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

Stamper, J. A.

doi:10.1017/s0263034600006595

Cited by 180 publications

(94 citation statements)

References 81 publications

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“…Such a field is higher than both the self-generated magnetic fields (see Ref. [5]) and the external fields that can be generated by coils. Magnetic-flux compression [6] is a viable path to generating tens of MG magnetic fields with adequate size compression of a metal liner driven by high explosives [7,8] or by pulsed power.…”

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confidence: 90%

Laser-Driven Magnetic-Flux Compression in High-Energy-Density Plasmas

et al. 2009

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The demonstration of magnetic field compression to many tens of megagauss in cylindrical implosions of inertial confinement fusion targets is reported for the first time. The OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] was used to implode cylindrical CH targets filled with deuterium gas and seeded with a strong external field (>50 kG) from a specially developed magnetic pulse generator. This seed field was trapped (frozen) in the shock-heated gas fill and compressed by the imploding shell at a high implosion velocity, minimizing the effect of resistive flux diffusion. The magnetic fields in the compressed core were probed via proton deflectrometry using the fusion products from an imploding D 3 He target. Line-averaged magnetic fields between 30 and 40 MG were observed. In the magnetic fusion energy (MFE) concept, a strong magnetic field confines the plasma and reduces the electron thermal conduction to the vessel wall [1]. The magnetic pressure of typical $0:1-MG fields is higher than the total energy density of the plasma (with ¼ 2 0 p=B 2 < 1). MFE plasmas are fully magnetized and characterized by a Hall parameter ! ce > 1 since the modest gyrofrequency ! ce is matched by long collision times . In contrast, typical inertial confinement fusion (ICF) plasmas have collision frequencies higher by 10 to 12 orders of magnitude because of their extreme density. In such systems, thermal conduction losses are a major factor in the energy balance of an implosion. While it can be more difficult, magnetizing the hot spot in ICF implosions can lead to improved gains and to a reduction of the energy required for ignition. A similar approach is used in the magnetized target fusion concept [2], where the fusion burn requires relatively low-implosion velocities, provided there is an adequate magnetic thermal insulation. In ICF implosions, lower implosion velocities lead to higher gains [3]. However, tens of MG are needed to achieve ! ce $ 1 in the hot spot of a typical, direct drive DT ignition target [4] with hot-spot density of $30 g=cc and a temperature of $7 keV. Such a field is higher than both the self-generated magnetic fields (see Ref. [5]) and the external fields that can be generated by coils. Magnetic-flux compression [6] is a viable path to generating tens of MG magnetic fields with adequate size compression of a metal liner driven by high explosives [7,8] or by pulsed power. The latter approach has been pursued by the Z-pinch [9] communities. The results from the first experiments on a new approach that provides very effective flux compression are reported here. The field is compressed by the ablative pressure exerted on an imploding ICF capsule by the driving laser [10]. This approach was proposed in the 1980s [11] as a way to achieve record compressed fields with possible applications for fusion [12] but no laser experiments were performed. There are numerous advantages to this approach as the implosion velocity is high (a few 10 7 cm=s) and the hot plasma is an effective conductor that tra...

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confidence: 90%

Laser-Driven Magnetic-Flux Compression in High-Energy-Density Plasmas

et al. 2009

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“…Radio-frequency electric currents were produced on metal targets in air at the 600-MHz modulation frequency of a biharmonic laser at 10 9 to 10 10 W/cm 2 [9]. Analysis of radiation at harmonics of electron waves in plasmas suggests that momentum exchange between electrons and ions can produce dipole power that is comparable to, or somewhat greater than, the quadrupole power, but not by many orders of magnitude [10].The model of this letter is based on a simple circuit model of a laser plasma [11,12]. This Tidman-Stamper circuit model estimates the voltage drop V along the length of a dense laser-plasma plume to be equal to T/e, where T is the plasma electron temperature in energy units, and e is the electron charge.…”

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confidence: 96%

“…Observations of rf emissions during the sublimation of metal targets by a 1 ms, low-intensity laser were attributed to the dipole moment of a double layer at the front of the turbulent expanding metal vapor [6]. At much higher laser intensities of 12 …”

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Dipole radio-frequency power from laser plasmas with no dipole moment

