Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session UI3: Basic Plasmas: Novel Plasmas in Laboratory and Space |
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Chair: Matthew Stoneking, Lawrence University Room: Centennial II |
Thursday, November 5, 2009 2:00PM - 2:30PM |
UI3.00001: Measurements of Correlation-Enhanced Collision Rates Invited Speaker: This talk presents the first detailed experimental measurements of the Salpeter collisional enhancement factor $g ( \Gamma )$ in strongly correlated plasmas. This factor is predicted to enhance the nuclear reaction rate in dense strongly-correlated plasmas, such as in giant planet interiors, brown dwarfs and degenerate stars;\footnote{E.E. Salpeter and H.M. Van Horn, Astrophys. J. {\bf 155}, 183 (1969).} and recent theory establishes that it also applies to the perpendicular-to-parallel collisions in magnetized plasmas described here.\footnote{D.H.E. Dubin, Phys. Rev. Lett. {\bf 94}, 025002 (2005).} The enhancement is caused by plasma screening of the repulsive Coulomb potential between charges, allowing closer collisions for a given particle energy. The enhancement factor is predicted to be large when the plasma correlation parameter $\Gamma \equiv e^2 /aT$ is larger than unity, scaling as $g ( \Gamma ) \sim e^\Gamma$. The perp-to-parallel collision rate is then $\nu_{\perp \|} = n \overline{\mathrm{v}} b^2 \, I ( \overline{\kappa} ) \, g ( \Gamma )$, where $I ( \overline{\kappa} )$ decreases precipitously below $( 8 \sqrt{\pi} / 15 ) \ln \Lambda$ in the highly magnetized regime of $\overline{\kappa} \equiv \sqrt{2} \, b / r_c \gg 1$. $\bullet$ Our measurements\footnote{F. Anderegg {\it et al.}, Phys. Rev. Lett. {\bf 102}, 185001 (2009); F. Anderegg {\it et al.}, Phys. Plasmas {\bf 16}, 055705 (2009).} of $\nu_{\perp \|}$ in Mg$^+$ pure ion plasmas are consistent with the predicted Salpeter correlation enhancement, with the comparison limited mainly by systematic spatial variations in the plasma temperature. The plasma temperatures are controlled over the range $4 \times 10^{-6} < T < 1$eV, with the outer radii being up to 2$\times$ hotter. Bulk-averaged collision rates of $1 < \nu_{\perp \|} < 2 \times 10^4$ sec$^{-1}$ are measured by 2 techniques: for slow collisions, $T_\|$ is heated or cooled, and the subsequent relaxation is directly observed; for rapid collisions, sinusoidal modulation of the plasma length at frequency $f_{\mathrm{mod}}$ gives maximal heating when $f_{\mathrm{mod}} = \nu_{\perp \|} / 2 \pi c (\Gamma)$, where $c ( \Gamma )$ is the specific heat. Two densities are used, 2.0 and $0.12 \times 10^7$ cm$^{-3}$; the lower density has $ \sim 2.5 \times$ less correlation at any temperature. Experiments clearly show the expected $\nu_{\perp \|} \propto T^{-3/2}$ regime at high temperatures, and show the strong $I ( \overline{\kappa} )$ suppression of $\nu_{\perp \|}$ for $b / r_c \gg 1$. At low temperatures and high density, the measured $\nu_{\perp \|}$ is enhanced by up to $g \sim 10^{12} $ over the uncorrelated prediction, consistent with the Salpeter-enhanced prediction. At low (uncorrelated) densities, no enhancement is observed. Future experiments may be able to image ``burn fronts'' propagating from hot regions to cold regions. [Preview Abstract] |
Thursday, November 5, 2009 2:30PM - 3:00PM |
UI3.00002: Noninvasive technique for studying plasma modes of ion Coulomb crystals using cavity quantum electrodynamics Invited Speaker: Cavity Quantum ElectroDynamics (CQED) is a research field which focuses on understanding the interactions between matter and the electromagnetic field in cavities at the quantum level. Currently, CQED is a very active research field due to the prospect of creating efficient light-matter quantum interfaces at the single photon level for quantum information science. Ion Coulomb crystals have a series of properties of particular interest for CQED studies, as demonstrated in recent CQED experiments [1]. The coupling strength between ions in the crystals and photons in the cavity strongly depend on the motion of the ions due to the Doppler-effect. Consequently, the CQED signals can be exploited to learn about excitations of plasma modes in ion Coulomb