Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session UP6: Poster Session VIII: Astrophysical and Space Plasmas; Laboratory Astrophysics; Beams, Coherent Radiation and Computation; Basic Plasma Experiment; Divertors, Edge Physics, and Fuelling |
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Room: Marsalis A/B, 2:00pm - 5:00pm |
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UP6.00001: ASTROPHYSICAL AND SPACE PLASMAS |
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UP6.00002: Hamiltonian Theory and Stochastic Simulation Methods for Radiation Belt Dynamics Xin Tao, Anthony Chan, Alain Brizard, Jay Albert, James Miller A general Hamiltonian theory for the adiabatic motion of relativistic charged particles in the radiation belts and numerical modeling of multi-dimensional diffusion due to interactions between electrons and waves are presented in this work. By using Lie-transform perturbation analysis with the Hamiltonian theory, three invariants of the adiabatic relativistic motion and the guiding center equations of motion of charged particles are obtained. Interactions with small amplitude waves are described using quasi-linear diffusion theory, and we note that in previous work numerical problems arise when solving the resulting multi-dimensional diffusion equations using standard finite difference methods. In this work we introduce two new methods based on stochastic differential equation theory to solve the multi-dimensional radiation belt diffusion equations. We use our new codes to assess the importance of cross diffusion, which is often ignored in previous work, and effects of ignoring oblique waves, which are omitted in the parallel-propagation approximation of calculating diffusion coefficients. Using an established wave model we show that ignoring cross diffusion or oblique waves may produce large errors at small pitch angels and high energies. Results of this work are useful for understanding radiation belt dynamics, which is crucial for predictability of radiation in space. [Preview Abstract] |
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UP6.00003: Ion temperatures in the terrestrial ring current: Comparison between remote measurements and simulation Amy Keesee, Judith Connelly, Earl Scime, Mei-Ching Fok The processes underlying the intensification of the ring current during geomagnetic storms are not completely understood. Strong electric fields transport ions from the plasma sheet to the ring current during geomagnetic storms, causing a depression in the geomagnetic field. However, these injected ions are initially cool and are heated up during the transport into the ring current. Magnetospheric modeling can aid in the determination of the underlying physics of ring current formation and heating by clarifying what processes are necessary for simulation results to match actual measurements. The simulation tool used in this study is the Comprehensive Ring Current Model (CRCM). Using data from the Medium Energy Neutral Atom (MENA) imager on the IMAGE spacecraft, two-dimensional maps of ion temperatures can be calculated throughout the evolution of geomagnetic storms. Ion temperature maps created using a CRCM simulation for a single storm and a superposed epoch analysis of MENA data from many storms both show a region of relatively cool ion temperatures at dawn surrounded by a ring of hotter ion temperatures. We will also present comparisons of time resolved ion temperature images using the two methods for individual geomagnetic storms. [Preview Abstract] |
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UP6.00004: What Supports the Parallel Electric Field in the Birkeland (Field-Aligned) Current Regions of the Earth's Magnetosphere John Jasperse, Bamandas Basu, Eric Lund, Neil Grossbard Quasi-steady electric fields parallel to the geomagnetic field exist in both upward and downward Birkeland (field-aligned) current regions above the aurora. These fields, together with the turbulence found on auroral field lines, energize the plasma particles as they flow either away from or toward the earth. In general, these parallel electric fields are supported by one or more strong double layers, mirror force, generalized pressure gradient, and anomalous resistivity due to the turbulence. Recently, and for the first time, we have developed a new kinetic and multi-moment fluid theory for the Birkeland current system that contains the effect of turbulence for the inhomogeneous, non-uniformly magnetized plasma. Applying the new theory to observations in a downward-current sheet, we show that anomalous resistivity accounts for only a small portion of the parallel electric field and that contributions from the double layer, mirror force, and generalized pressure gradient terms in the generalized Ohm's law for the problem are more important. These results have important implications in other regions of space such as magnetospheric reconnection sites and solar coronal loops where parallel electric fields are likely to exist. [Preview Abstract] |
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UP6.00005: Particle heating and density cavity formation by inertial Alfven waves Stephen Vincena, T.A. Carter, D.W. Auerbach, W. Gekelman Shear Alfv\'{e}n waves in the inertial regime $(\omega/k_{\parallel} > \sqrt{2}v_{Te} )$ with transverse wavelengths on the order of the electron inertial length are commonly observed in the earth's low-altitude auroral zones. These regions are also replete with observations of electron beams and transversely heated ions. The auroral plasma environment is further enriched by the presence of field-aligned depletions in plasma density, and it has been suggested\footnote{Chaston, {\it et al.}, ``Ionospheric erosion by Alfv\'{e}n Waves,'' JGR, V 111, A03206, 2006.} that the Alfv\'{e}n waves may, in fact, be the cause of the erosion of ionospheric density. Laboratory experiments aimed at modeling the inertial Alfv\'{e}n wave-plasma interaction are ongoing at UCLA's Basic Plasma Science Facility in the Large Plasma Device (LAPD). In LAPD, shear waves are launched using antennas which have a current path in the plasma parallel to the background magnetic field. The waves are shown to heat both ions and electrons, create depletions in plasma density, and modify the plasma potential profile. Measurements will be presented for Alfv\'{e}n wave propagation in a uniform magnetic field as well as an increasing field, which approximates propagation along the geomagnetic field. [Preview Abstract] |
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UP6.00006: Numerical simulation of constrained and unconstrained emission from an electron horseshoe distribution A.W. Cross, K.M. Gillespie, D.C. Speirs, K Ronald, A.D.R. Phelps, S.L. McConville, C.G. Whyte, C.W. Robertson, R. Bingham, B.J. Kellet, I. Vorgul, R.A. Cairns When an electron beam is subject to significant magnetic compression, conservation of the 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 and may be responsible for the generation of Auroral Kilometric Radiation (AKR) -- an intense RF emission sourced at high altitudes in the Earth's magnetosphere. We present results from a numerical investigation of RF emission from an electron beam with predefined horseshoe distribution injected into radially bounded and unbounded geometries. Both 2D and 3D versions of the particle-in-cell (PiC) code KARAT were used to conduct the analysis. RF emission was observed at a frequency close to the relativistic electron cyclotron frequency. 3D results from the bounded case show a backward wave instability which is more resilient to Doppler broadening than forward wave coupling. This has important implications where a cold tenuous plasma is present. [Preview Abstract] |
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UP6.00007: Cyclotron Maser Radiation From An Inhomogeneous Plasma R.A. Cairns, Irena Vorgul, Robert Bingham Cyclotron maser radiation is important in both laboratory devices such as gyrotrons and in space physics applications to phenomena such as auroral kilometric radiation. To understand the behaviour, especially in the latter case where there is generally a localised region of instability, requires an understanding of how such instabilities behave in an inhomogeneous plasma. Here we consider, for simplicity, a simple ring distribution of electrons in either a step function variation of magnetic field or a continuous gradient. In each case we show that there can exist localised regions of instability from which waves, growing in time, can be radiated outwards. [Preview Abstract] |
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UP6.00008: Understanding Ionospheric Effects on the LWA Christopher Watts, K.F. Dymond, Masaya Kuniyoshi The Long Wavelength Array (LWA) is a new telescope/interferometer facility being established to do astrophysical observations in the frequency range 10 MHz to 90 MHz. As such, measurements will be strongly affected by the ionosphere. In fact, part of the LWA mandate is to make highly precise measurements of the ionosphere. We present here preliminary modeling results of the effect of the ionosphere on the LWA for a single station beam. As expected, detrimental effects of a non-uniform ionosphere are most severe at lower frequency/longer wavelength. Beam divergences of as much as +/- 5 degrees are to be expected. We also present results from a recent experiment using the Very Large Array (VLA) at 74 MHz and COSMIC satellite data to reconstruct the ionospheric density profile using tomographic techniques. [Preview Abstract] |
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UP6.00009: Solar Coronal Heating By Plasma Waves Bengt Eliasson, Robert Bingham, Padma Shukla, Lennart Stenflo The solar coronal plasma is maintained at temperatures of millions of degrees, much hotter than the photosphere which is at a temperature of just 6000K. In this paper, the plasma particle heating based on the kinetic theory of wave-particle interactions involving the kinetic Alfven waves and lower-hybrid drift modes are presented. The solar coronal plasma is collisionless, and therefore the heating must rely on turbulent wave heating models, such as lower-hybrid drift models at reconnection sites or the kinetic Alfven waves. These turbulent wave modes are created by a variety of instabilities driven from below. The transition region at altitudes of about 2000 km is an important boundary chromosphere, since it separates the collision dominated photosphere/chromosphere and the collisionless corona. The collisionless plasma of the corona is ideal for supporting kinetic wave-plasma interactions. Wave-particle interactions lead to anisotropic non-maxwellian plasma distribution functions, which may be investigated by using spectral analysis procedures being developed at the present time. [Preview Abstract] |
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UP6.00010: Lundquist Number Scaling of Solar Coronal Heating Due to Random Photospheric Footpoint Motion in a Three-Dimensional Tectonics Model L. Lin, C.S. Ng, A. Bhattacharjee We have recently obtained new scaling results in 2D for a ``tectonics model'' of coronal heating which suggest that the heating rate becomes independent of resistivity in a statistical steady state [Ng \& Bhattacharjee, Astrophys. J., {\bf 675}, 899 (2008)]. Here we extend our 2D results to 3D by means of numerical simulations. Random photospheric footpoint motion is applied for a time much longer than the correlation time to obtain converged average coronal heating rates. Simulations are done for different values of the Lundquist number to determine scaling. In the large Lundquist number limit, we recover the case in which the heating rate is independent of the Lundquist number, predicted by previous analysis as well as 2D simulations. In the same limit the average magnetic energy built up by the random footpoint motion saturates at a constant level, apparently limited by nonlinear processes, such as instabilities and/or magnetic reconnection. [Preview Abstract] |
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UP6.00011: Alignment of Velocity and Magnetic Fluctuations in Anisotropic MHD Turbulence C.S. Ng, A. Bhattacharjee There has been recent theoretical interest in the effect of the alignment of velocity and magnetic fluctuations in three-dimensional (3D) MHD turbulence with a large-scale magnetic field [Boldyrev 2005, 2006]. This theory predicts that the angle $\theta$ between the velocity and magnetic fluctuation vectors has a scaling of $\theta \propto \lambda^{1/4}$, where $\lambda$ is the spatial scale of the fluctuations. There have also been simulations on 3D forced MHD turbulence that supports this prediction [Mason {\em et al.} 2006, 2007]. Based on decaying two-dimensional (2D) turbulence, we have found the scaling of $\theta \propto \lambda^{1/4}$ within a range of time interval and spatial scales, despite the fact that Boldyrev's phenomenological theory relies on physical mechanisms operative in fully 3D turbulence in the presence of a strong external field. Higher resolution simulations and scaling analysis, based on pseudo-Alfven waves in 2D, will be presented. [Preview Abstract] |
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UP6.00012: Current-Driven Instabilities and Coronal Heating Steven Spangler Radioastronomical observations of the solar corona have yielded measurements consistent with coronal currents $\simeq 2.5 \times 10^9$ Amperes inside an Amperian Loop with a width of about 35,000 km (Spangler, Astrophysical Journal, 670, 841, 2007). An estimate has been made of the coronal heating due to Joule heating by these currents. It is assumed that the current is concentrated in thin current sheets, as suggested by theories of MHD turbulence. If the Joule heating is to be astrophysically significant, the resistivity in the corona must be enhanced by about 6 orders of magnitude relative to the Spitzer value. In this paper, I explore the possibility that instabilities produced by these currents could be responsible for generating waves and turbulence which raise the resistivity to significant levels. Model-dependent calculations of the electron drift speed in the current sheets indicate that speeds of order the electron thermal speed are possible. Current-driven instabilities and their associated waves are therefore feasible. These drift speeds also exceed the ion acoustic speed, which would excite lower hybrid waves and enhance the resistivity. [Preview Abstract] |
