Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session BI3: Plasma Jets, Beams and Flows |
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Chair: Christoph Niemann, University of California, Los Angeles Room: Plaza F |
Monday, November 11, 2013 9:30AM - 10:00AM |
BI3.00001: Experimental Characterization of the Stagnation Layer between Two Obliquely Merging Supersonic Plasma Jets Invited Speaker: Elizabeth C. Merritt Experiments on the oblique merging of two supersonic argon plasma jets have been conducted at LANL in order to assess the use of such jets to form imploding spherical plasma liners for high energy density physics applications [1]. The plasma jets are formed and launched by pulsed-power-driven railguns and have initial jet parameters of $n \approx 2 \times 10^{16}$~cm$^{-3}$, $T_e \approx 1.4$~eV, ionization fraction $\approx 0.96$, velocity $\approx 30$~km/s, diameter = 5~cm, and length $\approx 20$~cm [2]. We have experimentally identified density increases that are consistent with shock formation, and a few-cm thick stagnation layer structure observed both in CCD camera images and interferometer density profiles. Although the jets are each individually collisional, the mean free path between counter-streaming ions is on the same order as the stagnation layer thickness, placing the jet merging in a semi-collisional regime. It was not known {\em a priori} whether the observations corresponded to hydrodynamic oblique shocks and whether two-fluid or kinetic effects played a role. Through careful analysis [3] of the stagnation layer density and emission profiles, and comparisons between the data and both analytic hydrodynamic shock theory and multi-fluid plasma simulations, we demonstrate that our observations are consistent with collisional shocks. \\[4pt] [1] S. C. Hsu et al., IEEE Trans. Plasma Sci.~{\bf 40}, 1287 (2012).\\[0pt] [2] S. C. Hsu et al., Phys. Plasmas~{\bf 19}, 123514 (2012).\\[0pt] [3] E. C. Merritt et al., ``Experimental characterization of the stagnation layer between two obliquely merging supersonic plasma jets,'' submitted for publication (2013). [Preview Abstract] |
Monday, November 11, 2013 10:00AM - 10:30AM |
BI3.00002: The impact of Hall physics on magnetized high energy density plasma jets Invited Speaker: Pierre-Alexandre Gourdain Magnetized high energy density (HED) plasma jets produced by radial foil explosions on pulsed power machines have improved our understanding of the fundamental mechanisms driving flowing matter under extreme conditions. Experiments and simulations indicate that magnetic fields are crucial in the formation and stability of strongly collimated plasma jets, a property also shared by astrophysical jets originating from black holes and protostars. It is understood that these magnetic fields also generate electric fields, often associated with the dynamo effect. In fact, when the Lundquist number is large enough, the dynamo effect is frequently seen as the dominant electric field driver of flowing plasmas. This is true inside the collimated jet where the density (\textgreater\ 10$^{19}$ cm$^{-3})$, velocity (\textless\ 200 km/s) and temperature (\textgreater\ 50 eV) are high enough to preclude the dominance of any other type of electric fields. However, the ion flow speed is much lower than the speed of light. As a result, dynamo electric fields do not impact noticeably fluid motion since electric stresses are negligible compared to magnetic stresses. On the other hand, Hall physics dominates the low density plasma surrounding the jet (\textless\ 10$^{18}$ cm$^{-3})$. In this region, electron speeds can be orders of magnitude higher than the bulk flow velocity as ion and electron fluids are decoupled. As a result, electric stresses can rival with magnetic stresses and Hall physics does impact the overall plasma dynamics. This talk will discuss how HED plasmas are subjected to Hall physics and how it impacts the particle confinement as well as the MHD stability of plasma jets. After focusing on experimental results and numerical simulations from the PERSEUS code, the talk will extend its conclusions to inertial fusion regimes where Hall physics could also alter plasma confinement and stability. [Preview Abstract] |
Monday, November 11, 2013 10:30AM - 11:00AM |
BI3.00003: Charged Particle Stopping Power in Dense Plasmas: Improvements, Validation, and Practical Implications Invited Speaker: Paul Grabowski Charged particle stopping power is an important quantity that arises in thermonuclear burn, particle beam experiments, and fast ignition. Because stopping power models arise from kinetic theory collision operators and stopping power is a velocity-resolved non-equilibrium statistical mechanics problem, exact values of stopping power are ideal quantities for validating collision models. By directly comparing classical molecular dynamics simulations with stopping models derived from both linear response and binary cross section pictures, we have quantified the accuracy of these models and determined which physics is needed as a function of Coulomb coupling, projectile charge, and velocity. We have found that for divergent linear response theories, a velocity-dependent cutoff works better than a simpler temperature-dependent cutoff, but both fail when the velocity of the projectile is low and the Coulomb coupling is large. This problem is somewhat rectified by the inclusion of local field corrections. Alternatively, one can use a binary cross section in constructing a collision operator for better inclusion of strong scattering. We find that low-velocity stopping can be significantly improved by including non-linear screening of the target particles when calculating this cross section. We extend this knowledge to the quantum case, giving the relative contributions of Heisenberg uncertainty, degeneracy, and quantum scattering. [Preview Abstract] |
Monday, November 11, 2013 11:00AM - 11:30AM |
