Session TI3: Invited HED: High Energy Density Science: Stix Award, Lab Astro II

Thomas H. Stix Award for Outstanding Early Career Contributions to Plasma Physics Research Talk: Magnetic reconnection, collisionless shocks, and cosmic particle acceleration in the laboratory

Magnetic reconnection and collisionless shocks are of great interest as fundamental physics processes which allow rapid conversion of energy in plasmas. Magnetic reconnection mediates a change of magnetic topology and the explosive release of stored magnetic energy, while shocks mediate a fast thermalization of colliding supersonic plasma flows. In collisionless plasmas, shocks must be mediated by electromagnetic interactions, rather than particle collisions. The recent generation of laboratory experiments, especially high-energy-density physics facilities, has opened significant avenues to experimentally study these fundamental processes. I will review recent experimental progress and future opportunities on several challenge problems, including observation of fast magnetic reconnection mediated by two-fluid effects and by current sheet or plasmoid instabilities, laboratory generation and observations of magnetized collisionless shocks, spontaneous magnetic field generation by Biermann battery and Weibel instability, and acceleration of energized particle populations. First-principles kinetic simulations provide invaluable insights to guide experiments and predictions for observables under experimental conditions.   

3-D magnetic reconnection in laser-driven plasmas: novel modeling provides insight into laboratory and astrophysical current sheets

Magnetic reconnection is a fundamental process throughout plasma physics that allows for the rapid conversion of magnetic energy into kinetic energy and particle flows. While a major challenge in studying reconnection has been bridging the gap between small-scale experiments and large astrophysical scenarios, high energy density (HED) plasma experiments driven by long pulse lasers have demonstrated magnetic reconnection between colliding plasma plumes, recently achieving reconnection current sheets significantly larger than any previous experiments (as normalized by the ion skin depth). In this talk, we present newly developed fully kinetic 3-D simulations,$^1$ which are able to directly compare to such HED experiments, and in turn give insight to reconnection dynamics in larger astrophysical reconnection scenarios. In particular, we have observed a novel, inherently 3-D reconnection mechanism where the Biermann battery effect plays a direct role in reconnection process.$^2$ Based on scaling laws, we find that this mechanism can also play a non-trivial role during turbulent reconnection in the Earth’s magnetosheath. Our simulations shed light on significant differences in 2-D versus 3-D reconnection, transitions of quiescent current sheets to a sea of plasmoids, as well as various magnetic field generation mechanisms in the presence of large temperature and density gradients. [1] W. Fox, J. Matteucci, C. Moissard, et al., Physics of Plasmas $\textbf{25}$, 102106 (2018) [2] J. Matteucci, W. Fox, A. Bhattacharjee, et al., Phys. Rev. Lett. $\textbf{121}$, 095001 (2018)   

Magnetic Reconnection in the High-Energy-Density Regime

Motivated by an improved understanding of reconnection theory and its application to extreme astrophysical environments and controlled thermonuclear fusion, much attention has been given to the development of techniques for studying magnetic reconnection in the laboratory. Important open questions concern the multiscale processes that lead to reconnection onset, magnetic connectivity changes, and the conversion of magnetic energy into other forms. Recreating and studying these processes in the laboratory, particularly under conditions of extreme energy density, provides an opportunity for testing the fundamental physics that underpin reconnection-model predictions. In this work, magnetic reconnection is demonstrated between two laser-produced plasmas that are generated one laser-spot diameter apart. The closer spot separation minimizes the time for plasma expansion before the interaction occurs and allows, for the first time, the Biermann fields at the edge of each laser spot to interact and reconnect. The significant new result is the clear and complete change in magnetic connectivity that occurs close to the laser focal regions. The importance of plasma accumulation and heat-flow effects on the transport of magnetic fields into the region where reconnection occurs will be discussed in the context of experimental data, theory, and numerical simulations.   