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UP6.00013: Particle acceleration by quasi-parallel shocks in the solar wind V.L. Galinsky, V.I. Shevchenko The theoretical study of proton acceleration at a quasi-parallel shock due to interaction with Alfven waves self-consistently excited in both upstream and downstream regions was conducted using a scale-separation model [1]. The model uses conservation laws and resonance conditions to find where waves will be generated or dumped and hence particles will be pitch--angle scattered as well as the change of the wave energy due to instability or damping. It includes in consideration the total distribution function (the bulk plasma and high energy tail), so no any assumptions (e.g. seed populations, or some ad-hoc escape rate of accelerated particles) are required. The dynamics of ion acceleration by the November 11-12, 1978 interplanetary traveling shock was investigated and compared with the observations [2] as well as with solution obtained using the so-called convection-diffusion equation for distribution function of accelerated particles [3]. [1] Galinsky, V.L., and V.I. Shevchenko, Astrophys. J., 669, L109, 2007. [2] Kennel, C.F., F.W. Coroniti, F.L. Scarf, W.A. Livesey, C.T. Russell, E.J. Smith, K.P. Wenzel, and M. Scholer, J. Geophys. Res. 91, 11,917, 1986. [3] Gordon B.E., M.A. Lee, E. Mobius, and K.J. Trattner, J. Geophys. Res., 104, 28,263, 1990. [Preview Abstract] |
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UP6.00014: Thomas Gold's Intense Solar Wind; It's evidence in prehistoric petroglyphs recorded along rivers in North and South America A.L. Peratt A past intense solar outburst and its effect on Earth circa 8,000 BCE was proposed by Gold who based his hypotheses on astronomical and geophysical evidence [1]. The discovery of high-current Z-pinch patterns in Neolithic petroglyphs provides evidence for this occurrence and insight into the origin and meaning of these ancient symbols produced by mankind. These correspond to mankind's visual observations of ancient aurora if the solar wind had increased between one and two orders of magnitude millennia ago [2]. Our data show identical MHD patterns from surveys along 300 km of the Orinoco River (Venezuela), the Chuluut River (Mongolia), the Columbia River (USA), Red Gorge (South Australia) and the Urubamba River (Peru). Three-dimensional, high-fidelity PIC simulations of intense Z-pinches replicate the carved data [3]. 1. T. Gold, \textit{Pontificiae Academiae Scientiarvm Scripta Varia}, 25, 159, 1962. 2. A. L. Peratt. \textit{Trans. Plasma Sci}. 35. 778. 2007. 3. A. L. Peratt and W. F. Yao, \textit{Physica Scripta}, T130, August 2008. [Preview Abstract] |
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UP6.00015: Relativistic Pulsar Winds with Pressure Anisotropy and Heat Flow Jason TenBarge, Richard Hazeltine, Swadesh Mahajan A newly developed covariant fluid model for magnetized plasmas\footnote{J. M. TenBarge, R. D. Hazeltine, and S. M. Mahajan, Phys. Plasmas \textbf{15}, 062112 (2008).}, incorporating anisotropy in both temperature and heat flow, is used to study equatorial radial profiles of density, velocity, magnetic field, pressure, and heat flow in the hot, strongly magnetized wind region beyond the light cylinder of pulsar magnetospheres. Radiative losses are assumed to have isotropized the wind region plasma so that $P_\parallel \gg P_\perp$. Fluid velocities are taken as mildly relativistic, while temperatures are ultra-relativistic. This study of pulsar magnetospheres extends the work by Tsikarishvili et al.\footnote{E. G. Tsikarishvili, A. D. Rogava, and D. G. Tsiklauri, Ap. J. \textbf{439}, 822 (1995).} to a more general fluid closure including heat flow. The general covariant fluid model in spherical geometry and equations of state for arbitrary temperature will also be presented for more general applicability. [Preview Abstract] |
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UP6.00016: Momentum transport from current-driven and flow-driven instabilities in astrophysical disks S.C. Prager, F. Ebrahimi, D.D. Schnack Rapid transport of angular momentum in astrophysical disks can be explained through the stresses arising from MHD instabilities. Here, we examine momentum transport from current-driven and flow-driven instabilities in disk geometry. We perform nonlinear MHD computations both for turbulence generated by tearing modes and by flow-driven Magneto-Rotational Instability (MRI). We find that in an MRI stable disk configuration, tearing modes can grow and cause transport of momentum. The effects of disk thickness and flow magnitude in momentum transport from tearing and MRI instabilities will be shown. We also examine the saturation mechanism of flow-driven instability through analytical quasilinear theory and through nonlinear computation of a single mode in a rotating disk. We show that the generation of large-scale magnetic field through the alpha effect causes the MRI mode to saturate. [Preview Abstract] |
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UP6.00017: Angular Momentum of Non-axisymmetric Global Modes in Magnetically Confined Plasmas and in Astrophysics* L. Fei, B. Coppi The angular momentum of typical tridimensional modes that can be excited in magnetically confined laboratory plasmas and in astrophysics is evaluated by extending pre-existing theories [1], that are applicable to ``conventional'' waves. For the former case, pressure gradient driven ballooning modes whose frequencies are larger than their growth rates (e.g. given by two-fluid theories) are considered in view of the transport of angular momentum out of the plasma column produced by them. This is one of the processes that can lead to a spontaneous rotation [2], by recoil, of the plasma column. For the latter case tridimensional spiral modes [3] are considered that can be excited in plasma disk structures around compact objects and transport angular momentum radially away from the radius where they co-rotate with the plasma. This allows for mass accretion toward the central object to occur. Two classes of spirals are considered: those that are radially standing and are unstable and those that are convective and oscillatory in the relevant co-rotating frame.{*}Sponsored in part by the U.S. D.O.E. \\ {[1] B. Coppi, M.N. Rosenbluth and R.N. Sudan, \textit{Ann. Phys.} \textbf{55}, 2; 207-248 (1969)}.\\ {[2] B. Coppi, \textit{Nucl. Fus.} \textbf{42}, 1 (2002)}.\\ {[3]B. Coppi, Paper P1.177, E.P.S. Inter. Conf. (Crete, Greece, 2008).}\\ [Preview Abstract] |
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UP6.00018: Twin-Peak Quasi Periodic Oscillations and Tri-dimen-sional Spiral Modes of Disks Around Black Holes* P. Rebusco, B. Coppi Existing explanations of high frequency Quasi Periodic Oscillations (QPO\textquoteright{}s) from compact objects have shortcomings [1] that a theory based on the excitation of tri-dimensional spiral modes [2] co-rotating with the plasma near a black hole can avoid. The modes that are likely to prevail, with the largest growth rates, are localized relatively close to the last stable orbit (a.k.a. ISCO). The modulation of the radiation due to the rotating plasma density enhancements associated with the spirals and reaching the observer, is evaluated by an appropriate extension of existing analyses [3] developed for a rotating ``hot spot'' model. As a result of relevant non-linear decays, the lowest harmonics $m_{\phi}=2$ and $m_{\phi}=3$ of the considered spiral modes (where $m_{\phi}$ is the toroidal mode number) are envisioned to acquire the largest amplitudes justifing the observed $3/2$ ratios of the two peaks of the frequency spectra of high frequency QPO\textquoteright{}s. {*}Sponsored in part by the U.S. D.O.E and the Pappalardo Fellowship. \\ {[1] B. Coppi and P. Rebusco, Paper P5.154, E.P.S. Inter. Conf. (Crete, Greece, 2008).}\\ {[2] B. Coppi, Paper P1.177, E.P.S. Inter. Conf. (Crete, Greece, 2008).}\\ {[3] J. D. Schmittman and E. Bertschinger, Ap. J. \textbf{606}, 1098 (2004).} [Preview Abstract] |
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UP6.00019: General Relativistic Plasma Disk Dynamics, Key Role of Topology and Interpretation of Black Hole Associated Phenomena* B. Coppi The geometry of the plasma disk structures surrounding black holes plays a key role in the excitation of collective modes [1] that are relevant to the global dynamics of these structures. A class of unstable tri-dimensional spirals [2] is identified that depend on the vertical profile of the associated plasma density perturbations, are localized around the radius where they co-rotate with the plasma and are driven by the local differential rotation and the vertical gradient of the plasma pressure. For relatively flat temperature profiles, spiral modes that are purely oscillatory in the frame co-rotating with the plasma and are convective (radially) are found. A considerable rate of radial transport of angular momentum can be associated with these modes. Coexisting unstable normal modes both axisymmetric [1] and of the spiral type [2] can produce significant vertical plasma outflows away from the equatorial plane. The effects of these modes provide a basis for interpreting experimental observations associated with black holes, in particular Active Galactic Nuclei Winds and high frequency Quasi Periodic Oscillations, and for justifying rates of angular momentum transport consistent with the inferred rates of mass accretion. {*}U.S. D.O.E. partially sponsored.\\ {[1] B. Coppi, \textit{Europhys. Letters} \textbf{82}, 19001 (2008).}\\ {[2] B. Coppi, Paper P1.177, E.P.S. Inter. Conf. (Crete, Greece, 2008).} [Preview Abstract] |
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UP6.00020: Long Term Evolution of Magnetized Bubbles in Galaxy Clusters Wei Liu, Hui Li, Shengtai Li, Scott Hsu We have performed nonlinear ideal magnetohydrodynamic simulations of the long term evolution of a magnetized low-density ``bubble" plasma formed by a radio galaxy in a stratified cluster medium. It is found that about 3.5\% of the initial magnetic energy remains in the bubble after $\sim 8 \times 10^{9}$~years, and the initial magnetic bubble expansion is adiabatic. The bubble can survive for at least $8 \times 10^9$~years due to the stabilizing effect of the bubble magnetic field on Rayleigh-Taylor and Kelvin-Holmholtz instabilities, possibly accounting for ``ghost cavities" as observed in Perseus-A\@. A filament structure spanning about 500~kpc is formed along the path of bubble motion. The mean value of the magnetic field inside this structure is $\sim 0.57$~$\mu$G at $\sim8\times10^9$~years. Finally, the initial bubble momentum and rotation have limited influence on the long term evolution of the bubble. [Preview Abstract] |
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UP6.00021: Magnetic Fields and Turbulence in the Intra-cluster Medium of Galaxy Clusters Hui Li, Hao Xu, Wei Liu Recent observations have revealed that the intra-cluster medium (ICM) of galaxy clusters could be significantly magnetized. Observations further revealed that magnetic fields in the ICM have large amount of fluxes, yet appearing to have a power-law spectrum. It is often suggested that such magentic fields could be generated via a turbulent dynamo. Here, we study a different scenario where significant magnetic fields are produced by supermassive black holes (SMBHs) in the centers of massive galaxies, then these magnetic fields are transported to the wider ICM via jets/lobes during the lifetime of active galaxies ($\sim$ 100 Myr). Subsequent cluster mergers during the cluster evolution (up to 10 Gyr) will stir, shear, and shock the ICM as well as the magnetic fields provided by SMBHs. We present numerical simulations of the evolution of clusters with magnetic fields using the newly developed cosmological MHD code with adaptive mesh refinement. The evolution of magnetic field energy and flux, along with the ICM dynamics, will be discussed in detail. By comparing our simulations with the observations, we will explore the implications for MHD turbulence and dynamo mechanisms in the ICM. [Preview Abstract] |
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UP6.00022: Simulations of Decaying Kinetic Alfv\'en Wave Turbulence: Intermittent and Coherent Structures Kurt Smith, Paul Terry We simulate decaying kinetic Alfv\'en wave turbulence in a strong guide field, appropriate for modeling interstellar turbulence at scales $\leq 10\rho_s$. Ion flow decouples from the system at these scales, while electron density ($n_e$) fluctuations equipartition with the magnetic field. Stable circularly symmetric structures form in $J$, $\mathrm{B}$ and $n_e$ fields after a few Alfv\'en times; nonlinear magnetic shear prevents turbulence from mixing the structures into the background and allow the structures to persist for many Alfv\'en times. $J$ filaments are large in amplitude and spatially localized, and their associated $\mathrm{B}$ and $n_e$ structures are less localized, consistent with the Biot-Savart law and KAW equipartitioning. Ensemble-averaged pdfs indicate $n_e$ and $\nabla n_e$ deviate strongly from Gaussian statistics following the onset of structure formation. The non-Gaussian $\nabla n_e$ statistics are especially of interest as a possible explanation of $\tau \propto D^4$ scaling of pulsar signal widths $\tau$ with distance-to-source $D$.---Work supported by NSF. [Preview Abstract] |
