGEF) values than radial SGEF values are correlated with the effective charge-separation mechanism of electrons and ions. The generation of a self-generated magnetic field is observed dominantly in the radial direction at increasing laser irradiance as compared to the axial one due to the deflection of fast-moving electrons and the persistence of two-electron temperature on the radial axis.

Model of self-generated magnetic field generation from relativistic laser interaction with solid targets

Generation of self-generated annular magnetic fields at the rear side of a solid target driven by relativistic laser pulse is investigated by using theoretical analysis and particle-in-cell simulations. The spatial strength distribution of magnetic fields can be accurately predicted by calculating the net flow caused by the superposition of source flow and return flow of hot electrons. The theoretical model established shows good agreement with the simulation results, indicating that the magnetic-field strength scales positively to the temperature of hot electrons. This provides us a way to improve the magnetic-field generation by using a micro-structured plasma grating in front of the solid target. Compared with that for a common flat target, hot electrons can be effectively heated with the well-designed grating size, leading to a stronger magnetic field. The spatial distribution of magnetic fields can be modulated by optimizing the grating period and height as well as the incident angle of the laser pulse.

Performance improvement studies of axially partitioned dual band magnetically insulated line oscillator

In magnetically insulated line oscillator (MILO)s, a proportion of device current is responsible for the self-generated magnetic field that aids electron flow confinement in the anode cathode gap, thereby suitable RF generation. The device currents should not be high due to its direct impact on device efficiency, and electron energy depositions at load can lead to anode plasma formation. Also, the lower currents can facilitate the device to operate at larger voltages and/or for longer operational times. In this article, couple of axially partitioned dual band MILOs (DBMILO) designs that operate at lower device currents in comparison with the existing axially partitioned DBMILOs in the literature is presented. For one design with the radius of the slow wave structure (SWS), i.e., RSWS = 40 mm, the injection of the DC electron beam parameters 505 kV and 49 kA, generated an output RF power of ∼5.8 GW oscillating at 3.71 and 10.16 GHz (S-and X- bands) concurrently with ∼23.8% conversion efficiency. However, the other design (RSWS = 40.6 mm) at beam voltage 505 kV and device current of 47 kA generated an output RF power of >3.85 GW oscillating at 3.5 and 10.5 GHz, concurrently, where the frequencies are harmonically related. Thus, the proposed designs yielded significant device current reduction to ∼47 and ∼49 kA from ∼56 kA (existed), respectively, thereby drop of 16% and 12.5% to its counter designs, respectively. Also, there is substantial reduction in the electron energy depositions, thus the occurrence of anode plasma formations. Moreover, the peak conversion efficiency is enhanced to 23.4% and 16.4%, from 12.8%.

Characterization of bright betatron radiation generated by direct laser acceleration of electrons in plasma of near critical density

Directed x-rays produced in the interaction of sub-picosecond laser pulses of moderate relativistic intensity with plasma of near-critical density are investigated. Synchrotron-like (betatron) radiation occurs in the process of direct laser acceleration (DLA) of electrons in a relativistic laser channel when the electrons undergo transverse betatron oscillations in self-generated quasi-static electric and magnetic fields. In an experiment at the PHELIX laser system, high-current directed beams of DLA electrons with a mean energy ten times higher than the ponderomotive potential and maximum energy up to 100 MeV were measured at 1019 W/cm2 laser intensity. The spectrum of directed x-rays in the range of 5–60 keV was evaluated using two sets of Ross filters placed at 0° and 10° to the laser pulse propagation axis. The differential x-ray absorption method allowed for absolute measurements of the angular-dependent photon fluence. We report 1013 photons/sr with energies >5 keV measured at 0° to the laser axis and a brilliance of 1021 photons s−1 mm−2 mrad−2 (0.1%BW)−1. The angular distribution of the emission has an FWHM of 14°–16°. Thanks to the ultra-high photon fluence, point-like radiation source, and ultra-short emission time, DLA-based keV backlighters are promising for various applications in high-energy-density research with kilojoule petawatt-class laser facilities.

Probing the peripheral self-generated magnetic field distribution in laser-plasma magnetic reconnection with Martin–Puplett interferometer polarimeter

Magnetic reconnection of the self-generated magnetic fields in laser-plasma interaction is an important laboratory method for modeling high-energy density astronomical and astrophysical phenomena. We use the Martin–Puplett interferometer (MPI) polarimeter to probe the peripheral magnetic fields generated in the common magnetic reconnection configuration, two separated coplanar plane targets, in laser-target interaction. We introduce a new method that can obtain polarization information from the interference pattern instead of the sinusoidal function fitting of the intensity. A bidirectional magnetic field is observed from the side view, which is consistent with the magneto-hydro-dynamical (MHD) simulation results of self-generated magnetic field reconnection. We find that the cancellation of reverse magnetic fields after averaging and integration along the observing direction could reduce the magnetic field strength by one to two orders of magnitude. It indicates that imaging resolution can significantly affect the accuracy of measured magnetic field strength.