First direct experimental observation of Weibel-mediated collisionless shock formation and the resulting nonthermal electron acceleration in laboratory experiments

Collisionless shocks are ubiquitous in astrophysics and are thought to be a source of magnetic field generation and particle acceleration in the universe. They can occur in supernova remnants, cataclysmic variables, protostellar jets, active galactic nuclei, and colliding galaxies. Laboratory experiments with high-Mach number plasma flows can provide critical information to help understand the shock formation mechanisms in these systems. High velocity, low density, interpenetrating plasma flows were studied on the Omega laser. These interpenetrating flows exhibited strong filamentation via the ion-Weibel instability, which in turn generated microscale magnetic fields that were observed with proton radiography and optical Thomson scattering \footnote{C. M. Huntington et al., \textbf{Nat. Phys.} 11, 173 (2015)}$^,$ \footnote{G.F. Swadling et al., \textbf{Phys. Rev. Lett.}, submitted (2019)}. No evidence for collisionless shock generation was observed in these Omega experiments. On the National Ignition Facility (NIF), the interpenetrating plasmas extended over much larger time intervals and interaction volumes. Under these conditions on NIF, we observed the first unambiguous experimental evidence of Weibel-mediated collisionless shock formation, as demonstrated by an abrupt ~3x increase in density, with significant increase in temperature. Detailed plasma characterization using optical Thomson scattering was carried out, and the first experimental observation of particle acceleration from these high velocity collisionless shocks on NIF was obtained\footnote{J. S. Ross et al., \textbf{Phys. Rev. Lett.}, 118, 185003 (2017)}$^,$ \footnote{F. Fiuza et al., \textbf{In Preparation} (2019)}. The role of the Weibel instability in creating seed magnetic fields, initiating plasma flow stagnation, and generating collisionless shocks in laboratory experiments on Omega and NIF will be discussed.   

Astrophysical Turbulent Dynamos in High Energy-Density Laboratory Plasmas

Turbulent dynamos exponentially amplify weak, primordial magnetic fields to produce the present-day magnetic universe on scales from the interplanetary to the intergalactic. Advances in HED experimental techniques make possible for astrophysical turbulent dynamos to be recreated in the laboratory. Taking advantage of this, we devised, using the FLASH MHD code, a new scheme to observe the astrophysical turbulent dynamo in action. The key to our scheme on the OMEGA-EP laser was to involve its long pulse beams in dual roles: First, the laser beams initiated a quasi-steady, turbulent, magnetized plasma by irradiating a simple target cone of machined plastic. Then, the laser beams continuously heated the ablated plasma, thereby sustaining the highly irresistive conditions necessary for turbulent dynamo activity through their 10 ns-long pulse. As the turbulent dynamo evolved, we sketched its trajectory by timing a TNSA proton beam to project deflection images revealing the plasma’s electric currents that underlay its magnetic field. We found that in its 5 ns lifetime the turbulent dynamo exponentially amplified the plasma’s magnetic energy on all resolved scales unto saturation at 10’s of times its initial amount. Hence, we matched our experiment to predictions with greatest fidelity. \\ \\ This submission has been approved for unlimited release, LA-UR-19-26288.   

Building Laboratory Astrophysics Experiments from Laser-Driven Jets

This talk will explore some of the advantages and disadvantages to using plasma jets driven by rear-surface irradiation in laboratory astrophysics experiments. In many respects, rear-driven jets are a natural choice for laboratory astrophysics experiments: they are reliable, and they require simple beam configurations---a distinct advantage in laboratory astrophysics investigations where the full experiment can be quite complex. Compared to jets driven by front surface irradiation, rear-driven jets are fairly long-lived, lasting for 10s of nanoseconds, which allows for astrophysically relevant structures to form in a laboratory experiment. However, one downside of rear-driven jets that they change significantly over the dynamic timescale of the jet. Our group observed this directly with optical Thomson scattering on OMEGA laser (Laboratory for Laser Energetics, Rochester, NY). Over a timeframe of just 6 nanoseconds, mass density increased by three orders of magnitude while velocity fell by a factor of three. This sort of rapid change can be scientifically useful, as it was when we used it to observe a transition between interpenetration and stagnation. But it can also present a serious challenge when attempting to scale astrophysical phenomena to the laboratory environment, as I will demonstrate with our group's experience investigating accretion shocks. This work is funded by the U.S. Department of Energy, through the NNSA-DS and SC-OFES Joint Program in High-Energy-Density Laboratory Plasmas, grant number DE-NA0002956, and the National Laser User Facility Program, grant number DE-NA0002719, and through the Laboratory for Laser Energetics, University of Rochester by the NNSA/OICF under Cooperative Agreement No. DE-NA0001944.