Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session UO7: Expanding Plasmas, Shocks, and Astrophysical Plasmas |
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Chair: Auna Moser, Los Alamos National Laboratory Room: Galerie 6 |
Thursday, October 30, 2014 2:00PM - 2:12PM |
UO7.00001: Instabilities observed at the bubble edge of a laser produced plasma during its expansion in an ambient tenuous plasma Bo Ram Lee, S.E. Clark, D.H.H. Hoffmann, C. Niemann The Raptor kJ class 1053nm Nd:Glass laser in the Phoenix laser laboratory at University of California, Los Angeles, is used to ablate a dense debris plasma from a graphite or plastic target embedded in a tenuous, uniform, and quiescent ambient magnetized plasma in the Large Plasma Device (LAPD) which provides a peak plasma density of n$_{\mathrm{i}}$ $\sim$ 10$^{13}$cm$^{-3}$. Its background magnetic field can vary between 200 and 1200G. Debris ions from laser produced plasma expand out conically with super-Alfv\'enic speed (M$_{\mathrm{A}}$ $\sim$ 2) and expel the background magnetic field and ambient ions to form a diamagnetic bubble. The debris plasma interacts with the ambient plasma and the magnetic field and acts as a piston which can create collisionless shocks. Flute-type instabilities, which are probably large Larmor radius Rayleigh Taylor instabilities or lower hybrid drift instabilities, are developed at the bubble edge and also observed in the experiment. The amplitude and wavelength dependence of the instabilities, which might be a strong function of debris to ambient mass to charge ratio, is studied and the experimental results are compared to the two dimensional hybrid simulations. [Preview Abstract] |
Thursday, October 30, 2014 2:12PM - 2:24PM |
UO7.00002: Spectroscopic Measurement of High-Frequency Electric Fields in the Interaction of Explosive Debris Plasma with Ambient, Magnetized Background Plasma Anton Bondarenko, Derek Schaeffer, Erik Everson, Eric Clark, Stephen Vincena, Bart Van Compernolle, Shreekrishna Tripathi, Carmen Constantin, Chris Niemann The explosive expansion of dense, high-beta debris plasma into relatively tenuous, magnetized background plasma is relevant to a wide variety of astrophysical and space environments. Electric fields play a fundamental role in the coupling of momentum and energy from debris to background, and emission spectroscopy provides a powerful diagnostic for assessing electric fields via the Stark effect. A recent experiment utilizing a unique experimental platform at UCLA that combines the Large Plasma Device and the Raptor laser facility has investigated the super-Alfv\'enic, quasi-perpendicular expansion of a laser-produced carbon (C) debris plasma through a preformed, ambient, magnetized helium (He) background plasma via emission spectroscopy. Spectral profiles of the He II 468.6 nm line have been analyzed via single-mode and multi-mode time-dependent Stark broadening models for hydrogen-like ions, yielding large magnitude ($\sim$100 kV/cm), high-frequency ($\sim$100 GHz) electric fields. The measurements suggest the development of an electron beam-plasma instability, and a simple instability saturation model demonstrates that the measured electric field magnitudes are feasible under the experimental conditions. [Preview Abstract] |
Thursday, October 30, 2014 2:24PM - 2:36PM |
UO7.00003: Laboratory Experiments on Propagating Plasma Bubbles into Vacuum, Vacuum Magnetic Field, and Background Plasmas Alan G. Lynn, Yue Zhang, Mark Gilmore, Scott Hsu We discuss the dynamics of plasma ``bubbles'' as they propagate through a variety of background media. These bubbles are formed by a pulsed coaxial gun with an externally applied magnetic field. Bubble parameters are typically $n_e \sim 10^{20}$ m$^{-3}$, $T_e \sim 5-10$ eV, and $T_i \sim 10-15$ eV. The structure of the bubbles can range from unmagnetized jet-like structures to spheromak-like structures with complex magnetic flux surfaces. Some of the background media the bubbles interact with are vacuum, vacuum with magnetic field, and other magnetized plasmas. These bubbles exhibit different qualitative behavior depending on coaxial gun parameters such as gas species, gun current, and gun bias magnetic field. Their behavior also depends on the parameters of the background they propagate through. Multi-frame fast camera imaging and magnetic probe data are used to characterize the bubble evolution under various conditions. [Preview Abstract] |
Thursday, October 30, 2014 2:36PM - 2:48PM |
UO7.00004: Towards the generation of collisionless electron-positron shocks in the laboratory: Ultrafast thermalisation of laser-driven relativistic plasma jets Mickael Grech, Mathieu Lobet, Charles Ruyer, Emmanuel d'Humi\`eres, Martin Lemoine, Arnaud Debayle, Laurent Gremillet Weibel-mediated collisionless shocks between high-velocity, counter-streaming (electron-ion or electron-positron) plasma flows have been extensively investigated over the past years to gain understanding of various extreme astrophysical scenarii. Here, we examine a concept of colliding pair plasmas that exploits the extreme electromagnetic fields envisioned on compressed LMJ-class laser projects. We present the first self-consistent numerical study, using QED-PIC simulations, of the creation (through the multi-photon Breit-Wheeler process) and subsequent interaction of two counter-streaming, relativistic pair flows driven from laser-irradiated thin Al foils. Fast-growing Weibel instabilities are found to induce ultra-fast thermalisation of the pair jets through the buildup of a MT magnetostatic barrier. The associated gamma-ray generation, its effect on electron-positron thermalisation, as well as the subsequent shock formation are analysed in detail. [Preview Abstract] |
