arXiv subjects

D. B. Schaeffer

Publications and source records attributed to D. B. Schaeffer.

22 records · Page 2Linked to original sources

arXiv2018

Particle Velocity Distributions in Developing Magnetized Collisionless Shocks in Laser-Produced Plasmas

We present the first laboratory observations of time-resolved electron and ion velocity distributions in forming, magnetized collisionless shocks. Thomson scattering of a probe laser beam was used to observe the interaction of a laser-driven, supersonic piston plasma expanding through a magnetized ambient plasma. 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 sweeping up and acceleration of ambient ions, magnetic field compression, and the subsequent deformation of the piston ion flow, key steps in shock formation. Even before the shock has fully formed, we observe strong density compressions and electron heating associated with the pile up of piston ions. The results demonstrate that laboratory experiments can probe particle velocity distributions relevant to collisionless shocks, and thus complement similar measurements undertaken by spacecraft missions.

physics.plasm-ph↗

arXiv2018

Regimes of magnetic reconnection in colliding laser-produced magnetized plasma bubbles

We conduct a multiparametric study of driven magnetic reconnection relevant to recent experiments on colliding magnetized laser produced plasmas using particle-in-cell simulations. Varying the background plasma density, plasma resistivity, and plasma bubble geometry, the 2D simulations demonstrate a rich variety of reconnection behavior and show the coupling between magnetic reconnection and the global hydrodynamical evolution of the system. We consider both the collision between two radially expanding bubbles where reconnection is seeded by the pre-existing X-point, and the collision between two flows in a quasi-1D geometry with initially anti-parallel fields where reconnection must be initiated by the tearing instability. In both geometries, at a baseline case of low-collisionality and low background density, the current sheet is strongly compressed to below scale of the ion-skin-depth scale, and rapid, multi-plasmoid reconnection results. Increasing the plasma resistivity, we observe a collisional slow-down of reconnection and stabilization of plasmoid instability for Lundquist numbers less than approximately $S \sim 10^3$. Secondly, increasing the background plasma density modifies the compressibility of the plasma and can also slow-down or even prevent reconnection, even in completely collisionless regimes, by preventing the current sheet from thinning down to the scale of the ion-skin depth. These results have implications for understanding recent and future experiments, and signatures for these processes for proton-radiography diagnostics of these experiments are discussed.

physics.plasm-ph↗

arXiv2017

Kinetic simulation of magnetic field generation and collisionless shock formation in expanding laboratory plasmas

Recent laboratory experiments with laser-produced plasmas have observed and studied a number of fundamental physical processes relevant to magnetized astrophysical plasmas, including magnetic reconnection, collisionless shocks, and magnetic field generation by Weibel instability, opening up new experimental platforms for laboratory astrophysics. We develop a fully kinetic simulation model for first-principles simulation of these systems including the dynamics of magnetic fields---magnetic field generation by the Biermann battery effect or Weibel instability; advection by the ion flow, Hall effect, and Nernst effect; and destruction of the field by dissipative mechanisms. Key dimensionless parameters describing the system are derived for scaling between kinetic simulation, recent experiments, and astrophysical plasmas. First, simulations are presented which model Biermann battery magnetic field generation in plasmas expanding from a thin target. Ablation of two neighboring plumes leads to the formation of a current sheet as the opposing Biermann-generated fields collide, modeling recent laser-driven magnetic reconnection experiments. Second, we simulate recent experiments on collisionless magnetized shock generation, by expanding a piston plasma into a pre-magnetized ambient plasma. For parameters considered, the Biermann effect generates additional magnetic fields in the curved shock front and thereby increases shock particle reflection. Both cases show the importance of kinetic processes in the interaction of plasmas with magnetic fields, and open opportunities to benchmark these important processes through comparison of theory and experiments.

physics.plasm-ph↗

arXiv2017

Observations of a Field-Aligned Ion/Ion-Beam Instability in a Magnetized Laboratory Plasma

Collisionless coupling between super Alfvénic ions and an ambient plasma parallel to a background magnetic field is mediated by a set of electromagnetic ion/ion-beam instabilities including the resonant right hand instability (RHI). To study this coupling and its role in parallel shock formation, a new experimental configuration at the University of California, Los Angeles utilizes high-energy and high-repetition-rate lasers to create a super-Alfvénic field-aligned debris plasma within an ambient plasma in the Large Plasma Device (LAPD). We used a time-resolved fluorescence monochromator and an array of Langmuir probes to characterize the laser plasma velocity distribution and density. The debris ions were observed to be sufficiently super-Alfvénic and dense to excite the RHI. Measurements with magnetic flux probes exhibited a right-hand circularly polarized frequency chirp consistent with the excitation of the RHI near the laser target. We compared measurements to 2D hybrid simulations of the experiment.

physics.plasm-ph↗