Fields In Laser-produced Plasmas Research Articles

High repetition rate exploration of the Biermann battery effect in laser produced plasmas over large spatial regions

Abstract In this paper we present a high repetition rate experimental platform for examining the spatial structure and evolution of Biermann-generated magnetic fields in laser-produced plasmas. We have extended the work of prior experiments, which spanned over millimeter scales, by spatially measuring magnetic fields in multiple planes on centimeter scales over thousands of laser shots. Measurements with magnetic flux probes show azimuthally symmetric magnetic fields that range from 60 G at 0.7 cm from the target to 7 G at 4.2 cm from the target. The expansion rate of the magnetic fields and evolution of current density structures are also mapped and examined. Electron temperature and density of the laser-produced plasma are measured with optical Thomson scattering and used to directly calculate a magnetic Reynolds number of $1.4\times {10}^4$ , confirming that magnetic advection is dominant at $\ge 1.5$ cm from the target surface. The results are compared to FLASH simulations, which show qualitative agreement with the data.

Mapping of azimuthal B-fields in Z-pinch plasmas using Z-pinch-driven ion deflectometry

B-field measurements are crucial for the study of high-temperature and high-energy-density plasmas. A successful diagnostic method, ion deflectometry (radiography), is commonly employed to measure MGauss magnetic fields in laser-produced plasmas. It is based on the detection of multi-MeV ions, which are deflected in B-fields and measure their path integral. Until now, protons accelerated via laser–target interactions from a point-like source have been utilized for the study of Z-pinch plasmas. In this paper, we present the results of the first Z-pinch-driven ion deflectometry experiments using MeV deuterium beams accelerated within a hybrid gas-puff Z-pinch plasma on the GIT-12 pulse power generator. In our experimental setup, an inserted fiducial deflectometry grid (D-grid) separates the imploding plasma into two regions of the deuteron source and the studied azimuthal B-fields. The D-grid is backlighted by accelerated ions, and its shadow imprinted into the deuteron beams demonstrates ion deflections. In contrast to the employment of the conventional point-like ion source, in our configuration, the ions are emitted from the extensive and divergent source inside the Z-pinch. Instead of having the point ion source, deflected ions are selected via a point projection by a pinhole camera before their detection. Radial distribution of path-integrated B-fields near the axis (within a 15 mm radius) is obtained by analysis of experimental images (deflectograms). Moreover, we present a 2D topological map of local azimuthal B-fields B(r,z) via numerical retrieval of the experimental deflectogram.

Extreme case of Faraday effect: magnetic splitting of ultrashort laser pulses in plasmas

The Faraday effect, caused by a magnetic-field-induced change in the optical properties, takes place in a vast variety of systems from a single atomic layer of graphenes to huge galaxies. Currently, it plays a pivot role in many applications such as the manipulation of light and the probing of magnetic fields and material's properties. Basically, this effect causes a polarization rotation of light during its propagation along the magnetic field in a medium. Here, we report an extreme case of the Faraday effect where a linearly polarized ultrashort laser pulse splits in time into two circularly polarized pulses of opposite handedness during its propagation in a highly magnetized plasma. This offers a new degree of freedom for manipulating ultrashort and ultrahigh power laser pulses. Together with technologies of ultra-strong magnetic fields, it may pave the way for novel optical devices, such as magnetized plasma polarizers. In addition, it may offer a powerful means to measure strong magnetic fields in laser-produced plasmas.

Space-time resolved measurements of spontaneous magnetic fields in laser-produced plasma

The first space-time resolved spontaneous magnetic field (SMF) measurements realized on Prague Asterix Laser System are presented. The SMF was generated as a result of single laser beam (1.315 μm) interaction with massive planar targets made of materials with various atomic numbers (plastic and Cu). Measured SMF confirmed azimuthal geometry and their maximum amplitude reached the value of 10 MG at the laser energy of 250 J for both target materials. It was demonstrated that spatial distributions of these fields are associated with the character of the ablative plasma expansion which clearly depends on the target material. To measure the SMF, the Faraday effect was employed causing rotation of the vector of polarization of the linearly polarized diagnostic beam. The rotation angle was determined together with the phase shift using a novel design of a two-channel polaro-interferometer. To obtain sufficiently high temporal resolution, the polaro-interferometer was irradiated by Ti:Sa laser pulse with the wavelength of 808 nm and the pulse duration of 40 fs. The results of measurements were compared with theoretical analysis.