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UP6.00023: MHD stability of extragalactic jets with azimuthal rotation C. Carey, C. Sovinec, J. Everett, S. Heinz Observations of extragalactic jets show highly collimated structures that extend over distances which are many orders of magnitude larger than the central object from which they emanate. Hydrodynamic and magnetohydrodynamic (MHD) launching scenarios have been investigated in various studies [1], but 3D effects have been tractable only relatively recently. An important question for Poynting-flux dominated jets is how they remain robust to kink-type instability. Here, we present nonlinear non-relativistic 3D MHD computations that produce collimated outflows over scales that are significantly larger than the shearing scale of the accretion disk. The calculations indicate that the stability of the column with respect to the kink mode depends on the rotation velocity of the accretion disk relative to the Alfven velocity in the column, similar to the findings of Ref. 2. Above a critical disk velocity, the column is observed to be stable. To confirm threshold rotation rates, we have performed an eigenmode analysis in periodic cylindrical geometry. 1D current profiles that are unstable to m=1 kink modes without flow are found to be linearly stable with rigid rotation at sub-Alfvenic speeds. 1. A. Ferrari, Annu. Rev. Astron. Astrophys. 36, 539 (1998). 2. M. Nakamura and D. L. Meier, ApJ 617, 123 (2004). [Preview Abstract] |
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UP6.00024: Astrophysical jet dynamos based on spheromak, dusty plasma, and Hamiltonian concepts Paul Bellan Experiments at Caltech demonstrate that spheromak formation physics and astrophysical jets are closely related [1] as both involve toroidal magnetic field pressure inflating poloidal flux surfaces. The use of capacitor banks to power the lab magnetic fields raises the question of what powers the magnetic fields in the astrophysical situation where gravity is presumably the ultimate power source. In answer to this question, the dust grain mass accretion rate is shown to be much greater than previously assumed [2]. Then, by considering Hamiltonian trajectories of charged dust grains in combined gravitational--magnetic fields, dynamos suitable for powering toroidal and poloidal magnetic fields are demonstrated. The toroidal field dynamo is powered by gravitational power liberated by dust grains having zero canonical momentum; these have spiral trajectories towards the central object [3]. The poloidal field dynamo results from dust grains with Speiser-type trajectories; these grains meander back and forth across a toroidal magnetic axis [3]. Supported in part by USDOE \newline [1] P. M. Bellan et al, J. Fusion Energy 10.1007/s10894-006-9048-z (2006) \newline [2] P. M. Bellan, ApJ 678, 1099 (2008) \newline [3] P. M. Bellan, ApJ (in press), http://arxiv.org/abs/0807.1373 [Preview Abstract] |
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UP6.00025: Magnetic Pattern Resulting from Particle Acceleration in the Cosmic Ray Shock Precursor V. Ivanov, M.A. Malkov, P.H. Diamond An acoustic instability of shocks which nonlinearly accelerate particles by the diffusive mechanism is studied in a nonlinear regime. The instability is driven by the pressure gradient of accelerated particles. The nonlinear term, included in the description of the instability, leads to the steepening of the unstable acoustic waves and to the formation of an ensemble of shocks (shocktrain). By compressing an ambient and turbulent magnetic field the shocktrain creates a distinct magnetic pattern. This changes the transport regime of accelerated particles from diffusive at lower momenta to fractionally kinetic (Levy flights and traps) over about two decades below the momentum cut-off. As a result, the spectral index of these particles steepens. [Preview Abstract] |
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UP6.00026: Cosmic ray effect on magnetization of a relativistic foreshock in an unmagetized and weakly magnetized plasmas Mikhail Medvedev Cosmic Rays (CRs) accelerated by a shock form a streaming distribution of outgoing particles in the foreshock region. If the ambient fields are small enough, compared to the shock and CR energetics, the magnetic fields are to be generated in the shock upstream via the Weibel instability. Here we demonstrate self-similar nature of the foreshock region and calculate its structure, e.g., the magnetic field strength, its correlation scale, etc., as a function of the distance from the shock. This result indicates that the entire foreshock region of thickness comparable to the shock radius may be populated with magnetic fields much stronger than the typical interstellar medium magnetic fields. The presence of such fields in the foreshock region can help to explain both the efficient particle acceleration and large radiative efficiency of a gamma-ray burst afterglow shock. [Preview Abstract] |
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UP6.00027: Energy transfer via Weibel and two-stream instabilities in two-temperature electron-ion plasmas Jaehong Park, Chuang Ren, Eric G. Blackman, Xianglong Kong Whether an efficient collisonless temperature equilibration mechanism exists for a two-temperature ion-electron plasma, with $T_i>T_e$, is important for understanding astrophysical phenomena such as radiatively inefficient accretion flows and relativistic collisionless shocks in GRBs. Here we study whether the two-stream and Weibel instabilities driven by proton counter-streaming and/or temperature anisotropy can be such a mechanism. Analysis and PIC simulations show that the Weibel instability alone induces only a weak electro-ion coupling in either non-magnetized [Ren, Blackman, and Fong, Phys. Plasmas, 14:012901 (2007)] or magnetized plasmas. The two-stream instability alone also provides a weak coupling [Davidson et al., PRL, 24:579 (1970)]. However, we will provide 2D PIC simulation results to show distinct stages with different dominant modes during the nonlinear evolution and that the interactions of the two instabilities can be more effective for the electron-ion. This work is supported by the U.S. Department of Energy under Grant Nos. DE-FC02-04ER54789 and DE-FG02-06ER54879. [Preview Abstract] |
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UP6.00028: Lower-Hybrid Wave Activity, Reconnection and the reactive Weibel Instability Robert Bingham, Luis Silva, V.D. Shapiro, Padma Shukla, Raoul Trines An isolated current sheet unsuitable to the filamentation of the current seeks to lower its energy and converts it into kinetic energy by the attraction of parallel current elements, in collisionless plasmas anomalous resistivity resulting from particle inertia or wave-particle interactions is required for current filamentation leading to the tearing mode instability or reconnection. We demonstrate that lower -hybrid activity can be responsible for anomalous resistivity and the resulting for anomalous diffusion rate driving magnetic reconnect ion. We further demonstrate that the current filamentation may also be associated with the reactive Weibel instability. Applications to laboratory and space plasmas will be presented. [Preview Abstract] |
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UP6.00029: Self-Regulating Reconnection in Marginally Collisionless Coronae of Accreting Black Holes Jeremy Goodman, Dmitri Uzdensky Hard X-ray (up to $\sim$100 keV) emission is commonly observed in accreting Galactic (stellar-mass) and extra-galactic (super-massive) black hole sources. This emission is often attributed to the Comptonization of soft accretion-disk photons by a hot overlying corona with a Thomson optical depth is of order~1. We show that this observational result suggests that the coronal plasma is roughly marginally collisionless with respect to magnetic reconnection. As has been recently suggested for the Sun's corona,\footnote{D.~Uzdensky, ApJ, 671, 2139 (2007); see also Cassak~et~al., ApJ, 644, L145 (2006).} such marginal states may naturally result from a combination of disk-corona mass- and energy-exchange processes and the condition for the onset of fast collisionless reconnection. We also analyze the electron and ion cooling processes in a reconnection-heated corona, investigate the roles of pair creation and ion thermal conduction, and explore observational implications of our physical picture. [Preview Abstract] |
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UP6.00030: LABORATORY ASTROPHYSICS |
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UP6.00031: A laboratory experiment for studying the eruption of a magnetic flux loop in a large magnetized plasma Shreekrishna Tripathi, Walter Gekelman A new experiment has been constructed to simulate the interaction of erupting coronal loops with the ambient solar plasma. The laboratory plasma loop is produced using an annular LaB$_6$ cathode and an annular anode mounted on two movable shafts in a large vacuum chamber (1.0 m diameter, 4.5 m long). Each electrode has an electromagnet to produce a vacuum magnetic field along the axis of the flux loop. The maximum magnetic field at the foot-points of the flux loop is $\sim$ 0.1 T. Two laser beams (1064 nm, $\sim$ 0.5 J/pulse) strike movable carbon targets placed behind the orifices of the electrodes to generate controlled flows. This set-up produces a magnetic flux loop with maximum density $\sim 5 \times 10^{19}$ m$^{-3}$ and maximum discharge current 250 A. The vacuum chamber has $\sim 0.03$ T axial magnetic field and additional source for producing the ambient plasma. Langmuir probe, magnetic loop probe, and fast imaging camera are main diagnostics. We plan to present the initial results from this experiment characterizing the flux loop and showing the details of its evolution in the ambient plasma. [Preview Abstract] |
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UP6.00032: Magnetically driven behavior of plasma loops Eve Stenson, Paul Bellan By studying one or two current-carrying arched flux tubes in a laboratory environment, much can be learned about fundamental plasma dynamics and, potentially, analogous features found in the solar corona. These flux tubes, filled with low-beta plasma, are created with a modified plasma gun. Similar to spheromak guns but possessing a different geometry, the gun comprises an arched vacuum field linking a coplanar anode and cathode. Neutral gas is supplied from nozzles in the electrodes as high voltage is applied, ionizing the gas to form a semicircular loop of plasma. Supplying more than one neutral gas allows the resulting portions of the plasma to be imaged separately with optical filters. When two gases are supplied to a single loop, one from each electrode, high-speed jets are seen to flow from both ends into the apex. This method was used to test an MHD theory explaining flux tube collimation (P. M. Bellan, Phys. Plasmas 10, 1999 (2003)). If instead a pair of loops is created, each from a different gas, the two twist around each other and/or merge; experiments of this type suggest reconnection effects (J. F. Hansen et al, Phys. Plasmas 11, 3177 (2004)). The plasma's changing magnetic field is measured with an array of ``B dot'' probes and compared to force-free models. [Preview Abstract] |
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UP6.00033: Characterization of the Turbulent Electromotive Force in the Madison Dynamo Experiment E.J. Kaplan, C.B. Forest, R.D. Kendrick, N.Z. Taylor, E.J. Spence The Madison Dynamo Experiment is a simply connected liquid sodium dynamo experiment. Two impellers driven by 100 horsepower motors drive the sodium with a mean velocity field that approximates either a t2s2 or t1s1 configuration. Two sets of inductive coils in a Helmholtz configuration provide seed fields in axial or transverse dipole and quadrupole fields. Previous experiments on the Madison Dynamo Experiment have shown the existence of growing mean fields in both Axial and Transverse dipole configurations. Both of these growth modes are attributed to turbulent EMFs in the experiment. A high current signal generator has been built to apply monochromatic, oscillating seed fields to the dynamo experiment. With this, the magnitudes and phases of the magnetic fields within the experiment can be compared to that predicted by laminar dynamo theory in order to characterize the effect of these EMFs. [Preview Abstract] |
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UP6.00034: Reduction of Turbulence in the Madison Dynamo Experiment N.Z. Taylor, C.B. Forest, N.S. Haehn, E.J. Kaplan, R.D. Kendrick, K. Reuter, E.J. Spence This poster describes efforts to observe spontaneous magnetic field generation in the Madison Dynamo Experiment. The experiment uses a turbulent flow of liquid sodium, driven by two counter-rotating impellers in a one meter-diameter sphere. The time-averaged flows are expected to be dynamos, but previous work has shown that turbulent fluctuations strongly increase the minimum speed required for self-excitation, beyond the design parameters of the experiment. Two approaches for accessing dynamos in the experiment seem feasible. First, a computational fluid dynamics code has shown that reduction of large scale turbulence and optimization of the helicity of the mean flow can be achieved through the addition of baffles to the experiment. Second, a sub-critical dynamo transition has recently been discovered using numerical simulations; by supplying a sufficiently strong magnetic field to the turbulent flow, the fluctuations are reduced, a dynamo grows and saturates, and can be sustained when the externally applied field is removed. The experimental modifications necessary to the experiment will be described, including a newly implemented set of magnetic probes for characterizing the eigenmodes. [Preview Abstract] |
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UP6.00035: MHD Modeling of a Plasma Dynamo Experiment Cary Forest, Adam Bayliss, Dalton Schnack, Erik Spence, Klaus Reuter A new plasma experiment to investigate the magnetorotational instability, dynamos, and other fundamental plasma processes for astrophysics is described and numerically modeled using MHD computation. Use of a plasma for such an experiment may allow the magnetic Reynolds number (the dimensionless parameter governing self- excitation of magnetic fields) to be approximately a factor of 10 larger than in liquid metal experiments The experiment uses an axisymmetric ring cusp geometry (poles facing inward with alternating polarity along the vessel wall) to confine a plasma in a large, magnetic field free region in the center of the device. To stir the plasma, cathodes positioned between the magnet rings are biased such that the resulting electric field induces plasma rotation through the ExB drift. This poster describes numerical simulations using {\sc NIMROD} and the incompressible MHD code {\sc DYNAMO} of the experiment that (1) establish the viability of driving the flows using the proposed electrode scheme in the multicusp geometry, and (2) that edge driven differential rotation can provide flows that can lead to bulk flows suitable for a broad range of turbulence and dynamo studies in astrophysically relevant parameters. [Preview Abstract] |
