Multi-megagauss Magnetic Field Research Articles

Explosion of the thick metallic surface during ultrafast rise of a multimegagauss magnetic field

The technique of producing a multimegagauss magnetic field on the surface of a cylindrical conductor with a rise time of less than 1 ns has been examined experimentally on a high-current generator with a peak current of about 2 MA. In experiments in the conductor region, a plasma with a density of 1016–1017 cm−3 is preliminarily injected. A fast increase in the magnetic field on the conductor surface occurs, as the J × B force sweeps up the injected plasma in the axial and radial directions. It is observed that such a high rise-rate of the field is accompanied by an explosion of the current skin layer with the formation of a surface layer of a dense high-temperature plasma and a powerful pulse of soft x-ray radiation. The measured power of radiation emitted from the surface of a 1.1-mm diameter tungsten rod is 0.7 TW/cm, which corresponds to a blackbody temperature of about 65 eV.

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.

Multi-megagauss magnetic field generation by amplitude modulated surface plasma wave over a rippled metal surface.

The mechanism of a self-generated megagauss magnetic field by an amplitude modulated surface plasma wave (SPW) over a rippled metallic surface is proposed. The amplitude modulated SPW exerts a ponderomotive force on free electrons in the metallic ripple, giving them an oscillatory velocity at the modulation frequency. The oscillatory velocity couples with the electron density in metallic ripple and drives a nonlinear current J⃗(NL) with ∇×J⃗(NL)≠0. This irrotational nonlinear current generates the megagauss magnetic field.

The Evolving Structure of Plasma Formed From the Surface of Aluminum Rods Driven to Megaampere Current

Plasma formation from thick Al rods ohmically heated during the diffusion of pulsed multimegagauss magnetic field is examined experimentally. Rods in the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Z</i> -pinch configuration are driven to 1.0-MA peak current. The evolution of the resultant surface plasma is complex yet highly reproducible. Surface plasmas first form at discrete hot spots. As the pulsed current grows, plasma filaments form, first along the current, and then transverse to it. As the plasma temperature rises, emissions become increasingly uniform until instabilities grow and modulate the surface.

Generation and measurement of multi megagauss fields in inertial magnets

We present here the development of a facility to generate high (multi megagauss) magnetic field of 4 to 5µs rise time, using inertial magnets. The facility includes a low inductance, high current capacitor bank (280 kJ/40 kV) and an inertial magnet, which is a copper disk machined to have a keyhole in it. As the high current from the capacitor bank is discharged through the copper disk, a high magnetic field is produced along its axis, before it is destroyed by the combined effect of the dynamic loading and skin effect. A maximum peak magnetic field of 257 T is realized, when the magnet with 3.6mm inner diameter, 35mm outer diameter and 5mmlength, is powered by the capacitor bank charged to 28 kV (134 kJ). The transient magnetic field is measured using a B dot probe with an error of ±25 T. The probe in most of high field shots (> 200 T) got destroyed before recording the peak field and the trailing edge of the magnetic field. Experimental evidence of enhancement of the probe survival for longer time in copper disks using spatial non-uniform conductivity with 1mm thick SS brazed to the inner wall of the inertial magnet is also reported.

Experimental investigation of thermal plasma formation from thick aluminum surfaces by pulsed multimegagauss magnetic field

The thermal ionization of a thick metal surface by pulsed multimegagauss magnetic field has been examined experimentally. Thick 6061-alloy Al rods with initial radii (R0) from 1.00 to 0.25 mm, larger than the magnetic field skin depth, are pulsed to 1.0 MA peak current in 100 ns. Surface fields (Bs) rise at 30−80 MG/μs and reach 1.5 and 4 MG, respectively. For this range of parameters, plasma forms at a threshold level of Bs=2.2 MG. Novel load hardware ensures that plasma formation is thermal, by Ohmic or compression heating. Surface-plasma formation is conclusively indicated through radiometry, extreme ultraviolet spectroscopy, and gated imaging. When R0=0.50 mm rods reach peak current, Bs=3 MG, the surface temperature is 20 eV, and Al3+ and Al4+ spectra and surface instabilities are observed. In contrast, R0=1.00 mm rod surfaces [Bs(t)&amp;lt;2.2 MG] reach only 0.7 eV and remain extremely smooth, indicating that no plasma forms.

