This manuscript is distributed under a Creative Commons Attribution-NonCommercial-NoDerivs License (https://creativecommons.org/licenses/by-nc-nd/4.0/), which permits distribution and reproduction for non-commercial purposes, provided the author and source are cited. General rightsCopyright for the publications made accessible via the Queen's University Belfast Research Portal is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. AbstractThe collimation of astrophysically-relevant plasma ejecta in the form of narrow jets via a poloidal magnetic field is studied experimentally by irradiating a target situated in a 20 T axial magnetic field with a 40 J, 0.6 ns, 0.7 mm diameter, high-power laser. The dynamics of the plasma shaping by the magnetic field are studied over 70 ns and up to 20 mm from the source by diagnosing the electron density, temperature and optical self-emission. These show that the initial expansion of the plasma is highly magnetized, which leads to the formation of a cavity structure when the kinetic plasma pressure compresses the magnetic field resulting in an oblique shock [A. Ciardi et al., Phys. Rev. Lett. 110, 025002 (2013)]. The resulting poloidal magnetic nozzle generates a standing conical shock that collimates the plasma into a narrow jet [B. Albertazzi et al., Science 346, 325 (2014).]. At distances far from the target, the jet is only marginally magnetized and maintains a high aspect ratio due to its high Mach-number (M ∼ 20) and not due to external magnetic pressure. The formation of the jet is evaluated over a range of laser intensities (10 12 -10 13 W/cm 2 ), target materials and orientations of the magnetic field. Plasma cavity formation is observed in all cases and the viability of long-range jet formation is found to be dependent on the orientation of the magnetic field.
The sheared-flow stabilized (SFS) Z-pinch has demonstrated long-lived plasmas with fusion-relevant parameters. This Letter presents the first experimental results demonstrating sustained, quasi-steady-state neutron production from the Fusion Z-pinch Experiment (FuZE), operated with a mixture of 20% deuterium/80% hydrogen by volume. Neutron emissions lasting approximately 5 µs are reproducibly observed with pinch currents of approximately 200 kA during an approximately 16 µs period of plasma quiescence. The average neutron yield is estimated to be (1.25 ± 0.45) × 10 5 neutrons/pulse and scales with the square of the deuterium concentration. Coincident with the neutron signal, plasma temperatures of 1 − 2 keV, and densities of approximately 10 17 cm −3 with 0.3 cm pinch radii are measured with fully-integrated diagnostics.
In many natural phenomena in space (cosmic-rays, fast winds), non-thermal ion populations are produced, with wave-particle interactions in self-induced electromagnetic turbulence being suspected to be mediators. However, the processes by which the electromagnetic energy is bestowed upon the particles is debated, and in some cases requires field compression. Here we show that laboratory experiments using high-power lasers and external strong magnetic field can be used to infer magnetic field compression in the interpenetration of two collisionless, high-velocity (0.01-0.1c) quasi-neutral plasma flows. This is evidenced through observed plasma stagnation at the flows collision point, which Particle-in-Cell (PIC) simulations suggest to be the signature of magnetic field compression into a thin layer, followed by its dislocation into magnetic vortices. Acceleration of protons from the plasma collision is observed as well. As a possible scenario, with 1D and 2D PIC simulations we consider a compression of the vortices against dense plasma remnants.
The structure of a strong collisional shock front forming in a plasma is directly probed for the first time in laser-driven gas-jet experiments. Thomson scattering of a 526.5 nm probe beam was used to diagnose temperature and ion velocity distribution in a strong shock (M∼11) propagating through a low-density (ρ∼0.01 mg/cc) plasma composed of hydrogen. A forward-streaming population of ions traveling in excess of the shock velocity was observed to heat and slow down on an unmoving, unshocked population of cold protons, until ultimately the populations merge and begin to thermalize. Instabilities are observed during the merging, indicating a uniquely plasma-phase process in shock front formation.
We investigate the formation of a laser-produced magnetized jet under conditions of a varying mass ejection rate and a varying divergence of the ejected plasma flow. This is done by irradiating a solid target placed in a 20 T magnetic field with, first, a collinear precursor laser pulse (10 12 W=cm 2 ) and, then, a main pulse (10 13 W=cm 2 ) arriving 9-19 ns later. Varying the time delay between the two pulses is found to control the divergence of the expanding plasma, which is shown to increase the strength of and heating in the conical shock that is responsible for jet collimation. These results show that plasma collimation due to shocks against a strong magnetic field can lead to stable, astrophysically relevant jets that are sustained over time scales 100 times the laser pulse duration (i.e., > 70 ns), even in the case of strong variability at the source. DOI: 10.1103/PhysRevLett.119.255002 Whereas a large amount of work has been done concerning the vacuum expansion of a laser-produced plasma and its inertial collimation through, for example, different geometrical target configurations or particular focusing of the laser [1][2][3][4], experiments with external magnetic fields strong enough to affect the plasma dynamics or energy transport have been made possible only recently. Such studies are pertinent to concepts related to both laser- [5] and magnetically driven [6] inertial confinement fusion that combine high-power lasers with strong magnetic fields to increase implosion stability [7,8] and improve yields [9]. Related experiments have already shown significant modifications to energy transport [10,11] due to the magnetic field. However, little attention has been paid so far to the role of a strong externally applied magnetic field on the laser ablation dynamics or on the influence of time-variable ejections on the plasma evolution.Within the context of high-energy density laboratory astrophysics, the coupling of laser-driven plasmas with an externally imposed magnetic field has proven successful in diverse areas, examples are the generation of collisionless shocks [12] and studies related to magnetized accretion columns [13] and magnetically collimated jets [14][15][16][17]. High-aspect-ratio, supersonic jets are ubiquitous in astrophysics (e.g., in young stellar objects [18] and active galactic nuclei [19]), and are the result of magnetic fields mediating the extraction of energy from an accreting system [20].While not yet studied in the laboratory, variability of the mass ejection rate plays an important role in structuring astrophysical jets, for example, by generating velocity fluctuations large enough to produce internal shocks in the flow [21].In this Letter we investigate the collimation, heating, long-range jet formation, and stability of plasma flows
Calibrated diagnostics for energetic particle detection allow for the systematic study of charged particle sources. The Fujifilm BAS-TR imaging plate (IP) is a reusable phosphorescent detector for radiation applications such as x-ray and particle beam detection. The BAS-TR IP has been absolutely calibrated to many low-Z (low proton number) ions, and extending these calibrations to the mid-Z regime is beneficial for the study of laser-driven ion sources. The Texas Petawatt Laser was used to generate energetic ions from a 100 nm titanium foil, and charge states Ti10+ through Ti12+, ranging from 6 to 27 MeV, were analyzed for calibration. A plastic detector of CR-39 with evenly placed slots was mounted in front of the IP to count the number of ions that correspond with the IP levels of photo-stimulated luminescence (PSL). A response curve was fitted to the data, yielding a model of the PSL signal vs ion energy. Comparisons to other published response curves are also presented, illustrating the trend of PSL/nucleon decreasing with increasing ion mass.
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