Calculation of Self-Generated Magnetic Fields in Laser-Produced Plasmas

We develop an interactive procedure for estimating the self-generated magnetic fields in laser-produced plasmas using MATLAB software. We use this procedure for analyzing the digital images recorded employing a three-channel polarointerferometer to estimate the self-generated magnetic fields. The three regions of an image recorded using the three-channel polarointerferometer contain all the required information in the form of Faraday rotation (polarimetry), spatial profile of the incident probe beam (shadowgraphy), and electron-plasma density (interferometry). The computer program displays different portions of the image and allows the user to track the interference fringes interactively and displays the calculated spatial profiles of the electron density and the magnetic fields. Using this program, we estimate magnetic fields of the order of 2–4 MG at electron densities ∼ 1017 cm−3 in an aluminum plasma. The program replaces many calculations that have to be done manually.

First Measurements of Rayleigh-Taylor-Induced Magnetic Fields in Laser-Produced Plasmas

The first experimental demonstration of Rayleigh-Taylor-induced magnetic fields due to the Biermann battery effect has been made. Experiments with laser-irradiated plastic foils were performed to investigate these illusive fields using a monoenergetic proton radiography system. Path-integrated B field strength measurements were inferred from radiographs and found to increase from 10 to 100 T μm during the linear growth phase for 120 μm perturbations. Proton fluence modulations were corrected for Coulomb scattering using measured areal density profiles from x-ray radiographs.

Laser-Driven Proton Beams: Acceleration Mechanism, Beam Optimization, and Radiographic Applications

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper reviews recent experimental activity in the area of optimization, control, and application of laser-accelerated proton beams, carried out at the Rutherford Appleton Laboratory and the Laboratoire pour l’Utilisation des Lasers Intenses 100 TW facility in France. In particular, experiments have investigated the role of the scale length at the rear of the plasma in reducing target-normal-sheath-acceleration acceleration efficiency. Results match with recent theoretical predictions and provide information in view of the feasibility of proton fast-ignition applications. Experiments aiming to control the divergence of the proton beams have investigated the use of a laser-triggered microlens, which employs laser-driven transient electric fields in cylindrical geometry, enabling to focus the emitted protons and select monochromatic beamlets out of the broad-spectrum beam. This approach could be advantageous in view of a variety of applications. The use of laser-driven protons as a particle probe for transient field detection has been developed and applied to a number of experimental conditions. Recent work in this area has focused on the detection of large-scale self-generated magnetic fields in laser-produced plasmas and the investigation of fields associated to the propagation of relativistic electron both on the surface and in the bulk of targets irradiated by high-power laser pulses. </para>

Review on spontaneous magnetic fields in laser-produced plasmas: Phenomena and measurements

Large (megagauss) “spontaneous” magnetic fields are produced by laser–plasma interactions when a short, powerful laser pulse is focused to a small diameter onto a solid target. The relevance of these magnetic fields to inertial confinement fusion applications depends on the numerous ways in which they can affect laser–plasma interactions and the resulting plasma. Theoretical studies have dealt with a variety (thermal, radiative, and dynamo) of generation mechanisms and with the associated transport and instability phenomena. The fields, originally observed with small induction probes placed near the target, have been studied in the focal region by optical methods. These optical diagnostics have used Faraday rotation of a probing laser beam and Zeeman profiles of emitted spectral lines.

Phase-amplitude imaging: The fully automated analysis of megagauss magnetic field measurements in laser-produced plasmas

This paper describes the application of phase-amplitude imaging to the measurement of self-generated magnetic fields in laser-produced plasmas. The magnitude of the magnetic field is determined by polarimetry and the plasma density profile by interferometry. Both pieces of information are recorded on a single interferogram in a form that can be analyzed automatically using computer-based image processing. We describe a method for successfully determining the magnetic field profile in plasmas generated by 70-ps Nd laser pulses at high intensity on planar targets when plasma motion or turbulence introduces an additional difficulty by reducing the visibility of the fringes.

Scaling laws for a self-generated axial magnetic field in laser-produced plasma

The generation of an axial magnetic field caused by the interaction of standing waves formed by two counterpropagating, elliptically polarized electromagnetic waves with a laser-produced plasma has been investigated. Study of the variation of magnetization with different powers and wavelengths available from Nd–glass and CO2 lasers shows that induced axial magnetization increases linearly with an increase in power for all wavelengths and increases exponentially with the wavelengths for different laser powers at a particular plasma density. The variation of magnetization with plasma density reveals that the peak value of magnetization occurs below the critical density (nc) for a particular wavelength and power. Scaling laws for the induced axial magnetic field have been given. The poloidal fields arising from such mechanisms are important in the lateral energy and axial energy transport mechanisms. Such a field inhibits lateral energy transport, but axial energy propagation may be increased.

Comment on "Free-electron laser and laser electron acceleration based on the megagauss magnetic fields in laser-produced plasmas."