Measurement of hot electron transport in overdense plasma VIA self induced giant magnetic pulses

Spatial and temporal resolved ultrashort(8ps) multimegagauss(65 MG) magnetic field has been measured in plasma produced on Al-coated BK-7 glass by the interaction of a relativististic intensity laser(4x1018W/cm2, 30 fs) using pump-probe polarimetry. The 2D profile of magnetic field is captured using a CCD camera. Mapping of this magnetic field maps the transport of relativistic electrons in the plasma. The magnetic field profiles indicate filamentary behavior (Weibel-like instability). Particle in cell simulation are used to explain the result obtained.

Polarimetric detection of laser induced ultrashort magnetic pulses in overdense plasma

The interaction of intense (∼1016 W cm−2), subpicosecond pulses with solid targets can generate highly directional jets of hot electrons. These electrons can propagate in the solid along with the counterpropagating return shielding currents. The spontaneous magnetic field that is generated by these currents, captures in its time evolution, important information about the dynamics of the complex transport processes. By using a two pulse pump-probe polarimetric technique the temporal evolution of multimegagauss magnetic fields is measured for optically polished BK7 glass targets, each coated with a thin layer of either copper or silver. A simple model is then used for explaining the observations and for deducing quantitative information about the transport of hot electrons.

Temporally and spatially resolved measurements of multi-megagauss magnetic fields in high intensity laser-produced plasmas

We report spatially and temporally resolved measurements of self-generated multi-megagauss magnetic fields produced during ultrahigh intensity laser plasma interactions. Spatially resolved measurements of the magnetic fields show an asymmetry in the distribution of field with respect to the angle of laser incidence. Temporally resolved measurements of the self-generated third harmonic suggest that the strength of the magnetic field is proportional to the square root of laser intensity (i.e., the laser B-field) during the rise of the laser pulse. The experimental results are compared with numerical simulations using a particle-in-cell code which also shows clear asymmetry of the field profile and similar magnetic field growth rates and scalings.

Micronscale spatially resolved and femtosecond time resolved megagauss magnetic pulse in hot, dense plasmas

We report the measurement of spatially and temporally resolved evolution of the self generated magnetic field in laser generated solid density plasma on A1 coated glass target. In a two pulse experiment we capture the magnetic field generated by the main pulse in the change in ellipticity of polarisation of the probe pulse, which is incident at various time delays. The observed multi megagauss magnetic field has a rise time which is very short. The decay of the field occurs over a much longer time scale.

Pulsed-Power Hydrodynamics: An Application of Pulsed-Power and High Magnetic Fields to the Exploration of Material Properties and Problems in Experimental Hydrodynamics

Pulsed-power hydrodynamics (PPH) is an evolving application of low-impedance pulsed-power technology. PPH is particularly useful for the study of problems in advanced hydrodynamics, instabilities, turbulence, and material properties. PPH techniques provide a precisely characterized controllable environment at the currently achievable extremes of pressure and material velocity. The Atlas facility, which is designed and built by Los Alamos National Laboratory, is the world's first, and only, laboratory pulsed-power system designed specifically for this relatively new family of pulsed-power applications. Atlas joins a family of low-impedance high-current drivers around the world, which is advancing the field of PPH. The high-precision cylindrical magnetically imploded liner is the tool most frequently used to convert electromagnetic energy into the hydrodynamic (particle kinetic) energy needed to drive strong shocks, quasi-isentropic compression, or large-volume adiabatic compression for the experiments. At typical parameters, a 30-g 1-mm-thick liner with an initial radius of 5 cm and a moderate current of 20 MA can be accelerated to 7.5 km/s, producing megabar shocks in medium density targets. Velocities of up to 20 km/s and pressures of > 20 Mbar in high-density targets are possible. The first Atlas liner implosion experiments were conducted in Los Alamos in September 2001. Sixteen experiments were conducted in the first year of operation before Atlas was disassembled, moved to the Nevada Test Site (NTS), and recommissioned in 2005. The experimental program resumed at the NTS in July 2005. The first Atlas experiments at the NTS included two implosion dynamics experiments, two experiments exploring damage and material failure, a new advanced hydrodynamics series aimed at studying the behavior of particles of damaged material ejected from a free surface into a gas, and a series exploring friction at sliding interfaces under conditions of high normal pressure and high relative velocities. Longer term applications of PPH and the Atlas system include the study of material interfaces subjected to multimegagauss magnetic fields, material strength at high strain rate, the properties of strongly coupled plasmas, and the equation of state of materials at pressures approaching 10 Mbar.