crystals. Since the method relies on having one or less photons in the cavity at any time, it constitutes a noninvasive alternative to the Doppler-fluorescence method previous demonstrated in Penning trap experiments [2]. So far, CQED signal has been used to characterize how several normal mode frequencies depend on the aspect ratio of Coulomb crystals, and how the so-called micromotion of ions confined in rf traps influences the damping of the mode [3]. The observed mode frequencies are in remarkable agreement with theoretical prediction based on uniformly charged fluids [4]. \\[4pt] [1] P. F. Herskind, A. Dantan, J. P. Marler, M. Albert, and M. Drewsen, to appear in Nature Physics (2009). \\[0pt] [2] T. B. Mitchell, J. J. Bollinger, X.-P. Huang, and W. M. Itano, Opt. Express \textbf{2}, 314 (1998). \\[0pt] [3] J. P. Marler, M. Albert, D. Guenot, P. F. Herskind, A. Dantan and M. Drewsen, manuscript in preparation. \\[0pt] [4] D. H. E. Dubin, Phys. Rev. Lett. \textbf{66}, 2076 (1991). [Preview Abstract] |
Thursday, November 5, 2009 3:00PM - 3:30PM |
UI3.00003: Shock Experiments on Pre-Compressed Fluid Helium Invited Speaker: Hugoniot data were obtained for ?uid He in the 100 GPa pressure range by shock compression of samples statically pre-compressed in diamond-anvil cells. The initial (pre-compressed) He density for each experiment was tuned to a value between 1 and 3.3 times the cryogenic liquid density. Maximum observed shock-compression ratios range from 4 to 6 and show an increase in compressibility at the onset of ionization, similar to theoretical predictions. Simultaneous temperature and reflectivity data suggest that ionization is primarily temperature driven, but has an identifiable and significant density component. Fits to a modified Drude model to allow for forbidden electronic energy gaps suggest that the energy gap is relatively independent of temperature and closes with density at about 1.8 g/cc. [Preview Abstract] |
Thursday, November 5, 2009 3:30PM - 4:00PM |
UI3.00004: High Energy Plasmas in the Surroundings of Black Holes: Composite Disk Structures and Characteristic Modes$*$ Invited Speaker: Theoretically finding of composite disk structures around compact objects (e.g. black holes) and recent experimental observations indicate that highly coherent and dynamically important magnetic field configurations exist in the core of these structures [1]. These coherent configurations provide a means to resolve the ``accretion paradox'' for a magnetized disk [2] while the formation of jets that are emitted in the close vicinity of the compact object is related to them. The absence of vigorous accretion activity in the presence of black holes in old galaxies can be attributed to the loss of the surrounding coherent magnetic configurations during their history. As for relevant dynamics, axisymmetric (ballooning) modes as well as tri-dimensional spirals can be excited from disks with a ``seed'' magnetic field, under the effects of differential rotation and of the vertical plasma pressure gradient. The properties of these spirals are strongly dependent on their vertical structure. Axisymmetric modes can produce vertical flows of thermal energy [3] and particles in opposing directions that can be connected to the winds emanating from disks in Active Galactic Nuclei (AGN's). A similarity with the effects of temperature gradient driven modes in magnetically confined laboratory plasmas is pointed out. Spiral modes that are oscillatory in time and in the radial direction can produce transport of angular momentum toward the outer region of the disk structure, a necessary process for the occurrence of accretion [3]. The excitation of radially localized density spirals co-rotating with the plasma, at a distance related to the Schwartzchild radius $R_{S}=2GM_{*}/c^{2}$ where $M_{*}$ is the black hole mass, is proposed [4] as the explanation for High Frequency Quasi Periodic Oscillations (HFQPOs) of non-thermal X-ray emission from compact objects. $*$Sponsored in part by the U.S. Department of Energy.\\[4pt] [1] B. Coppi and F. Rousseau \textit{Ap. J.} \textbf{641} 458 (2006)\\[0pt] [2] B. Coppi to be published in \textit{Pl. Phys. Cont. Fus.} (2009)\\[0pt] [3] B. Coppi \textit{Europhys. Letters} \textbf{82} 19001 (2008)\\[0pt] [4] Coppi B and P. Rebusco, Paper P5.154, EPS Int. Conf. Pl. Phys. (Crete, Greece, 2008). [Preview Abstract] |