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UP6.00036: Spinning a Hot, Unmagnetized Plasma C. Collins, C.B. Forest, R. Kendrick, J. Jara-Almonte A Plasma Couette Experiment is under construction to investigate a nearly unmagnetized, differentially rotating plasma. A host of astrophysically motivated processes can be studied, including the magnetorotational instability, a mechanism that may account for outward transport of angular momentum in accretion disks. The plasma is confined by a cylindrical, axisymmetric, highly localized ring cusp magnetic field at the boundary. Electrodes positioned between the magnet rings are biased with alternating polarity so that the resulting electric field induces ExB drift. This poster discusses the initial diagnostics for measuring plasma parameters, including rotation. Density and temperature profiles will be measured using a single tip Langmuir probe, and plasma potential will be determined using an emissive probe. As plasma begins to rotate, the plasma density and plasma potential are expected to hollow out, with the electron pressure gradient balancing the outward centrifugal force. Plasma flow will also be measured with a Mach probe. Evidence of rotation will be presented, and the efficiency of plasma spin-up through edge-applied ExB drift will be assessed. [Preview Abstract] |
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UP6.00037: Overview of the Princeton MagnetoRotational Instability Experiment E. Schartman, M. Nornberg, A. Roach, H.T. Ji, D. Coster, W. Liu, J. Goodman, M.J. Burin A turbulent viscosity is required to explain the observationally-inferred rates of angular momentum transport in accretion disks. Investigation of thin disks has focused on two sources of instability to drive the turbulence: the MagnetoRotational Instability (MRI) and Subcritical Hydrodynamic Instability (SHI). In MRI a weak ambient magnetic field causes the radially-decreasing angular velocity to become a source of free energy. In SHI, stable perturbations allow access to unstable modes. This experiment investigates both of these instabilities in a Couette-Taylor flow. Using water or liquid Gallium alloy we generate rotating shear flows with linear stability properties analagous to astrophysical disks. Differentially rotatable end-rings reduce boundary effects. We found no evidence of SHI, up to Reynolds number of order one million. During the MHD experiments a solenoidal magnetic field of up to 5~kG is applied. Radially-aligned induction coils detect magnetic perturbations generated by the liquid metal. Initial magnetized experiments focussed on magneto-Coriolis waves which at large magnetic Reynolds number are expected to transition into MRI modes. Results of the current search for the MRI will be presented. [Preview Abstract] |
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UP6.00038: Angular Momentum Transport Studies in the Princeton MRI Experiment Austin Roach, Mark Nornberg, Hantao Ji, Jeremy Goodman The Princeton MRI experiment seeks to understand the effect of the magnetorotational instability on angular momentum transport in rotating MHD systems. This poster will discuss progress in numerical and experimental work to better understand angular momentum transport in the device. A global stability code is being developed to numerically investigate nonaxisymmetric modes of nonideal MHD, which have been observed in the experiment. Experimentally, development of an ultrasound doppler velocimetry system is being pursued in order to measure azimuthal velocity profiles and velocity fluctuation levels. Results from a trial of an ultrasound system in a liquid gallium channel flow will be presented. Finally, the results of an effort to seek consistency between experiments and numerical simulations of the effect of boundary layer perturbations on bulk velocity profiles and angular momentum transport in water experiments will be discussed. Supported by DOE, NASA, and NSF. [Preview Abstract] |
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UP6.00039: Free-surface MHD channel flow experiments M.D. Nornberg, H. Ji, J. Luc Peterson, J.R. Rhoads Surface waves and turbulence are essential components to processes in both astrophysical and laboratory plasmas. Energetic events such as X-ray bursts from neutron stars are thought to be related to the waves generated by accretion of material onto the dense plasma ocean on the star surface. Interest in using liquid metals in a first-wall application in fusion devices raises important questions about the stability of a flowing liquid metal subject to strong magnetic fields and high heat flux. A liquid metal channel experiment is used to study the basic physics of free-surface MHD effects in turbulent channel flow. The design of the channel, pneumatic transfer system, and pumping scheme is presented. Laser Doppler Velocimetry measurements using tracer particles in water are used to model the velocity profile as a turbulent boundary layer. Measurements of the wave propagation characteristics in the liquid metal demonstrate the surfactant effect of surface oxides and the reduction of turbulent fluctuations by a cross-channel magnetic field. Although streamwise waves are not damped, the turbulent structures generating them are suppressed. Implications for turbulent mixing will be discussed. [Preview Abstract] |
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UP6.00040: Effects of transverse magnetic field on channel flow of liquid gallium John Rhoads, Hantao Ji, Mark Nornberg, Scott Pfeffer Interest in using liquid metals as first walls in fusion devices requires understanding of their behavior in strong magnetic fields. The effects of such a field applied orthogonal to the direction of flow of liquid gallium in a wide aspect ratio channel were studied through several diagnostics. Magnetohydrodynamic (MHD) theory was tested for surface waves in the deep liquid limit along with the cross-channel velocity profile. A non-invasive diagnostic consisting of an intensified-CCD camera capturing the positions of an array of reflected lasers was employed. The resulting dispersion relation was found to agree with linear MHD theory. Strong damping of turbulent structures was observed along the field lines, while no damping was observed in longitudinal waves. A second non-invasive diagnostic using a position sensitive photodiode was used to obtain the full frequency response of the surface waves, which implicates a transition to two-dimensional turbulence. An invasive potential probe diagnostic was developed to measure the local velocity to examine boundary layer characteristics. Experimental results and conclusions will be discussed. [Preview Abstract] |
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UP6.00041: Magnetorotational Instability in Multiple-fluid Plasmas H. Ji, H. Rinderknecht, J. McDonald, M. Nornberg, M. Yamada, A. Gurak, E.L. Foley Fast angular momentum transport in accretion disks has been an outstanding problem in astrophysics for more than three decades. The magnetorotational instability (MRI) has been identified as a powerful mechanism to transport angular momentum. Experiments using liquid metal are underway to study the MRI in incompressible MHD limit. A new frontier in accretion disk research is to explore physics beyond incompressible MHD. Possible new effects include compressibility, multiple-fluid effects, kinetic effects, ion-neutral collisions, radiation pressure, and dust grains. Theoretical and experimental aspects of multiple-fluid effects of MRI will be discussed in this poster. Traditional two-fluid (electron and ion fluids) effects manifest as the Hall effect, which leads to qualitative differences between the cases when magnetic field is parallel and anti-parallel to the rotation axis. Including a third fluid of neutral particles leads to the so-called ``ambipolar diffusion,'' adding further complexity in the dynamics of MRI. Experimentally, some of these effects can be studied in laboratory plasmas under some specific conditions. A newly constructed small-scale experiment using a helicon plasma has been used to explore these possibilities. The detailed analyses and experimental results will be presented. [Preview Abstract] |
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UP6.00042: Simulations of waves in magnetised spherical Couette flow Erik Spence, Klaus Reuter The observation of hydromagnetic waves in electrically-conducting spherical Couette flow experiments has recently been reported. Here we present simulations of such experiments in the presence of a dipolar applied magnetic field, and report the observation of non-axisymmetric propagating disturbances in the simulated experiment's velocity and magnetic fields. The energy source which fuels the growth of the disturbances is described. Analysis of the waves is presented, and a dispersion relation calculated. The simulated waves are compared to the waves observed in experiments. [Preview Abstract] |
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UP6.00043: A Simplified Method to Estimate Transport Coefficients of Laser Plasmas and H-Rich White Dwarf Stars Vithal L. Patel, Jaechul Oh High intensity laser-matter interactions generate large magnetic fields of several 100s MG [1]. Hydrogen-rich white dwarf stars are known to exhibit large magnetic fields of 10kG to 10MG. Laser generated laboratory plasmas as well as astrophysical plasmas such as the ones in the crust of white dwarf stars may be weakly Landau quantized. Estimates of transport properties are important for the energy transport in the laboratory plasmas and thermal evolution of these stars. A complex weakly Landau quantization problem can be simplified [3] by reducing it to simple equations without evaluating elaborate integrals. We calculate transport coefficients for hydrogen and helium plasmas for a range of magnetic fields relevant to laser plasmas and white dwarf stars. This research was performed in the Laser Plasma Branch, Plasma Physics Division, Naval Research Laboratory and supported by DOE/NNSA. [1] U. Wagner et. al., Phys. Rev. E, 70, 026401, 2004 [2] A. Kawka et. al., Astrophys. J., 654, 499, 2007 [3] A. Y. Potekhin, Astron. Astrophys., 346, 345, 1999 [Preview Abstract] |
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UP6.00044: Colliding Laser-Produced Plasmas on LaPD Andrew Collette, Walter Gekelman The expansion and interaction of dense plasmas in the presence of a magnetized background plasma is important in many astrophysical processes. We describe a series of experiments which involve the collision of two dense (initially $n > 10^{15}{\mbox{cm}}^{-3}$) laser-produced plasmas within an ambient, highly magnetized background plasma at the UCLA Large Plasma Device facility. These plasmas form diamagnetic cavities in which a large fraction of the background field (600G) has been expelled. Fast (3ns) camera observations of this experiment recorded complicated structures, including coherent corrugated structures on the bubble surfaces. The data hint at the presence of turbulence in the interaction. In order to directly investigate the evolution of the magnetic field, we developed a novel diagnostic system composed of small (1-mm) 3-axis differential magnetic field probes, in conjunction with a vacuum ceramic motor system capable of sub-micron positioning accuracy. Using an ensemble of magnetic field data from fixed and movable probes, we calculate the cross-spectral function, from which the dominant modes and ultimately the dispersion relation of waves in this region may be deduced. [Preview Abstract] |
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UP6.00045: Experiments to Observe the Weibel Instability: The Origin of Gamma Ray Burst Afterglow C.M. Huntington, T. Matsuoka, C. McGuffy, A. Maksimchuk, V. Yanovsky, K. Krushelnick, T. Katsouleas, M. Medvedev, L.O. Silva, W.B. Mori, R. Bingham, R.P. Drake Recent theory suggest that the radiation signature of gamma ray burst afterglow may be the result of the interaction of ultrarelativistic electrons, ejected from supernova shocks, with small-scale magnetic fields. These tiny ``tangled'' magnetic fields are thought to be created by the two-stream filamentation instability, or Weibel Instability, of the beaming electrons. Using the Hercules laser facility at the University of Michigan, we are conducting an experiment to create an electron beam by the laser wakefield technique, produce such filaments by passing the electron beam through another plasma, and image the resulting structure. This experiment provides one of the first direct observations of Weibel filamentation in a relativistic electron beam. [Preview Abstract] |
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UP6.00046: Magnetic Bubble Expansion as an Experimental Model for Extra-Galactic Radio Lobes Alan G. Lynn, Yue Zhang, Scott Hsu, Hui Li, Wei Liu, Mark Gilmore, Christopher Watts The Plasma Bubble Expansion Experiment (PBEX) has begun laboratory experiments and coordinated nonlinear MHD simulations to address outstanding nonlinear plasma physics issues related to how magnetic energy and helicity carried by extra-galactic jets interacts with the intergalactic medium to form radio lobe structures. Experiments are being conducted in the 4 meter long, 50 cm diameter HELCAT linear plasma device at UNM. A pulsed magnetized coaxial gun ($\sim $10 kV, $\sim $100 kA, $\sim $2 mWb) forms and injects magnetized plasma bubbles perpendicularly into a lower pressure weakly magnetized background plasma formed by a helicon and/or hot cathode source in HELCAT. Experimental parameters can be adjusted so that important dimensionless parameters are relevant to the astrophysical context. Ideal MHD simulations show that an MHD shock develops ahead of the bubble as it propagates, and that the bubble develops asymmetries due to the background field [1]. First experimental data, including magnetic probe measurements and high-speed camera imaging, will be presented. [1] W. Liu et al., Phys. Plasmas \textbf{15}, 072905 (2008). [Preview Abstract] |