Isochoric heating in heterogeneous solid targets with ultrashort laser pulses

We study ultrafast heating of thin plastic foils by intense laser irradiation theoretically using collisional two-dimensional particle-in-cell simulations. We find that the laser-generated hot electrons are confined laterally by self-generated resistive magnetic fields, heating the laser focal area beyond keV electron temperatures isochorically in a few picoseconds. Using this confinement one can excite shock waves that compress the plasma beyond solid density and achieve keV thermal plasmas before the plasma disassembles. Such shocks can be launched at material interfaces inside the target where jumps in the average ionization state and thus electron density lead to gigabar pressure. They propagate stably over picoseconds accompanied by multi-megagauss magnetic fields, and thus have a potential for various applications in high energy density physics.

Ultrahigh magnetic field diagnostic with spectral profile calculation

We report the theoretical results on use of neon as a tracer element to measure the multi-megagauss magnetic field, which is induced in the ultrahigh intense laser–matter interactions. The shape of Zeeman splitting of spectral line for 1s2p( 1P 1)→1s 2 transition of He-like neon are calculated for high-intensity laser produced quasi one-components plasma with the consideration of the electron collision broadening, electron collision shift and magnetic field splitting. The results show that all of the Zeeman splitting spectrum can be identified under Rayleigh criterion for the plasma with the electron temperature from 10 to 100 eV , the magnetic field from 10 6 to 10 8 G and the electron density 2×10 20 cm −3 . With both the electron temperature and magnetic field increasing, the requirement for the resolution power of the spectrometer decreases. If a spectrometer with the resolution power of 1/1000 is used, the measurement of the quasistatic magnetic field by Zeeman splitting of spectral lines is applicable when quasistatic magnetic field is larger than some tens of megaGauss.

Intense laser pulse propagation and channel formation through plasmas relevant for the fast ignitor scheme

Measurements of self-channeling of picosecond laser pulses due to relativistic and ponderomotive expulsion effects have been obtained in preformed plasmas at laser irradiances between 5–9×1018 Wcm−2. The self-focused channel was surrounded by a multi-megagauss magnetic field. The orientation of the field was consistent with a forward going relativistic electron beam propagating along the laser pulse. Self-channeling and magnetic field generation mechanisms were modeled by multidimensional particle-in-cell (PIC) simulations and good agreement was obtained with the experimental observations. Measurements of the channel expansion after the interaction were obtained and the rate of expansion was consistent with a blast wave solution. The level of transmission of an intense laser pulse through such performed density channels was observed to increase significantly compared to the case without a channel. High levels of transmission of an intense laser pulse through microtubes were also observed. The relevance of these results to the fast ignitor is discussed.

The cascade magnetocumulative generator of ultrahigh magnetic fields — A reliable tool for megagauss physics

The multimegagauss magnetic fields MC-1-generator in the form of several coaxial liners made of material with controlled conductivity and enveloped in a HE ring is described. The generator is characterized by exceptional reproducibility of initial and final parameters, is produced in series, is easy and simple in arrangement for experiments at any firing test-sites. Use of this generator for the study of properties of substance at the upper limit in magnetic field range known to the science is suggested and can be a basis for a broad international collaboration of specialists in ultrahigh magnetic field production and application.

Megagauss by implosion

The Magnet Laboratory of the Department of Physics of Illinois Institute of Technology, in conjunction with the Explosives Research Laboratory of the Illinois Institute of Technology Research Institute, is constructing a pulse-implosion facility for the production of multimegagauss magnetic fields. The principle used increases field strength by holding magnetic flux constant while the area available to the field is decreased. The procedure is to set up an initial transitory magnetic field, typically about 100 kG maintained over a few microseconds, in a solenoid which is contained in a cylindrical jacket having explosives in its walls. When the field is present, the explosives are triggered to compress the jacket and provide the necessary space contraction.