Thursday, November 5, 2009 4:00PM - 4:30PM |
UI3.00005: Astrophysically relevant radiatively cooled hypersonic bow shocks in nested wire arrays Invited Speaker: We have performed laboratory experiments which introduce obstructions into hypersonic plasma flows to study the formation of shocks. Astrophysical observations have demonstrated many examples of equivalent radiatively cooled bow shocks, for example the head of protostellar jets or supernova remnants passing through the interstellar medium or between discrete clumps in jets. Wire array z-pinches allow us to study quasi-planar radiatively cooled flows in the laboratory. The early stage of a wire array z-pinch implosion consists of a steady flow of the wire material towards the axis. Given a high rate of radiative cooling, these flows reach high sonic- Mach numbers, typically up to 5. The 2D nature of this configuration allows the insertion of obstacles into the flow, such as a concentric ``inner'' wire array, as has previously been studied for ICF research. Here we study the application of such a nested array to laboratory astrophysics where the inner wires act as obstructions perpendicular to the flow, and induce bow shocks. By varying the wire array material (W/Al), the significance of radiative cooling on these shocks can be controlled, and is shown to change the shock opening angle. As multiple obstructions are present, the experiments show the interaction of multiple bow shocks. It is also possible to introduce a magnetic field around the static object, increasing the opening angle of the shocks. Further experiments can be designed to control the flow density, magnetic field structure and obstruction locations. In collaboration with: S.V. Lebedev, M.E. Cuneo, C.A. Jennings, S.N. Bland, J.P. Chittenden, A. Ciardi, G.N. Hall, S.C. Bott, M. Sherlock, A. Frank, E. Blackman [Preview Abstract] |
Thursday, November 5, 2009 4:30PM - 5:00PM |
UI3.00006: Experimental and numerical investigation of auroral cyclotron maser processes Invited Speaker: When an electron beam with initial spread in rotational velocity is subject to significant magnetic compression, conservation of magnetic moment results in the formation of a horseshoe shaped velocity distribution. It has been shown that such a distribution is unstable to cyclotron emission [1] and may be responsible for the generation of Auroral Kilometric Radiation (AKR) -- an intense RF emission sourced at high altitudes in the terrestrial auroral magnetosphere. In a scaled laboratory reproduction of this process, a 75-85keV electron beam of 5-40A was magnetically compressed by a system of solenoids and electromagnetic emissions observed for cyclotron frequencies of 4.42GHz and 11.7GHz [2]. A comparison of these experimental measurements with the results of 2D and 3D numerical simulations will be presented, showing the effect of cyclotron-wave detuning on the efficiency of forward and backward wave coupling. The experiment presently differs from the astrophysical case in that it has a well defined radiation boundary. PiC code calculations have been undertaken to investigate the dynamics of the cyclotron emission process in the absence of such metallic boundaries and incorporating a background plasma of variable density. Computations reveal that the cyclotron emission process persists although its spatial growth is reduced. A quenching of the instability is also apparent as the ratio $\omega _{ce}$ / $\omega _{p}$ is reduced to be $<$ 1. This is consistent with the predictions of theory [3] and satellite observations that suggest AKR emission is localized within a region of plasma depletion where $\omega _{ce}$ / $\omega _{p} \quad >$ 1. \\[4pt] [1] D. Gurnett, ``Waves in Space Plasmas'', 50th Annual meeting of the APS Division of Plasma Physics, 2008. \\[0pt] [2] K. Ronald et al, Physics of Plasmas, 15, 056503, 2008. \\[0pt] [3] R. Bingham and R. A. Cairns, Physics of Plasmas, 7, 3089, 2000. [Preview Abstract] |
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