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UP6.00047: Design of a Compact Coaxial Magnetized Plasma Gun for Magnetic Bubble Expansion Experiments Yue Zhang, Alan G. Lynn, Scott C. Hsu, Hui Li, Wei Liu, Mark Gilmore, Christopher Watts We will discuss the design of a compact coaxial magnetized plasma gun and its associated hardware systems in detail. The plasma gun will be used for experimental studies of magnetic bubble expansion into a lower pressure background plasma, as a model for extragalactic radio lobes. The gun is powered by an ignitron-switched capacitor bank. High-pressure gas will be puffed into an annular gap between inner and outer coaxial electrodes. An applied high voltage ionizes the gas and creates a radial current sheet. The 100kA discharge current generates toroidal flux; poloidal flux is provided by using an external bias magnet. The axial J$\times $B force ejects plasma out of the gun. If the J$\times $B force exceeds the magnetic tension of the poloidal flux by a sufficient amount then a detached magnetized plasma will be formed. The poster will discuss the plasma bubble formation system including the power system, gas valve control system, bias flux power system, and the magnetic probe diagnostic in detail. Experimental data will be provided. [Preview Abstract] |
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UP6.00048: Critical ionization velocity in plasma-neutral collisions on the Caltech spheromak experiment A.L. Moser, P.M. Bellan Alfv\'en predicted [1] that when a neutral gas impacts a magnetized plasma it will be ionized and entrained if its velocity across magnetic field lines exceeds the critical ionization velocity, $v=\sqrt{2 E/m}$, where $E$ is the ionization energy of a neutral atom with mass $m$. In experiments using the coplanar spheromak gun at Caltech, a magnetized plasma jet collides with a target cloud of neutral gas at a relative velocity that can be adjusted above or below the critical ionization velocity of the neutral cloud. The jet and its associated magnetic field deform as the jet impacts the neutral cloud. At relative velocities exceeding the critical velocity, the plasma jet slows dramatically and spectroscopic measurements show ionization of the target gas. The ratio of ion to neutral lines increases as relative velocity increases. Future experiments will look for a lack of ionization (neutral spectral lines only) of the neutral target cloud when the relative velocity is below the critical ionization velocity. [1] H. Alfv\'en, Rev. Mod. Phys., {\bf 32}, 710, (1960) [Preview Abstract] |
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UP6.00049: Kelvin-Helmholtz Instability in a Sheared Flow Actuated by a Magnetic Field Sandra Wright, Radu Presura, Stephan Neff, Christopher Plechaty, David Martinez The Kelvin-Helmholtz instability can lead to plasma transport across a magnetic field; one example is the solar wind transport across the earth's magnetotail. In an experiment done at the Nevada Terawatt Facility, we observed the Kelvin-Helmholtz instability using a laser produced plasma flowing across an external magnetic field. This instability is evidenced by the presence of evenly spaced vortices on the boundary parallel to the normal to the laser target. Due to the interaction with the external magnetic field, a velocity gradient perpendicular to the plasma velocity forms at this boundary. The presence of vortices in a region of sheared flow indicates the development of the Kelvin-Helmholtz instability. The details of the mechanism producing the sheared flow and the resulting instability will be discussed. [Preview Abstract] |
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UP6.00050: Modeling of Multi-Interface, Diverging, Hydrodynamic Experiments for the National Ignition Facility M.J. Grosskopf, R.P. Drake, C.C. Kuranz, A.R. Miles, J.F. Hansen, T. Plewa, N. Hearn, D. Arnett, J.C. Wheeler The National Ignition Facility (NIF) will soon provide experiments with far more than ten times the energy than has been previously available on laser facilities. In the context of supernova-relevant hydrodynamics, this will enable experiments in which hydrodynamic instabilities develop from multiple, coupled interfaces in a diverging explosion. This presentation discusses the design of such blast-wave-driven explosions in which the relative masses of the layers are scaled to those within the star. It reports scaling simulations with CALE to model the global dynamics of such an experiment. The simulations probed the instability growth and multi-interface interactions in mass-scaled systems to assess the diagnosability and experimental value of different designs using a variety of materials. Initial conditions in the simulation near the irradiated surface have been shown to lead to spurious structure on the shock; therefore, a series of simulations to understand this structure is also discussed. [Preview Abstract] |
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UP6.00051: ABSTRACT WITHDRAWN |
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UP6.00052: Shock experiments with magnetically accelerated flyer plates Stephan Neff, Sandra Wright, David Martinez, Christopher Plechaty, Radu Presura The interaction of shock waves with inhomogeneous media is important in many astrophysical phenomena. Modelling these phenomena in the laboratory yields additional information to improve both simulations and the interpretation of astrophysical observations. Scaled experiments using magnetically accelerated flyers impacting on low density foam targets have been proposed for the Z machine at the Sandia National Laboratories (R.P. Drake, Phys. Plasmas 2002). Carrying out such experiments on smaller machines like the UNR pulsed power generator Zebra reduces the costs significantly and thus enables a broader scan of experimental parameters. Our experiments at the Nevada Terawatt Facility study the flyer acceleration (reaching flyer velocities of up to 5 km/s) and the impact of the flyers on transparent targets with low sound speeds in order to create shock waves. Optical diagnostics are used to study the interaction of the flyers with the transparent target. [Preview Abstract] |
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UP6.00053: Radiation production in Weibel-generated magnetic fields in relativistic astrophysical shocks Sarah Reynolds, Mikhail Medvedev Radiation produced by charged relativistic particles undergoing small randomly-oriented accelerations correlated on a sub-Larmor scale is referred to as the Jitter radiation. It is emitted from small-scale turbulent electromagnetic fields, such as those generated in relativistic collisionless shock fronts of gamma-ray bursts (GRBs) and in Petawatt-scale laser-produced plasmas by the Weibel instability. The spectral characteristics of jitter radiation are distinct from the synchrotron case and intimately related to the magnetic field spectrum at small scales. Conventionally, in the Jitter regime, the particle deflections are considered to be smaller than the relativistic beaming angle of 1/$\gamma$ ($\gamma$ being the Lorentz factor of an emitting particle) and the particle distribution is assumed to be isotropic. Here we relax both assumptions and present the extension of the jitter theory amenable for comparisons with experimental data. We demonstrate the spectral sensitivity to anisotropy in the Weibel-generated magnetic field orientation. We also discuss applications to laboratory studies of the Weibel instability and to certain astrophysical phenomena. [Preview Abstract] |
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UP6.00054: 1 D analysis of Radiative Shock damping by lateral radiative losses Michel Busquet, Edouard Audit We have demonstrated the effect of the lateral radiative losses in radiative shocks propagative in layered quasi-planar atmospheres.[1,2] The damping of the precursor is sensitive to the fraction of self-emitted radiation reflected by the walls (called albedo) We have given recently an experimental determination of the wall albedo.[2] For parametric analysis of this effect, we implement lateral losses in the 1D hydro-rad code MULTI [3] and compared results with 2D simulations. \newline [1] S.Leygnac, et al., Phys. Plasmas \textbf{13}, 113301 (2006) \newline [2] M.Busquet, et al, High Energy Density Plasmas\textbf{ 3}, 8-11 (2007); M.Gonzalez, et al, Laser Part. Beams\textbf{ 24}, 1-6 (2006) \newline [3] Ramis et al, Comp. Phys. Comm., \textbf{49}, 475 (1988) [Preview Abstract] |
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UP6.00055: BEAMS, COHERENT RADIATION AND COMPUTATION |
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UP6.00056: Analysis and Experiments On Peer-to-Peer Locking of Magnetrons E.J. Cruz, P. Pengvanich, Y.Y. Lau, R.M. Gilgenbach, B. Hoff, J.W. Luginsland Locking of multiple magnetrons remains an important topic in contemporary applications of high power microwaves. We report our recent findings in both theory and experiments. Starting with the mutual, or peer-to-peer, locking of two magnetrons, we derived a novel condition for phase locking. This condition reduces to Adler's classical condition when the coupling is one way, where one magnetron becomes the *master* and the other becomes the *slave*. The formulation is extended to N magnetrons, under the assumption that the mutual coupling is modeled as an N-port network. Experimental results on the peer-to-peer phase-locking of two, 1-kW magnetrons will be reported. [Preview Abstract] |
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UP6.00057: Continuing Studies of a Simple Gyrotron Model Equation Harold Weitzner Following the analysis with other authors in J. Phys. A Math. Theor. \textbf{40}, 2203 (2007) three topics are explored. The model is linearized and for a given profile function solved exactly. This work leads to a revision in the previously proposed upstream conditions for the gyrotron. A variation of the linearized model allows for the introduction of particle bunching. In this case the dispersion relation is studied and some effects in the mode structure of particle bunching are given. The general linearized model can also be treated by geometrical optics methods. These methods can also then be extended to the non-linear problem. One can infer a number of interesting general results from these approximate solutions. [Preview Abstract] |
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UP6.00058: Experimental Research on a 1.5 MW, 110 GHz Gyrotron M.A. Shapiro, Y. Hidaka, E.M. Choi, I. Mastovsky, J.R. Sirigiri, D.S. Tax, R.J. Temkin, J. Neilson We report experimental research on the after cavity interaction (ACI) in a 1.5-MW, 110-GHz gyrotron with an internal mode converter operating in 3 microsecond pulses. Recent experiments with a single-stage depressed collector revealed the effects of the ACI, a second interaction that occurs after the electron beam passes through the intended primary interaction region. The ACI causes re-absorption of the microwave power and broadening of the spent electron beam energy distribution, thus leading to a significant reduction in gyrotron efficiency. The broadening of the spent electron beam energy spectrum, which results in degraded performance of the depressed collector, has been experimentally verified. Ways to minimize the ACI are currently being investigated. Also, a new internal mode converter, consisting of a helically-cut launcher and four smooth curved mirrors, has been designed and fabricated. The cold test shows a good agreement with the theoretical Gaussian beam pattern. This new converter will be hot tested shortly in the gyrotron. [Preview Abstract] |
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UP6.00059: Experimental Verification of Magnetron Operation with a Transparent Cathode Sarita Prasad, Mikhail Fuks, C. Jerald Buchenauer, Edl Schamiloglu Magnetrons are considered to be one of the most efficient sources of high power microwaves. However, the rather slow start of oscillations makes them unattractive for short-pulse applications. At the University of New Mexico (UNM) we proposed a new cathode design, namely the transparent cathode that provides fast start of oscillations in magnetrons [1]. Furthermore, we have been able to demonstrate via 3-dimensional particle-in-cell simulations that the magnetron output characteristic is significantly improved using the transparent cathode [2]. We have successfully modified the short-pulse electron beam accelerator ``SINUS-6'' at UNM that will be used for experimental verification of our simulation results. SINUS-6 has a pulse duration of 16 ns and is now capable of delivering 320 kV and a total current of 13 kA to the magnetron load. Experimental results of magnetron operation with a transparent cathode and solid cathode will be presented. [1] M. Fuks et al, ``Rapid Start of Oscillations in a magnetron with a Transparent Cathode'', Phys. Rev. Lett., vol 95, pp. 205101-1-205101-4, 2005. [2] H. Bosman et al., ``Improvement of the Output Characteristics of Magnetrons Using the Transparent Cathode'', IEEE Trans. Plasma Sci, 34 (4), 606 (2006). [Preview Abstract] |
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UP6.00060: Dielectric Window Breakdown on the UM/L-3 Relativistic Magnetron B.W. Hoff, R.M. Gilgenbach, Y.Y. Lau, D. French, M.A. Franzi, M.D. Haworth, P.J. Mardahl Experiments were performed on the UM/L-3 (6-vane, L-band) relativistic magnetron to test a new microwave window configuration designed to limit vacuum side breakdown. Originally, each microwave window was mounted 3 cm from its corresponding anode cavity, separated by an aperture. In this case, vacuum side window breakdown was observed to initiate at single waveguide output powers close to 20 MW. Moving the microwave windows further away from the anode apertures and redesigning the window mounts eliminated window breakdown at powers of 120+ MW. Examination of window damage suggests that impacts from electrons emanating from the magnetron assembly are responsible for initiating the vacuum side microwave window breakdown in the baseline case. PIC modeling is in progress to investigate probable electron sources responsible for initiating the window breakdown events. [Preview Abstract] |