The importance of Righi–Leduc heat flux to the ablative Rayleigh–Taylor instability during a laser irradiating targets

Abstract The Righi–Leduc heat flux generated by the self-generated magnetic field in the ablative Rayleigh–Taylor instability driven by a laser irradiating thin targets is studied through two-dimensional extended-magnetohydrodynamic simulations. The perturbation structure gets into a low magnetization state though the peak strength of the self-generated magnetic field could reach hundreds of teslas. The Righi–Leduc effect plays an essential impact both in the linear and nonlinear stages, and it deflects the total heat flux towards the spike base. Compared to the case without the self-generated magnetic field included, less heat flux is concentrated at the spike tip, finally mitigating the ablative stabilization and leading to an increase in the velocity of the spike tip. It is shown that the linear growth rate is increased by about 10% and the amplitude during the nonlinear stage is increased by even more than 10% due to the feedback of the magnetic field, respectively. Our results reveal the importance of Righi–Leduc heat flux to the growth of the instability and promote deep understanding of the instability evolution together with the self-generated magnetic field, especially during the acceleration stage in inertial confinement fusion.

Wavefront distortion and compensation for weakly relativistic vortex beams propagating in plasma

The propagation of electromagnetic wave in plasma is one of the long-standing concerns in the field of laser plasma, and it is closely related to the researches of radiation source generation, particle acceleration, and inertial confinement fusion. Recently, the proposal of various schemes for generating intense vortex beams has led to an increasing number of researchers focusing on the interaction between intense vortex beams and plasmas, resulting in significant research progress in various areas, such as particle acceleration, high-order harmonic generation, quasi-static self-generated magnetic fields, and parametric instability. Compared with traditional Gaussian beams, vortex beams, featuring their hollow amplitudes and helical phases, can exhibit novel phenomena during propagating through plasma. In this work, we primarily focus on studying the influence of the propagation process on the wave structure of vortex beams before filamentation occurs. The three-dimensional particle-in-cell simulations show that weakly relativistic vortex beams exhibit wavefront distortion during their propagation in plasma. The distortion degree is closely related to the intensity of the electromagnetic wave and the propagation distance for a given plasma density. This phenomenon is theoretically explained by using a phase correction model that considers the relativistic mass correction of electrons. Additionally, we demonstrate that the wavefront distortion can be compensated for and suppressed by appropriately modulating the initial plasma density, as confirmed by three-dimensional particle simulations. The results of decomposing the wavefront into Laguerre-Gaussian (LG) mode components indicate that the wavefront distortion is primarily caused by high-order <i>p</i> LG modes, and it is independent of other <i>l</i> LG modes. Additionally, we extend the present investigation to the propagation of vortex beams in axially magnetized plasma, where the phase correction model can also effectively explain the occurrence of wavefront distortion. Our work can deepen the understanding of the interaction between plasma and strong vortex beams, and provide some valuable references for designing plasma devices serving as the manipulation of intense vortex beams in future research.

Screening-current-induced magnetic fields and strains in a compact REBCO coil in self field and background field

REBa2Cu3O7−x (REBCO) coated conductors, owing to its high tensile strength and current-carrying ability in a background field, are widely regarded a promising candidate in high-field applications. Despite the great potentials, recent studies have highlighted the challenges posed by screening currents, which are featured by a highly nonuniform current distribution in the superconducting layer. In this paper, we report a comprehensive study on the behaviors of screening currents in a compact REBCO coil, specifically the screening-current-induced magnetic fields and strains. Experiments were carried out in the self-generated magnetic field and a background field, respectively. In the self-field condition, the full hysteresis of the magnetic field was obtained by applying current sweeps with repeatedly reversed polarity, as the nominal center field reached 9.17 T with a maximum peak current of 350 A. In a background field of 23.15 T, the insert coil generated a center field of 4.17 T with an applied current of 170 A. Ultimately, a total center field of 32.58 T was achieved before quench. Both the sequential model and the coupled model considering the perpendicular field modification due to conductor deformation are applied. The comparative study shows that, for this coil, the electromagnetic–mechanical coupling plays a trivial role in self-field conditions up to 9 T. In contrast, with a high axial field dominated by the background field, the coupling effect has a stronger influence on the predicted current and strain distributions. Further discussions regarding the role of background field on the strains in the insert suggest potential design strategies to maximize the total center field.

Self-similar collapse in a circular magnetic field and electron jets by hybrid transverse plasmon

Based on a set of nonlinear coupling equations describing the interaction of the HF field, self-generated magnetic field, and ion-acoustic wave, the dispersion relation of hybrid transverse plasmon under the circular self-generated magnetic field is obtained. The analysis of magnetic modulation instability shows that the circular self-generated magnetic fields have the tendency to self-similar collapse which makes the electron escape along the axial region and form a collimated jet. In addition, the velocity of the electron jets is calculated, and the result is consistent with experimental observation. The present research may be applied to understand the dynamic process of electron jets produced in laser plasma.

