Detailed characterization of laser-produced astrophysically-relevant jets formed via a poloidal magnetic nozzle

Abstract

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.

Laboratory unraveling of matter accretion in young stars

“…This laser beam was focused on target over a 700-μm diameter flat-top spot, which gives an intensity of I max = 1.6 × 10 13 W cm −2 . The target was a flat solid from which, following laser irradiation, a hot plasma was created, was expanded into a vacuum, and was funneled over 3 mm into a collimated stream ( 26 , 36 , 44 ) by the action of a large-scale, steady poloidal external magnetic field ( 35 ) having a strength from 6 to 30 T, 20 T being the magnetic strength used for the results reported in the main text. The collimated plasma stream followed the magnetic field lines to hit a secondary solid target at a distance of 11.7 mm from the first target.…”

“…When hitting this secondary obstacle target, the stream had a constant (over more than 100 ns) diameter of ~1.4 mm and a plasma electron density of ~1.5 × 10 18 cm −3 when using 20 T for the magnetic field and ~1.5 mm and ~1.2 × 10 18 cm −3 for the same quantities when using 6 T for the magnetic field, as detailed in tables S3 and S4. At 20 T, the measured plasma electron temperature was ~10 eV or 0.1 MK ( 37 , 44 ). We used two different materials, that is, polyvinyl chloride [PVC (C 2 H 3 Cl) n ] and Teflon (CF 2 ), for the two targets to be able to distinguish, through x-ray spectroscopy of the F-emitting ions (see below), the characteristics of the plasma coming from the impacting stream from the characteristics of the plasma ablated from the obstacle.…”

Enhancement of Quasistationary Shocks and Heating via Temporal Staging in a Magnetized Laser-Plasma Jet

“…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].…”

“…As shown in Fig. 1, both beams irradiated a ðC 2 F 4 Þ n target immersed in a 1 μs pulsed, 20 T external magnetic field aligned along the plasma expansion axis [17,23]. The plasma electron density evolution was investigated via a Mach-Zehnder interferometer using a 5 ps (λ L ¼ 528 nm) probe beam.…”

“…This method has been benchmarked against a variety of laser and target conditions such as those presented in Refs. [15,17]. To introduce the second pulse, we implemented a laser deposition module in GORGON that assumes linear inversebremsstrahlung absorption and the geometric optics approximation.…”