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UP6.00061: Accurate Computations of Cavity Frequencies from time domain VORPAL simulations Travis Austin, Serguei Ovtchinnikov, John Cary, Leo Bellantoni We have applied the Werner-Cary method (J. Comp. Phys. 227, 5200-5214 (2008)) for extracting modes and mode frequencies from time-domain simulations of crab cavities, as are needed for the ILC and the beam delivery system of the LHC. This method for frequency extraction relies on a small number of simulations and post-processing using the SVD algorithm with Tikhonov regularization. The time domain simulations were carried out using the VORPAL computational framework. Comparisons with measurements of the A15 cavity show that this method can provide accuracy to within 0.01\% of experimental results after accounting for manufacturing imperfections. This method has applications across many areas including obtaining MHD spectra from time-domain simulations. [Preview Abstract] |
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UP6.00062: Dynamic Control of Virtual Cathode for Improved IEC-POPS Operation Yongho Kim, Hans Herrmann, Greg Dale, Keenan Pepper The widely known characteristic of inertial electrostatic confinement (IEC) is that high energies are easy to achieve, but reasonable densities are considerably more difficult. LANL has developed a density enhancement scheme, the Periodically Oscillating Plasma Sphere (POPS). A spherical ion cloud in a uniform electron background may undergo a self-similar collapse that can result in the periodic and simultaneous attainment of ultra-high densities and temperatures. Over the past three years, a great deal of experimental evidence for the POPS oscillation has been obtained [1]. However, abrupt virtual cathode decay is also observed when the potential well depth drops below a certain level. To improve IEC-POPS operation, the creation of stable, deep potential wells produced by virtual cathode is an essential element. To keep deep potential wells, we are in the process of upgrading the IEC-POPS device to ramp the electron emitter bias voltage. Influence of dynamic control of electron injection on the virtual cathode stability will be presented. [1] J. Park, R. A. Nebel, S. Stange, and S. Krupakar Murali, Physical Review Letters \textbf{95}, 015003 (2005). [Preview Abstract] |
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UP6.00063: Excitation of surface plasma wave over a plasma cylinder by a relativistic electron beam via Cerenkov interaction Gagan Kumar, Vipin Kumar Tripathi A relativistic electron beam propagating in a plasma cylinder excites a surface plasma wave (SPW) via Cerenkov interaction. The wave frequency decreases with beam velocity. The growth rate, however, initially increases with frequency $\omega $, attains a maximum and then falls off due to the localization of the SPW near the surface. With the increase in the radius of plasma, the optimum growth rate increases in magnitude. The annular beam propagating outside the plasma cylinder excites the SPW with larger growth rate. The study is relevant to capillary plasma created by an intense short pulse laser. The energetic electrons accelerated by the laser wake fields can drive the surface plasma waves. [Preview Abstract] |
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UP6.00064: Building Novel RF Sources out of Optimized Photonic Crystals Gregory R. Werner, Tobin Munsat, Carl A. Bauer, John R. Cary RF sources, such as klystrons, use an electron beam to excite a mode of an RF resonant cavity; RF power is subsequently extracted from the cavity. Traditionally RF sources have used metal cavities, which can have higher order modes that disrupt production of the desired frequency; these can be particularly destructive in high-power multi-cavity systems. A cavity in a photonic crystal (e.g., a lattice of dielectric rods in vacuum), however, can be designed to support only a single mode; electromagnetic waves of other frequencies simply pass through the photonic crystal walls. Such a cavity should be ideal for an RF source, since there is only one mode an electron beam can excite. A feasible cavity based on a 2D photonic structure can be created using dielectric rods with metal endplates. Moreover, optimization of the rod positions leads to configurations that break lattice symmetry but yield more practical and better-performing structures. A design for a klystron using RF cavities made out of a small number of sapphire rods and metal end-plates is presented, along with computer simulations of electron beams exciting the cavities. [Preview Abstract] |
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UP6.00065: Optimized Photonic Crystal Accelerating Cavities Carl Bauer, Gregory Werner, John Cary Through computer simulation, a 2D photonic crystal (PhC) cavity formed from a truncated triangular lattice of dielectric rods is optimized to confine a single accelerating mode efficiently. Photonic crystals have the ability to reflect radiation within only certain frequency ranges, called bandgaps; the bandgaps are determined by the geometry and material of the PhC and so are tunable. For truncated PhCs, reflection is incomplete. Therefore, the confinement of bandgap frequencies to a cavity within a truncated PhC is weakened by the severity of the truncation. For a cavity made of 18 dielectric rods in a truncated triangular lattice arrangement, the desired accelerating cavity mode is weakly confined. Adjusting the positions and sizes of the dielectric rods away from the original lattice configuration within an optimization procedure gives unintuitive structures, ultimately increasing the confinement of the accelerating mode by a factor of 100. [Preview Abstract] |
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UP6.00066: Simulation of x-ray generation by betatron oscillation in a laser-plasma based accelerator Hae June Lee, Seok Won Hwang, Sang Young Chung, Dong Eon Kim, Cha Su Park The injection of an ultra-intense femtosecond laser pulse or a strong electron beam into a plasma generates a laser or a plasma wake field which can be used for electron acceleration with a strong acceleration gradient. It was observed that the off-axis injection of the electron beam in a laser or plasma wake field generates betatron oscillation. From the betatron oscillation, X-ray radiation was detected. The two-dimensional simulation results for the betatron oscillation of the electron beam are reported as well as the investigation of radiation properties. Besides, The simulations for femtosecond X-ray generation from Compton back-scattering interaction between electron beam and a laser are presented. [Preview Abstract] |
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UP6.00067: Ion Source and Laser-Induced Fluorescence Diagnostic System Development for the Paul Trap Simulator Experiment H. Wang, M. Chung, R.C. Davidson, M. Dorf, P.C. Efthimion, E.P. Gilson, R. Majeski, E.A. Startsev, N. Thomas, A. Arora The Paul Trap Simulator Experiment (PTSX) is a cylindrical Paul trap that simulates the nonlinear dynamics of intense charged particle beam propagation in an alternating-gradient magnetic transport system. Cesium has been used as the ion source in the initial phase of PTSX for its operational convenience, as well as its favorable stability. However, the cesium source is to be replaced by a barium source in order to develop a laser-induced fluorescence (LIF) diagnostic to study the ion density profile and ion velocity distribution function. The features of the cesium and barium sources are presented. The feasibility of the LIF diagnostic using the barium source and the development of the LIF diagnostic system are also discussed, including the installation of an excimer-pumped dye laser to allow a variety of fluorescence schemes to be pursued. [Preview Abstract] |
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UP6.00068: Distribution Function Effects on the Stability of Plasmas in the Paul Trap Simulator Experiment E.P. Gilson, M. Chung, R.C. Davidson, M. Dorf, P.C. Efthimion, R. Majeski, E.A. Startsev, H. Wang, N. Thomas, A. Arora Initial results are reported from experiments to study the effects of modified distribution functions on the stability of plasmas trapped in the Paul Trap Simulator Experiment (PTSX). The PTSX system is a compact laboratory Paul trap that simulates a long, thin charged-particle bunch coasting through a kilometers-long magnetic alternating-gradient transport system by putting the physicist in the frame-of-reference of the beam. The transverse dynamics of particles in both systems are described by the same sets of equations - including all nonlinear space-charge effects. By masking the PTSX ion source, various transverse plasma density profiles can be injected into the machine such as: a hollow profile, an off- axis profile, and an array of small beamlets. The shape of the long-time transverse plasma density profile is a measured and used to determine the effect of the initial modified distribution on the stability of the plasma. The results are compared to WARP particle-in-cell simulations. [Preview Abstract] |
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UP6.00069: Progress with an Implicit Drift-Lorentz mover R.H. Cohen, B.I. Cohen, A. Friedman, D.P. Grote, J.-L. Vay It is often desirable to follow charged-particle trajectories through regions where the particles are strongly magnetized as well as regions with little or no magnetic field, with timesteps large compared to the smallest cyclotron period. To address this need we developed the drift-Lorentz particle mover\footnote{R.H. Cohen {\it et al.}, Phys. Plasmas {\bf 12}, 056708 (2005)}, which interpolates between full particle dynamics and drift kinetics in such a way as to preserve proper particle drifts, motion along the magnetic field, and gyroradius in the large-B limit, while smoothly matching on to full-particle dynamics at small B. In order to be able to apply the mover to high-density problems (where the plasma frequency is comparable to or exceeds the cyclotron frequency) we have formulated and implemented a fully implicit (electrostatic) version. We describe this implementation, as well as successful tests on a magnetized Buneman instability. We also discuss the status of combining this mover with collisions, as well as electromagnetic extensions. [Preview Abstract] |
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UP6.00070: Hardware acceleration of PIC codes: tapping into the power of state of the art processing units R.A. Fonseca, P. Abreu, S.F. Martins, L.O. Silva There are many astrophysical and laboratory scenarios where kinetic effects play an important role. Further understanding of these scenarios requires detailed numerical modeling using fully relativistic three-dimensional kinetic code such as OSIRIS [1]. However, these codes are computationally heavy. Explicitly using available hardware resources such as SIMD units (Altivec/SSE3) [2], cell processors or graphics processing units (GPUs) may allow us to significantly boost performance of these codes. For the most cases, the processing units are limited to single precision arithmetic, and require specific C/C++ code to be used. We present a comparison between double precision and single precision results, focusing both on performance and on the effects on the simulation in terms of algorithm properties. Details on a framework allowing the integration of hardware optimized routines with existing high performance codes in languages other than C is given. Finally, initial results of high performance modules of the PIC algorithm using SIMD units and GPU's will also be presented. [1] R. A. Fonseca et al., LNCS 2331, 342, (2002) [2] K. J. Bowers et al., Phys Plasmas vol. 15 (5) pp. 055703 (2008) [Preview Abstract] |
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UP6.00071: Enhanced quasi-static PIC simulation with pipelining algorithm for e-cloud instability Bing Feng, Chengkun Huang, Viktor Decyk, Warren Mori, Patric Muggli, Tom Katsouleas Simulating the electron cloud effect on a beam that circulates thousands of turns in circular machines is highly computationally demanding. A novel algorithm, the pipelining algorithm is applied to the fully parallelized quasi-static particle-in-cell code QuickPIC to overcome the limit of the maximum number of processors can be used for each time step. The pipelining algorithm divides the processors into subgroups and each subgroup focuses on different partition of the beam and performs the calculation in series. With this novel algorithm, the accuracy of the simulation is preserved; the speed of the simulation is improved by one order of magnitude with more than 10$^{2}$ processors are used. The long term simulation results of the CERN-LHC and the Main Injector at FNAL from the QuickPIC with pipelining algorithm are presented. This work is supported by SiDAC and US Department of Energy [Preview Abstract] |
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UP6.00072: Beam Steering, Focusing and Compression for Warm-Dense Matter Experiments S.M. Lidia, A. Anders, R.H. Cohen, J.E. Coleman, M. Dorf, E.P. Gilson, D.P. Grote, J.Y. Jung, M. Leitner, B.G. Logan, P.K. Roy, A.B. Sefkow, P.A. Seidl, W.L. Waldron, D.R. Welch The Heavy-Ion Fusion Sciences Virtual National Laboratory is pursuing an approach to target heating experiments in the Warm Dense Matter regime, using space-charge-dominated ion beams that are simultaneously longitudinally bunched and transversely focused. Axial compression leading to $\sim $100X current amplification and simultaneous radial focusing have led to encouraging energy deposition approaching, but still short of, the intensities required for eV-range target heating experiments. We present measurements from the Neutralized Drift Compression Experiment to reach the necessary higher beam intensities, including: (1) axial compression and radial focusing; (2) spatial and temporal distribution of energy deposition at the target plane; and (3) centroid motion of the beam spot through the pulse. [Preview Abstract] |