BI3.00004: Initial experimental evidence of self-collimation of TNSA proton beam in a stack of conducting foils Invited Speaker: Pavel Ni Phenomena consistent with self-collimation (or weak self-focusing) of laser target-normal-sheath-accelerated (TNSA) protons was experimentally observed for the first time, in a specially engineered structure (``lens'') consisting of a stack of 300 thin aluminum foils separated by 50 $\mu$m vacuum gaps. The experiments were carried out in a ``passive environment,'' i.e. no external fields applied, neutralization plasma or injection of secondary charged particles was imposed. Experiments were performed at the petawatt ``PHELIX'' laser user facility (E$=$100 J, $\Delta$t$=$400 fs, $\lambda=$1062 nm) at the ``Helmholtzzentrum f\"{u}r Schwerionenforschung--GSI'' in Darmstadt, Germany. The observed rms beam spot reduction depends inversely on energy, with a focusing degree decreasing monotonically from 2 at 5.4 MeV to 1.5 MeV at 18.7 MeV. The physics inside the lens is complex, resulting in a number of different mechanisms that can potentially affect the particle dynamics within the structure. We present a plausible simple interpretation of the experiment in which the combination of magnetic self-pinch forces generated by the beam current together with the simultaneous reduction of the repulsive electrostatic forces due to the foils are the dominant mechanisms responsible for the observed focusing/collimation. This focusing technique could be applied to a wide variety of space-charge dominated proton and heavy ion beams and impact fields and applications, such as HEDP science, inertial confinement fusion in both fast ignition and heavy ion fusion approaches, compact laser-driven injectors for a LINAC or synchrotron, medical therapy, materials processing, etc. [Preview Abstract] |
Monday, November 11, 2013 11:30AM - 12:00PM |
BI3.00005: Observation and modeling of mixing-layer development in HED blast-wave-driven shear flow Invited Speaker: Carlos Di Stefano This talk describes work exploring the sensitivity to initial conditions of hydrodynamic mixing-layer growth due to shear flow in the high-energy-density regime. This work features an approach in two parts, experimental and theoretical. First, an experiment, conducted at the OMEGA-60 laser facility, seeks to measure the development of such a mixing layer. This is accomplished by placing a layer of low-density (initially of either 0.05 or 0.1 g/cm$^{\mathrm{3}}$, to vary the system's Atwood number) carbon foam against a layer of higher-density (initially 1.4 g/cm$^{\mathrm{3}})$ polyamide-imide that has been machined to a nominally-flat surface at its interface with the foam. Inherent roughness of this surface's finish is precisely measured and varied from piece to piece. Ten simultaneous OMEGA beams, comprising a 4.5 kJ, 1-ns pulse focused to a roughly 1-mm-diameter spot, irradiate a thin polycarbonate ablator, driving a blast wave into the foam, parallel to its interface with the polyamide-imide. The ablator is framed by a gold washer, such that the blast wave is driven only into the foam, and not into the polyamide-imide. The subsequent forward motion of the shocked foam creates the desired shear effect, and the system is imaged by X-ray radiography 35 ns after the beginning of the driving laser pulse. Second, a simulation is performed, intending to replicate the flow observed in the experiment as closely as possible. Using the resulting simulated flow parameters, an analytical model can be used to predict the evolution of the mixing layer, as well as track the motion of the fluid in the experiment prior to the snapshot seen in the radiograph. The ability of the model to predict growth of the mixing layer under the various conditions observed in the experiment is then examined. [Preview Abstract] |
Monday, November 11, 2013 12:00PM - 12:30PM |
BI3.00006: Formation of reverse shocks in magnetized high energy density supersonic plasma flows Invited Speaker: Sergey Lebedev There has been considerable effort in developing experiments for studies of both collisionless and radiative shocks in high energy density plasmas (HEDP), but there is still very limited experimental information the concerning properties of HEDP shocks in the presence of a magnetic field. A new experimental platform, based on the use of supersonic ablation plasma flows in inverse wire array z-pinches, was developed for studies of shocks in magnetized HEDP plasmas in a well-defined and diagnosable 1-D interaction geometry. The mechanism of flow generation ensures that the plasma flow (M$_{\mathrm{A}}\approx 5-6$, V$_{\mathrm{flow}} \approx$ 100km/s, n$_{\mathrm{i}}\approx$ 10$^{17}$cm$^{-3})$ has a frozen-in magnetic field at a level sufficient to affect the shocks formed in the interaction with conducting obstacles. Experiments show that in addition to the formation of a ''standard'' reverse shock in a stagnated HEDP plasma, the presence of the magnetic field leads to the formation of an additional shock-like feature in the upstream plasma. This shock is triggered by the pile-up of magnetic flux diffusing into the upstream flow, despite a relatively small initial level of the frozen-in magnetic field (the flow ram pressure being much greater than the magnetic field pressure). The thickness of this shock is much smaller than the m.f.p. for the ion-ion collisions, the shock is formed at a distance of $\approx $c/$\omega _{\mathrm{pi}}$ from the foil and remains stationary for the duration of the experiment ($\approx $100ns). The plasma parameters in the flow and in the shock are measured using optical Thomson scattering, two-color laser interferometry, monochromatic X-ray radiography and miniature magnetic probes. The quantitative data from this experiment, especially the spatial profiles of the density and of the flow velocity measured simultaneously in the upstream and downstream of the shock, will allow detailed verification of MHD and PIC codes used by the HEDP community.\\[4pt] In collaboration with L. Suttle, L. Pickworth, G.F. Swadling, G.N. Hall, G. Burdiak, M. Bennett, A. Ciardi, A. Harvey-Thompson, F. Suzuki-Vidal, J.P. Chittenden, S.N. Bland, P. De Grouchy, J. Skidmore, N. Niasse, A. Frank, R.A. Smith, N. Stuart, S. Patankar. [Preview Abstract] |
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