ABSTRACT
We present the first laboratory observations of time-resolved electron and ion velocity distributions in magnetized collisionless shock precursors. Thomson scattering of a probe laser beam was used to observe the interaction of a laser-driven, supersonic piston plasma expanding through an ambient plasma in an external magnetic field. From the Thomson-scattered spectra we measure time-resolved profiles of electron density, temperature, and ion flow speed, as well as spatially resolved magnetic fields from proton radiography. We observe direct evidence of the coupling between piston and ambient plasmas, including the acceleration of ambient ions driven by magnetic and pressure gradient electric fields, and deformation of the piston ion flow, key steps in the formation of magnetized collisionless shocks. Even before a shock has fully formed, we observe strong density compressions and electron heating associated with the pileup of piston ions. The results demonstrate that laboratory experiments can probe particle velocity distributions relevant to collisionless shocks, and can complement, and in some cases overcome, the limitations of similar measurements undertaken by spacecraft missions.
ABSTRACT
Recent experiments have demonstrated magnetic reconnection between colliding plasma plumes, where the reconnecting magnetic fields were self-generated in the plasma by the Biermann-battery effect. Using fully kinetic 3D simulations, we show the full evolution of the magnetic fields and plasma in these experiments, including self-consistent magnetic field generation about the expanding plume. The collision of the two plasmas drives the formation of a current sheet, where reconnection occurs in a strongly time- and space-dependent manner, demonstrating a new 3D reconnection mechanism. Specifically, we observe a fast, vertically localized Biermann-mediated reconnection, an inherently 3D process where the temperature profile in the current sheet coupled with the out-of-plane ablation density profile conspires to break inflowing field lines, reconnecting the field downstream. Fast reconnection is sustained by both the Biermann effect and the traceless electron pressure tensor, where the development of plasmoids appears to modulate the contribution of the latter. We present a simple and general formulation to consider the relevance of Biermann-mediated reconnection in general astrophysical scenarios.
ABSTRACT
We present the first laboratory generation of high-Mach-number magnetized collisionless shocks created through the interaction of an expanding laser-driven plasma with a magnetized ambient plasma. Time-resolved, two-dimensional imaging of plasma density and magnetic fields shows the formation and evolution of a supercritical shock propagating at magnetosonic Mach number M_{ms}≈12. Particle-in-cell simulations constrained by experimental data further detail the shock formation and separate dynamics of the multi-ion-species ambient plasma. The results show that the shocks form on time scales as fast as one gyroperiod, aided by the efficient coupling of energy, and the generation of a magnetic barrier between the piston and ambient ions. The development of this experimental platform complements present remote sensing and spacecraft observations, and opens the way for controlled laboratory investigations of high-Mach number collisionless shocks, including the mechanisms and efficiency of particle acceleration.
ABSTRACT
Observations of magnetic reconnection between colliding plumes of magnetized laser-produced plasma are presented. Two counterpropagating plasma flows are created by irradiating oppositely placed plastic (CH) targets with 1.8-kJ, 2-ns laser beams on the Omega EP Laser System. The interaction region between the plumes is prefilled with a low-density background plasma and magnetized by an externally applied magnetic field, imposed perpendicular to the plasma flow, and initialized with an X-type null point geometry with B=0 at the midplane and B=8 T at the targets. The counterflowing plumes sweep up and compress the background plasma and the magnetic field into a pair of magnetized ribbons, which collide, stagnate, and reconnect at the midplane, allowing the first detailed observations of a stretched current sheet in laser-driven reconnection experiments. The dynamics of current sheet formation are in good agreement with first-principles particle-in-cell simulations that model the experiments.
ABSTRACT
Filamentation due to the growth of a Weibel-type instability was observed in the interaction of a pair of counterstreaming, ablatively driven plasma flows, in a supersonic, collisionless regime relevant to astrophysical collisionless shocks. The flows were created by irradiating a pair of opposing plastic (CH) foils with 1.8 kJ, 2-ns laser pulses on the OMEGA EP Laser System. Ultrafast laser-driven proton radiography was used to image the Weibel-generated electromagnetic fields. The experimental observations are in good agreement with the analytical theory of the Weibel instability and with particle-in-cell simulations.
ABSTRACT
Recent experiments have observed magnetic reconnection in high-energy-density, laser-produced plasma bubbles, with reconnection rates observed to be much higher than can be explained by classical theory. Based on fully kinetic particle simulations we find that fast reconnection in these strongly driven systems can be explained by magnetic flux pileup at the shoulder of the current sheet and subsequent fast reconnection via two-fluid, collisionless mechanisms. In the strong drive regime with two-fluid effects, we find that the ultimate reconnection time is insensitive to the nominal system Alfvén time.
ABSTRACT
Direct numerical simulations of the line-tied Parker (or magnetic Rayleigh-Taylor) instability, based on the fully compressible ideal magnetohydrodynamic equations, are presented. In the intermediate nonlinear phase, the instability continues to grow exponentially in time and the plasma tends to develop convection-induced discontinuities in the form of shocklike coherent structures. No evidence of finite-time singularities is seen.
ABSTRACT
The theory of focusing light pulses in Kerr media with normal group-velocity dispersion in (2+1) and (3+1) dimensions is revisited. It is shown that pulse splitting introduced by this dispersion follows from shock fronts that develop along hyperbolas separating the region of transverse self-focusing from the domain of temporal dispersion. Justified by a self-similar approach, this property is confirmed by numerical simulations using an adaptive-mesh refinement code.