Comment on "Free-electron laser and laser electron acceleration based on the megagauss magnetic fields in laser-produced plasmas."

Effect of small-scale de magnetic fields on filamentation in laser-produced plasmas

A mechanism driving filamentation instability by self-generated small-scale magnetic fields in laser-produced plasmas is suggested with use of a simplified model. The wavelength dependence of the filamentation mechanism is very strong. The predictions of the model are consistent with experimental observations.

Convective Amplification of Magnetic Fields in Laser-Produced Plasmas by the Nernst Effect

A magnetic field $B$ is found to be convected by the electron heat flux ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{e}$ because of the Nernst effect, i.e., with a velocity ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{V}}}_{T} (\ensuremath{\sim}\frac{2{\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{e}}{3{n}_{e}{T}_{e}})$. In laser-driven ablation the convection is towards the overdense region and $B$ can be amplified by 10-100 times because $\ensuremath{\nabla}\ifmmode\cdot\else\textperiodcentered\fi{}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{V}}}_{T}&lt;0$, making $\frac{B}{{n}_{i}}$ and hence ${\ensuremath{\omega}}_{e}{\ensuremath{\tau}}_{e}$ almost constant for ${\ensuremath{\lambda}}_{\mathrm{mfp}}&gt;\frac{c}{{\ensuremath{\omega}}_{\mathrm{pe}}}$. The spatially extended and amplified magnetic field can inhibit hot-electron transport, reducing preheat. For ${\ensuremath{\omega}}_{e}{\ensuremath{\tau}}_{e}&gt;1$, the critical and ablation surfaces are brought closer together, which increases hydrodynamic coupling.

Decay instability of plasma waves into electron Bernstein waves in a laser-produced plasma

In the presence of self-generated magnetic fields in laser-produced plasmas, the mode-converted plasma waves near the critical layer decay parametrically into electron Bernstein waves and Langmuir sidebands. The threshold for this instability is easily exceeded at the currently employed power densities and the growth rate above the threshold ${\ensuremath{\gamma}}_{0}\ensuremath{\simeq}0.06{\ensuremath{\omega}}_{c}\frac{{v}_{0}}{{v}_{e}}$ for typical plasmas; ${v}_{0}$ and ${v}_{e}$ are the oscillatory and thermal speeds of electrons and ${\ensuremath{\omega}}_{c}$ is the cyclotron frequency. The electron cyclotron turbulence thus generated may have significant consequences on the energy deposition rate.

Comments on "Self-generated magnetic fields in laser-produced plasmas for metallic targets"

It is shown that the measurements of Edwards et al. do not demonstrate the existence of two independent self-generated magnetic fields in the laser-target interaction region. On the contrary, these results illustrate, once more, the two-phase expansion of the magnetic fields spatial configuration: diffusion within the photoionized background followed by convection with the laser plasma.

Studies of Spontaneous Magnetic Fields in Laser-Produced Plasmas by Faraday Rotation

An extensive study has been made of the large magnetic fields in the plasma near the focus of a high-irradiance Nd-laser beam. The dependence of the magnetic field on many experimental conditions is noted---particularly timing and the presence of a preformed plasma. The data were obtained with a Raman-shifted, three-channel Faraday-rotation diagnostic system.

Self-generated magnetic fields in laser-produced plasmas for metallic targets

Direct evidence is reported that associated with laser-produced plasma for metallic targets, the self-generated magnetic field consists of two identifiable parts having different radial and background pressure dependences as well as propagation speeds. For background pressures greater than about 0.1 mTorr and probe distances greater than 5 mm, two independent $\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{B}$ signals are observed. The ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{B}}_{1}$ field is laser driven and originates from a stream of electrons (${T}_{e}\ensuremath{\sim}10$ eV) moving counter to the incident laser beam. The ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{B}}_{2}$ field originates from the Korobkin effect and is associated with the expanding plasma. For the experiments reported here the magnetic fields are ${10}^{3}$ G or less.

Magnetic field generation due to radiation pressure in a laser-produced plasma

The role of radiation pressure gradients in directly generating magnetic fields in laser-produced plasmas is discussed. The mechanism is related to the thermal sources of magnetic field discussed in earlier work in that a solenoidal electric field is produced by the radiation pressure gradient.

Pressure Dependence of Self-Generated Magnetic Fields in Laser-Produced Plasmas

The systematic dependence of the magnitude of the self-generated magnetic fields of a laser-produced plasma on nitrogen background pressure has been investigated. At expansion distances of a few millimeters or more, the strongest fields were found to reside at the front of the streaming laser plasma. Magnetic fields were created in the laser plasma long after laser shut off by allowing the streaming plasma to impinge upon a glass plate.