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UP6.00073: Simulating Acceleration Schedules for NDCX-II W.M. Sharp, A. Friedman, D.P. Grote, E. Henestroza, M.A. Leitner, W.L. Waldron The Virtual National Laboratory for Heavy-Ion Fusion is developing a physics design for NDCX-II, an experiment to study warm dense matter heated by ions near the Bragg-peak energy. Present plans call for using about thirty induction cells to accelerate 30 nC of Li$^{+}$ ions to more than 3 MeV, followed by neutralized drift-compression. To heat targets to useful temperatures, the beam must be compressed to a sub-millimeter radius and a duration of about 1 ns. An interactive 1-D particle-in-cell simulation with an electrostatic field solver, acceleation-gap fringe fields, and a library of realizable analytic waveforms has been used for developing NDCX-II acceleration schedules. Multidimensional simulations with WARP have validated this 1-D model and have been used both to design transverse focusing and to compensate for injection non-uniformities and 3-D effects. Results from this work are presented, and ongoing work to replace the analytic waveforms with output from circuit models is discussed. [Preview Abstract] |
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UP6.00074: Ion Beam Pulse Propagation through a Neutralizing Background Plasma along a Solenoidal Magnetic Field: Whistler Wave Excitation and Beam Self-focusing Mikhail Dorf, Igor Kaganovich, Edward Startsev, Ronald Davidson The electromagnetic field perturbations excited by an ion beam propagating through a neutralizing background plasma along a solenoidal magnetic field is studied analytically by solving the Vlasov-Maxwell equations. Analytical predictions are compared with the results of particle-in-cell simulations using the LSP code. It is found that the plasma response to the ion beam pulse is significantly different depending on whether the value of the solenoidal magnetic field is below or above a threshold value corresponding to the resonant excitation of large-amplitude whistler waves. The self-pinching force acting on the beam particles is calculated. It is demonstrated that even a weak solenoidal magnetic field affecting only the plasma electron dynamics can significantly enhance the pinching force in the limit where the beam radius is small compared to the electron skin depth. Therefore, this effect can be used for effective ion beam focusing. Intense resonant whistler wave excitation can be also used for diagnostic and communication purposes. [Preview Abstract] |
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UP6.00075: Multi- Meter-Long Plasma Source for Heavy Ion Beam Charge Neutralization P.C. Efthimion, E.P. Gilson, R.C. Davidson, B.G. Logan, P.A. Seidl, W. Waldron Plasmas are a source of unbound electrons for charge neutralizing intense heavy ion beams to focus them to a small spot size and compress their axial length. To produce long plasma columns, sources based upon ferroelectric ceramics with large dielectric coefficients have been developed. The source utilizes the ferroelectric ceramic BaTiO$_{3}$ to form metal plasma. The drift tube inner surface of the Neutralized Drift Compression Experiment (NDCX) is covered with ceramic material. High voltage ($\sim $ 8 kV) is applied between the drift tube and the front surface of the ceramics. A BaTiO$_{3}$ source comprised of five 20-cm-long sources has been tested and characterized, producing relatively uniform plasma in the 5x10$^{10}$ cm$^{-3}$ density range. The source has been integrated into the NDCX device for charge neutralization and beam compression experiments. Initial beam compression experiments yielded current compression ratios $\sim $ 120. Recently, an additional 1 meter long source was fabricated to produce a 2 meter source for NDCX compression experiments. Present research is developing higher density sources to support beam compression experiments for high energy density physics applications. [Preview Abstract] |
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UP6.00076: Phase Space Distribution of the University of Maryland Electron Ring (UMER) Source I. Haber, S. Bernal, B. Beaudoin, K. Fiuza, R.A. Kishek, P.G. O'Shea, C. Papadopoulos, M. Reiser, D. Stratakis, C. Wu Because the downstream characteristics of a space-charge-dominated beam can be sensitive to details of the phase-space distribution of the beam emerging from the electron gun, experimental and simulation studies have been conducted to characterize the UMER source. Measurements of the beam distribution function at the gun exit have been conducted using a pinhole that is scanned across the beam, as well as tomographic reconstruction of the distribution using downstream phosphor screen images. And these have been found to be in good agreement. PIC simulations using the WARP code have also been used to understand the gun characteristics and have correctly predicted measured characteristics, including the substantial reduction in beam halo that resulted from repositioning the cathode. [Preview Abstract] |
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UP6.00077: Stability of the return-current-two-stream surface mode Edward Startsev, Mikhail Dorf, Ronald Davidson When intense charged particle beam with sharp edge propagates in the background plasma, its current is neutralized by the return plasma current everywhere except at the beam edge over a characteristic transverse distance $\Delta x_\perp \sim\delta_{pe}$, where $\delta_{pe}=c/\omega_{pe}$ is the collisionless skin depth. Because the background plasma electrons neutralizing the beam current inside of the beam stream relative to the background plasma electrons outside of the beam, the background plasma can support an electrostatic surface mode with a wavelength larger than the collisionless skin depth. Such mode has been studied previously and it has been shown to be strongly unstable. In this paper we study the stability properties of this two-stream surface mode in detail. In particular, it is shown that the magnetic field inside of the unneutralized current layer, which has not been taken into account previously, stabilizes the surface mode. [Preview Abstract] |
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UP6.00078: Non-Abelian Courant-Snyder Theory for Coupled Transverse Dynamics of Charged Particles in Electromagnetic Focusing Lattices Hong Qin, Ronald Davidson The Courant-Snyder theory for charged particle dynamics in electromagnetic focusing lattices is the fundamental theory that underlies the design of modern accelerators and beam transport systems. The basic elements of the Courant-Snyder theory consist of the envelope equation, the phase advance, and the Courant-Snyder invariant. However, the standard Courant- Snyder theory applies only to the 1D transverse dynamics. We have extended the Courant-Snyder theory to 2D coupled transverse dynamics. It is surprising that the concepts of envelope equation, phase advance, and Courant-Snyder invariant can be elegantly generalized to the 2D case, but with the interesting feature of being non-Abelian. The theory is constructed through a time-dependent symplectic transformation that belongs to a non-invariant subset of the symplectic group $Sp(4,R)$. This non-Abelian Courant-Snyder theory provides a non-perturbative theoretical tool for designing focusing lattices with coupled transverse dynamics, which was previously treated primarily by perturbative methods. [Preview Abstract] |
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UP6.00079: Nonlinear evolution of a relativistic electron beam in a plasma Xianglong Kong, Chuang Ren, John Tonge The most unstable modes in a relativistic electron beam-plasma return current system are oblique [Bret et al. PRE '04]. Nonlinear evolution of these modes is important to fast ignition and can only be simulated with the beam propagating in the simulation plane. Two-dimensional PIC simulations with this `in-plane' configuration show that the system evolves to a quasi-steady state stable to the two-stream instability with no bump on tail in the system distribution function. The most energetic field component is the transverse electric field. The beam retains more than 50{\%} of its initial energy. The beam does not become localized in the transverse direction through filamenting and merging, as observed in previous simulations with the beam propagating out of the simulation plane [Lee and Lampe PRL '73]. The simulations also show that as the beam and plasma temperatures increase, the dominant mode becomes increasingly longitudinal. [Preview Abstract] |
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UP6.00080: Numerical study of the propagation of positron beams in plasmas Xiaoying Li, Patric Muggli We study the propagation of ultra-relativistic positron bunches in long dense plasmas using numerical simulations. Emittance preservation of the incoming beam is essential for all particle accelerators. In the case of the blowout regime of the plasma wakefield accelerator (PWFA), the electron bunch essentially propagates in a uniform ion column, and its emittance is preserved along the plasma. Such a favorable regime does not exist for positron bunches and the preservation of the bunch emittance is an open question. It was shown experimentally and numerically that single positron bunches suffer halo formation and emittance growth when propagating along a uniform plasma [P. Muggli et al., to appear in Phys. Rev. Lett. 2008]. We therefore investigate the possibility of mitigating these deleterious effects and of preserving the incoming bunch emittance, for example by using a hollow plasma channel. We also investigate emittance preservation in the case of a drive-bunch/witness-bunch PWFA system. Preliminary simulation results will be presented. [Preview Abstract] |
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UP6.00081: Preservation of Ultra Low Emittances in Future High Energy Plasma Wakefield-based Colliders Reza Gholizadeh, Tom Katsouleas, Patric Muggli, Warren Mori Plasma Wakefield Accelerator has been proven to be a promising technique to lower the cost of the future high energy colliders by offering orders of magnitude higher gradients than the conventional accelerators. However, it has been shown that ion motion is an important issue to account for in the extreme regime of ultra high intensities and ultra low emittances, characteristics of future high energy colliders. In this regime, the transverse electric field of the beam is so high that the plasma ions cannot be considered immobile at the time scale of electron plasma oscillations, thereby leading to a nonlinear focusing force. Therefore, the transverse emittance of a beam matched to the initial linear focusing will not be preserved under these circumstances. However, Vlasov equation predicts a matching profile even in the nonlinear regime. Furthermore, we extend the idea and introduce a plasma section that can match the entire beam to the mobile-ion regime of plasma. We also find the analytic solution for the optimal matching section. Simulation results will be presented. [Preview Abstract] |
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UP6.00082: Simulation and Investigation of Weibel Instability for LWFA and PWFA Electron Beams Brian Allen, Tom Katsouleas, Bing Feng, Anatoly Maksimchuk, Vitaly Yakimenko, Patric Muggli Weibel instability (or current filamentation instability) is of central importance for relativistic beams in plasmas for the laboratory, ex. fast-igniter concept for inertial confinement fusion, and astrophysics, ex. cosmic jets. Simulations, with the particle-in-cell code QuickPic, of beams produced by Laser Wakefield and RF accelerators show the appearance of Weibel instability. The appearance of the instability is investigated as a function of electron beam parameters (including charge, transverse size and length) and plasma parameters (density and length). We present preliminary simulation results, discuss further simulation refinements, suggest criteria and threshold parameters for observing the presence of Weibel and outline potential future experiments. [Preview Abstract] |
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UP6.00083: 2D limiting current of a finite-width electron pulse in a parallel-plate gap Wee Shing Koh, Lay-Kee Ang, Shih-Hung Chen, Ling-Chieh Tai, Lin Wu The generation of pC-nC electron pulses is an important technique to produce a high-energy electron beam in applications, such as photoinjectors and laser wakefield accelerators. For an electron pulse with a large charge number, the space-charge effect is probably the main factor limiting the maximum charge number that can be packed in a pulse. The 1D space-charge-limited (SCL) current for a short pulse emitted from a planar cathode was first proposed in 2002 [1]. This 1D SCL model is later extended to the quantum, relativistic [2] and multi-dimensional (2D/3D) [3] regimes. In previous models [1-3], uniform electron beam is assumed, but in reality, the injected current of the short pulse is usually non-uniform. Therefore, we present the 2D non-uniform short-pulse model and compare the numerical results with the uniform injection model in Ref. [3]. [1] \'{A}. Valfells, et.al., Phys. Plasmas 9, 2377 (2002). [2] L. K. Ang, et. al., Phys. Rev. Lett. 98, 164802 (2007). [3] W.S. Koh, et. al., Phys. Plasmas, 13, 063102 (2006). [Preview Abstract] |
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UP6.00084: Designing Neutralized Drift Compression for Focusing of Intense Beam Pulses in a Background Plasma Igor D. Kaganovich, Michael Dorf, Edward A. Startsev, Ronald C. Davidson Neutralized drift compression offers effective particle beam focusing and current amplification. In the neutralized drift compression method, a linear radial and longitudinal velocity drift is applied to a beam pulse, so that the beam pulse compresses during its drift in the focusing section. The beam intensity can increase more than 100 times in both radial and longitudinal directions, totaling more than 10,000 times increase in the beam density during this process. The optimal configuration of focusing elements to mitigate a time-dependant focal plane will be discussed. Self-electric and magnetic fields can prevent tight ballistic focusing and have to be neutralized by supplying neutralizing electrons. The source of electrons can come from emitting electrodes, gas ionization by the beam ions, plasma plug region, and volumetric plasma. This paper presents a survey of the present numerical modeling techniques and theoretical understanding of plasma neutralization of intense particle beams. [Preview Abstract] |