Thursday, October 30, 2014 2:48PM - 3:00PM |
UO7.00005: Collisionless Weibel shocks: Full formation mechanism and timing Antoine Bret, Anne Stockem, Ramesh Narayan, Luis O. Silva Collisionless shocks in plasmas play an important role in space physics (Earth's bow shock) and astrophysics (supernova remnants, relativistic jets, gamma-ray bursts, high energy cosmic rays). While the formation of a fluid shock through the steepening of a large amplitude sound wave has been understood for long, there is currently no detailed picture of the mechanism responsible for the formation of a collisionless shock. We unravel the physical mechanism at work and show that an electromagnetic Weibel shock always forms when two relativistic collisionless, initially unmagnetized, plasma shells encounter. The predicted shock formation time is in good agreement with 2D and 3D particle-in-cell simulations of counterstreaming pair plasmas. By predicting the shock formation time, experimental setups aiming at producing such shocks can be optimised to favourable conditions [1]. \\[4pt] [1] Bret et al. PHYSICS OF PLASMAS 21, 072301 (2014) [Preview Abstract] |
Thursday, October 30, 2014 3:00PM - 3:12PM |
UO7.00006: Particle acceleration in non-relativistic collisionless shocks Frederico Fiuza Supernova remnant shocks are thought to be the dominant source of cosmic rays up to PeV energies; however, the mechanisms for shock formation, magnetic field amplification and particle acceleration in these scenarios are not yet fully understood. I will present detailed multi-dimensional particle-in-cell simulations of shock formation and particle acceleration in non-relativistic scenarios, both unmagnetized and magnetized. These first principles simulations, for unprecedented temporal and spatial scales, help bridge the gap between fully kinetic and hybrid modeling. The results show that electron acceleration is favored at quasi-perpendicular shocks, whereas ion acceleration is more efficient at quasi-parallel shocks. Moreover, it is possible to observe that in initially unmagnetized plasmas, where the shocks are mediated by the Weibel instability, particle acceleration can also occur. I will discuss the importance of these results for current astrophysical models and the possibility of observing particle acceleration in shocks in near future laboratory experiments. [Preview Abstract] |
Thursday, October 30, 2014 3:12PM - 3:24PM |
UO7.00007: Observing the two-photon Breit-Wheeler process for the first time Oliver Pike, Edward Hill, Steven Rose, Felix Mackenroth As the inverse of Dirac annihilation, the Breit-Wheeler process [1], the production of an electron-positron pair in the collision of two photons, is the simplest mechanism by which light can be transformed into matter. It is also of fundamental importance in high-energy astrophysics, both in the context of the dense radiation fields of compact objects [2] and the absorption of high-energy gamma rays travelling intergalactic distances [3]. However, in the 80 years since its theoretical prediction, this process has never been observed. Here, we present the design of a new class of photon-photon collider [4], which is capable of detecting significant numbers of Breit-Wheeler pairs using current-generation technology. We further show how our scheme could be implemented on existing laser facilities; successfully achieving this would represent the advent of a new type of high-energy physics experiment. \\[4pt] [1] G. Breit and J.A. Wheeler, \textit{Phys.~Rev.} \textbf{46,} 1087 (1934)\\[0pt] [2] S. Bonometto and M.J. Rees, \textit{MNRAS} \textbf{152,} 21 (1971)\\[0pt] [3] R.J. Gould and G. Schr\'eder, \textit{Phys.~Rev.~Lett.} \textbf{16,} 252 (1966)\\[0pt] [4] O.J. Pike \textit{et al}, \textit{Nature~Photon.} \textbf{8,} 434 (2014) [Preview Abstract] |
Thursday, October 30, 2014 3:24PM - 3:36PM |
UO7.00008: Theory of the leptonic cascade in magnetospheres of Kerr black holes Mikhail Medvedev, Alex Ford, Brett Keenan It is believed that relativistic jets observed in Active Galactic Nuclei, blazars, quasars and micro-quasars, radio-active galaxies and some other systems host rapidly spinning (Kerr) black holes (BH) and are powered by Blandford-Znajek mechanism, which converts the BH rotational energy into Poynting flux. For this process to occur, the BH mays be immersed into the external magnetic field (presumably brought in by the accreting plasma) and plasma must be {\em created} around a BH (in vacuum, the B-field takes the Ward solution, which delivers zero Blandford-Znajek power). This, plasma production in the so-called ``gap'' region of the BH magnetosphere is crucial for the jets to exist. Here we present analytical theory of the plasma production via the leptonic cascade. We present conditions (ambient photon spectrum, luminosity, magnetic field strength, BH spin) needed for the cascade multiplicity to exceed unity, i.e., for the astrophysical systems to exhibit powerful jets. We discuss how temporal variations of these parameters can turn the jets off and on. [Preview Abstract] |
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