Study on the characteristics of different species in the vacuum arc devices with deuteride cathode

To study the physical mechanism of the separation between heavy and light species in the vacuum arc devices with deuteride cathodes, a three-fluid model based on magnetohydrodynamic (MHD) theory is established. In the model, different kinds of species are considered to be different kinds of fluids, and their physical parameters are calculated separately. Moreover, the distribution of arc current is calculated by the generalized Ohm's law, and the ionization and recombination of species are taken into account. In the paper, the two cases where the cathode is Zr or ZrD0.67 are simulated, respectively. The results show that in the case of ZrD0.67 cathode, the separation of light and heavy species is remarkable. Because of D's lighter mass and lower mass-to-charge ratio, the distribution of it is more uniform. In addition, the differences between species also lead to large differences in other physical characteristics, such as ion velocity, ion temperature, and so on. Notably, the desorption and ionization of deuterium lead to a decrease in plasma temperature. The self-generated magnetic field of the arc has an inhibitory effect on the expansion of each species, and it is more obvious for ions with lower mass-to-charge ratio. The simulation results are consistent with the experimental results. The theoretical analysis can provide theoretical guidance for the improvement of vacuum arc devices with composite or gas-saturated cathodes.

Emergence of superconductivity in single-crystalline LaFeAsO under simultaneous Sm and P substitution

We report on the high-pressure growth, structural characterization, and investigation of the electronic properties of single-crystalline LaFeAsO co-substituted by Sm and P, in both its normal- and superconducting states. Here, the appearance of superconductivity is attributed to the inner chemical pressure induced by the smaller-size isovalent substituents. X-ray structural refinements show that the partial substitution of La by Sm and As by P in the parent LaFeAsO compound leads to a contraction in both the conducting Fe2(As,P)2 layers and the interlayer spacing. The main parameters of the superconducting state, including the critical temperature, the lower- and upper critical fields, as well as the coherence length, the penetration depth, and their anisotropy, were determined from magnetometry measurements on a single-crystalline La0.87Sm0.13FeAs0.91P0.09O sample. The critical current density (jc), as resulting from loops of magnetization hysteresis in the self-generated magnetic field, is 2 × 106 A/cm2 at 2 K. Overall, our findings illustrate a rare and interesting case of how superconductivity can be induced by co-substitution in the 1111 family. Such approach delineates new possibilities in the creation of superconductors by design, thus stimulating the exploration of related systems under multi-chemical pressure conditions.

Theory of magnetic turbulence and shock formation induced by a collisionless plasma instability

Magnetic fields are ubiquitous in universe, space, and laboratory plasmas. Especially, self-generated magnetic fields are important to know the mind of nature. The formation of Weibel-mediated collisionless shock is studied theoretically as a structure formation by the linear plasma wave growth, nonlinear saturation, and mode–mode coupling. Following a series of computer simulations and experimental studies of the physics, a simple model equation is proposed here to describe the time evolution of magnetic turbulence. Weibel instability is saturated by magnetic pressure, and thicker filaments continue to be generated by current coalescence (magnetic reconnection) mechanism. The model equation concludes the fact that the filament spacing increases linearly in time, and the magnetic energy power spectrum is given as Bk2 ∝ k−2. The time evolution of the turbulence is characterized with the cascade toward smaller k. Such inverse cascade is well-known in 2D hydrodynamic turbulence such as a typhoon or hurricane formation and is known to have Kolmogorov spectrum k−5/3. Although only a small difference in power, the physics of inverse cascades is very different as shown in the present paper. With use of Alfvén current limit condition, the criteria of collisionless shock formation are evaluated. The present theory is compared to corresponding experiments done with Omega and NIF lasers and a variety of PIC simulations. The theory is also applied to evaluate the strength of magnetic field near the shock front of the supernova remnant SN1006. The enhancement of magnetic field of about 25 μG is concluded in the present theory. Finally, a universality of the model equation is shown by applying the theory to the turbulent mixing due to Rayleigh–Taylor instability at the contact surface of two fluids in a gravitational or inertial force, which is very important in compressing plasma such as inertial confinement fusion by implosion. It is shown that the well-known evolution physics, mixing layer of the two fluids grows in proportion to (time)2, can be explained with the same model equation.

The effect of longitudinal static magnetic field on the radiation efficiency of high harmonics from overdense plasma

The effect of the magnetic field applied along the laser propagation direction on the radiation efficiency of high-order harmonics generated from laser-irradiated overdense plasma is investigated theoretically and numerically. We find that the external magnetic field can increase the transmittance of the overdense target, thereby dramatically enhancing the energy coupling between the laser and target. While for high-order harmonics of the laser reflected from the oscillating target, the radiation efficiency reaches the maximum when the cyclotron frequency of the electrons in the magnetized target approaches the laser frequency. This conclusion applies only to overdense plasmas targets. For targets with low reflectivity, the application of the magnetic field reduces the harmonic radiation efficiency due to the decrease of both the oscillating coherence and opacity of the target. This work provides a reasonable approach to improving the radiation efficiency of high-order harmonics and a method to estimate the magnitude of the self-generated magnetic field during intense laser–plasma interactions.