Felber

2005

Applied Physics Letters

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The radio-frequency power radiated from laser-target plasmas in a vacuum can be orders of magnitude greater than expected from such sources that have a negligible electric dipole moment. A model combining the Tidman-Stamper circuit model of a laser-target plasma with the theory of radiation from currents immersed in plasmas, however, predicts scaling of electric-dipole power radiated from laser plasmas in agreement with experiments. This paper presents a simple model of electron currents immersed in laser plasmas that estimates the power, angular distribution, and spectrum of radio-frequency (rf) radiation from laser plasmas produced on solid surfaces in vacuum. The estimates of rf power are compared with experimental data from CO 2 -laser interactions with copper targets at incident intensities from about 60 MW/cm 2 to 40 GW/cm 2 . The model agrees with the limited observations of rfpower scaling with laser power, with the radial and angular dependence of the near-zone fields, and with the correspondence of rf radiation with the formation of a critical-density surface. The model should be valid over a range of laser wavelengths from the ultraviolet to the long-wavelength infrared. The model assumes quasi-steady-state, one-dimensional ablation, which is valid as long as the laser pulses are not too short and the laser spots are not too small to establish nearly steady ablation across a nearly planar critical-density surface. The model should also be valid over a range of incident laser intensities that produce plasma electron temperatures of a few eV, just above the plasma ignition threshold, to 1 keV or higher. At higher laser intensities, double layers and hot electrons may affect rf production through their dipole moments.Although laser plasmas have a negligible electric dipole moment at these low intensities, the theory of currents immersed in plasmas suggests that the rf radiation from the current in the diffuse plasma surrounding the dense plasma plume is not cancelled by radiation from the equal and opposite current in the plume. If the rf radiation from such laser plasmas can be understood from a model such as this, then the radiation might serve as a diagnostic tool for laser plasmas. For example, the rf radiation can be used as a sensitive indicator for the threshold of formation of critical-density surfaces.The first observations of voltages on isolated laser targets [1] and of rf emissions from laser plasmas [2] were tentatively attributed to charge-separation (capacitive) electric fields. At about the same time, nonthermal microwaves observed from a plasma focus were attributed to the Buneman instability in the pinched plasma [3]. Microwave emissions from a plasma point and a plasma focus were investigated to study localized high-temperature plasma formations in high-current pinched discharges [4,5]. Observations of rf emissions during the sublimation of metal targets by a 1 ms, low-intensity laser were attributed to the dipole moment of a double layer at the front of the turbulent expanding metal v...

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“…17,18 In this paper, the SMF generation is studied in a plasma produced by interaction of the beam of the iodine laser PALS with a solid target, operating in the first harmonic. In many experiments carried out with terawatt nanosecond laser pulses in the absence of the efficient fast electron generation, magnetic fields in a range of several MGs were recorded (see, e.g., review 19 ). Under these conditions, the thermo-electric current corresponding to the vector product of pressure and density gradients in a plasma torch was considered as a source of SMF.…”

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Kinetic magnetization by fast electrons in laser-produced plasmas at sub-relativistic intensities

et al. 2017

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The problem of spontaneous magnetic field generation with nanosecond laser pulses raises a series of fundamental questions, including the intrinsic magnetization mechanisms in laser-driven plasmas and the understanding of charge-discharge processes in the irradiated target. These two issues are tightly bound as the charge-discharge processes are defined by the currents, which have in turn a feedback by magnetic fields in the plasma. Using direct polaro-interferometric measurements and theoretical analysis, we show that at parameters related to the PALS laser system (1.315 μm, 350 ps, and 1016 W/cm2), fast electrons play a decisive role in the generation of magnetic fields in the laser-driven plasma. Spatial distributions of electric currents were calculated from the measured magnetic field and plasma density distributions. The obtained results revealed the characteristics of strong currents observed in capacitor-coil magnetic generation schemes and open a new approach to fundamental studies related to magnetized plasmas.

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

Cited by 180 publications

References 81 publications

Laser-Driven Magnetic-Flux Compression in High-Energy-Density Plasmas

Laser-Driven Magnetic-Flux Compression in High-Energy-Density Plasmas

Dipole radio-frequency power from laser plasmas with no dipole moment

Kinetic magnetization by fast electrons in laser-produced plasmas at sub-relativistic intensities

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