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UP6.00085: Energy Spectra of RITS-6 Electron Beam Derived from Depth Dose Measurements Timothy Webb, Bryan Oliver, Nichelle Bruner Until recently the main methods of determining the output voltage of the diode of the RITS-6 accelerator (nominal 7-12 MV) has been through parapotential flow theory of the magnetically insulated transmission line (MITL), a radiographers equation based on radiation transport calculations for a particular diode configuration, or particle-in-cell simulations of various regions of the accelerator. Time integrated measurements of the depth-dose profile using radiochromic films have been performed on RITS for large area diodes. Comparisons to theoretical predictions for monoenergetic beams and empirical relations for the average electron energy are presented as well as a potential method for unfolding the electron energy distribution. [Preview Abstract] |
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UP6.00086: Calculation of charge-changing cross sections of ions or atoms colliding with fast ions using the classical trajectory method Ariel Shnidman, Harrison E. Mebane, Igor D. Kaganovich, Ronald C. Davidson Evaluation of ion-atom charge-changing cross sections is needed for many accelerator applications. A classical trajectory Monte Carlo (CTMC) simulation has been used to calculate ionization and charge exchange cross sections. For benchmarking purposes, an extensive study has been performed for the simple case of hydrogen and helium targets in collisions with various ions. Despite the fact that the simulation only accounts for classical mechanics, the calculations are comparable to experimental results for projectile velocities in the region corresponding to the vicinity of the maximum cross section. Shortcomings of the CTMC method for multielectron target atoms are discussed. [Preview Abstract] |
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UP6.00087: Electron Beam Design and Calibration for the Solid/Liquid Lithium Divertor Experiment Michael Jaworski, R. Flauta, T.K. Gray, J. Kim, C.Y. Lau, M.B. Lee, M.J. Neumann, V. Surla, D.N. Ruzic An electron beam has been developed as part of the Solid/Liquid Lithium Divertor Experiment (SLiDE) at the University of Illinois at Urbana-Champaign. The purpose of the SLiDE apparatus is to examine the motion of liquid lithium under fusion relevant heat loads and magnetic fields. To mimic the heat fluxes present in the divertor of a fusion machine, a linear sheet beam is utilized which can operate over a range of applied magnetic fields and power levels. With steady state operation up to 15kW input power, the beam can produce peak heat fluxes of 10 MW/m$^{2}$ and heat flux gradients comparable to those found in fusion experiments. The design of the electron beam was developed using commercial beam transport codes and the final design is diagnosed with a two-lead Faraday cup. Beam performance and characteristics are presented. [Preview Abstract] |
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UP6.00088: Multi Bunch Plasma Wakefield Acceleration Experiments Patric Muggli, Vitaly Yakimenko, Efftimios Kallos, Karl Kusche, Jangho Park, Marcus Babzien, Adam Lichtl We have demonstrated a method to produce a train of equidistant drive electron bunches (P. Muggli et al, Phys. Rev. Lett. 2008) followed by a witness bunch, suitable for multi-bunch plasma wakefield acceleration experiments (MB-PWFA).The drive bunches are separated by an adjustable distance d=100 to 450 microns. The witness bunch follows the last drive bunch by 1.5d. At the Brookhaven National Laboratory Accelerator Test Facility this bunch train is sent into a 2cm-long capillary discharge. The electron plasma density is adjusted by varying the timing between the firing capillary discharge and the arrival time of the electron bunch train. Calculations show that the acceleration of the witness bunch is maximum when the plasma wavelength is equal to d (resonant excitation of the wakefield). In the experiment we measure the energy gained by the witness bunch as a function of the plasma density and of the number of drive bunches. This scheme could be used to multiply the energy of the witness bunch in a single PWFA stage. Preliminary experimental results will be presented. [Preview Abstract] |
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UP6.00089: Scalings in a Plasma Wakefield Accelerator Ian Blumenfeld, F.J. Decker, M.J. Hogan, R. Ischebeck, R. Iverson, N. Kirby, R. Siemann, D. Walz, C.E. Clayton, C. Huang, C. Joshi, W. Lu, K.A. Marsh, W.B. Mori, M. Zhou, T.C. Katsouleas, P. Muggli, E. Oz High gradient acceleration of electrons has recently been achieved in meter scale plasmas at SLAC. Results from these experiments show that the wakefield is sensitive to parameters in the electron beam which drives it. In the experiment the bunch lengths were varied systematically at constant charge. Here we investigate the correlation of peak beam current to the wake amplitude. The results are compared to simulation. [Preview Abstract] |
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UP6.00090: The Dependence of Electron Trapping on Plasma Density in a Plasma Wakefield Accelerator Neil Kirby, Ian Blumenfeld, Franz-Josef Decker, Mark Hogan, Rasmus Ischebeck, Richard Iverson, Robert Siemann, Dieter Walz, David Auerbach, Christopher Clayton, Chengkun Huang, Chandrashekhar Joshi, Devon Johnson, Wei Lu, Kenneth Marsh, Warren Mori, Miaomiao Zhou, Thomas Katsouleas, Patric Muggli, Erdem Oz Plasma density is one factor that contributes to the onset of ionization induced electron trapping in a plasma wakefield accelerator. Here, experimental measurements and theory exhibit the dependence of trapping on plasma density. [Preview Abstract] |
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UP6.00091: Positron acceleration and beam loading in a beam-driven plasma wakefield Weiming An, Wei Lu, Chengkun Huang, Xiaoxiao Xu, Warren Mori A positron beam accelerated by a beam-driven plasma wakefield is investigated by numerical simulation. Both of the electron driving beam and positron driving beam are used. A preliminary parameters design is obtained for such acceleration scheme. [Preview Abstract] |
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UP6.00092: Radiation Pressure Acceleration by Circularly Polarized Pulses: Three-Dimensional Dynamics and Angular Momentum Absorption T.V. Liseykina, D. Bauer, A. Macchi, F. Pegoraro Radiation Pressure Acceleration of thin plasma targets by Circularly Polarized laser pulses is studied by three-dimensional particle-in-cell simulations. The use of flat-top intensity profiles is found to be important to avoid self-induced transparency and to reach high ion energies. A significant degree of absorption of the angular momentum of the laser pulse is observed, giving a signature of irreversible, non-adiabatic effects during the acceleration process. [Preview Abstract] |
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UP6.00093: BASIC PLASMA EXPERIMENT |
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UP6.00094: Electrostatic ion cyclotron waves in a plasma with heavy negative ions Robert L. Merlino, Su-Hyun Kim, Jonathon Heinrich, Marlene Rosenberg Results of a laboratory study of electrostatic ion cyclotron waves (EIC) in a plasma containing $K^+$positive ions (39 amu), electrons, and $C_7 F_{14}^- $(350 amu) negative ions are presented. The negative ions were produced in a single-ended Q machine in which $C_7 F_{14} $was introduced [1]. Excitation of the fundamental and higher harmonic light and heavy ion EIC modes were observed. The presence of negative ions has a significant effect on the excitation of the light ion EIC modes, with higher harmonics excited as the fraction of negative ions increases. The results are compared with those of linear kinetic theory of current-driven EIC waves in a plasma containing heavy negative ions. [1] S-H. Kim and R. L. Merlino, Phys. Rev. E 76, 035401(R) (2007). [Preview Abstract] |
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UP6.00095: Observation of Fast-Ion Doppler-Shifted Resonance with Shear Alfven Waves S. Zhou, Y. Zhang, H. Boehmer, W. Heidbrink, R. McWilliams, S. Vincena, T. Carter, W. Gekelman, D. Leneman, P. Pribyl The Doppler-shifted cyclotron resonance ($\omega_{Alfven}-k_zv_z=\omega_{fast-ion}$) between fast ions and shear Alfv\'en waves (SAW) is experimentally investigated. A test particle beam of fast ions is launched by a Li$^+$ source [1] in the helium plasma of the Large Plasma Device (LAPD), with ion energy around 600eV. Both single loop antenna and dual polarization antenna are used to launch linear or circular polarized SAW (amplitude $\delta B/B$ up to 1\%). A collimated fast-ion energy analyzer measures the non-classical spreading of the beam, which is proportional to the resonance with the wave. Theoretically, when launched in (out of) phase with perpendicular wave electric field, fast ions gain (lose) energy from (to) the wave. The energy change of fast ions in presence of the wave is measured by changing the grid potential of the energy analyzer. A resonance spectrum is observed by launching SAWs at 0.3-0.8$\omega_{ci}$. A Monte Carlo code simulates the fast-ion particle orbit, wave-particle interaction, and energy-analyzer properties. Both the magnitude and frequency dependence of the beam-spreading agree with the simulation. [1] Y. Zhang {\it et al.}, Rev. Sci. Instrum. {\bf78} (2007) 013302. [Preview Abstract] |
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UP6.00096: Generation of Polarized Shear Alfven Waves by a Rotating Magnetic Field Source Alex Gigliotti, Walter Gekelman, Patrick Pribyl, Stephen Vincena, Alex Karavaev, Xi Shao, Dennis Papadopoulos We report on the generation and characterization of polarized kinetic shear Alfven waves radiated from a rotating magnetic field created via a novel phased orthogonal loop antenna. Both right and left hand polarization's are generated at a wide range of frequencies from 0.01 $< \quad \omega $/$\Omega _{ci} \quad <$ 1.0. Propagation parallel to the background magnetic field at the Alfven velocity is observed along with a negligible parallel wave magnetic field component implying a shear mode. The magnitude of the waves magnetic field is on the order of 0.4{\%} of the background field. Small amplitude second harmonic generation is seen along with indirect evidence of electron heating and/or fast electrons during the pulse implying non-linear response. Three-dimensional data of the wave fields and currents will be presented. [Preview Abstract] |
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UP6.00097: Simulations of drift-Alfven turbulence in LAPD using BOUT Pavel Popovich, Maxim Umansky, Troy Carter, Steve Cowley The LArge Plasma Device (LAPD) at UCLA is a 17~m long, 60~cm diameter magnetized plasma column with typical plasma parameters $n_e \sim 1\times 10^{12}$cm$^{-3}$, $T_e \sim 10$eV, and $B \sim 1$kG. The simple geometry and extensive measurement capability on LAPD allows for detailed comparison with and validation of numerical simulations of turbulence and transport. We analyse the LAPD results using simulations with the boundary plasma turbulence code BOUT. BOUT models the 3D electromagnetic plasma turbulence solving a system of fluid moment equations in a general tokamak geometry near the boundary. We will discuss the physical model and modifications of the BOUT code required for the LAPD configuration, and present the first results of the simulations and comparison to experimental measurements. In particular, a confinement transition is observed in LAPD under the application of bias-driven rotation.\footnote{E. Maggs, T.A. Carter, and R.J. Taylor, Phys. Plasmas 14, (2007)} Also, intermittent generation and convection of filamentary structures (``blobs'' and ``holes'') is observed in the LAPD edge.\footnote{T.A. Carter, Phys. Plasmas 13, (2006)} Application of BOUT to modeling of these two phenomena will be discussed. [Preview Abstract] |
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UP6.00098: Convective Generation of Lorentzian Pulses in Cross-Field Pressure Gradients Meixuan Shi, David Pace, James Maggs, George Morales, Troy Carter This numerical study explores the effects of large amplitude ExB plasma flows on cross-field pressure gradients in a magnetized plasma. The model considers two radially-localized potential eigenmodes with different azimuthal mode numbers and radial profiles that undergo sinusoidal oscillation at the same frequency. The convective ExB flow is incorporated into the 2- D, cross-field energy and particle transport equations. In both cases studied, an electron temperature filament, and a density filament, fine-scale spatial structure develops for sufficiently large field amplitude. The required amplitude is consistent with the observation of the onset of broadband turbulence in temperature filaments studied in the LAPD-U. The temporal behavior of the density or temperature, observed at a fixed spatial position, contains non-sinusoidal pulses. In addition, different types of pulses are found in different regions of the system; negative in the inner and positive in the outer. The temporal pulses may be fit with a Lorentzian shape. These results are consistent with the observation of an exponential frequency spectrum in the broadband turbulence found in the temperature filament and limiter-edge experiments performed in the LAPD-U. [Preview Abstract] |
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UP6.00099: Interaction of fast ion beam with plasma turbulence in the TORPEX simple magnetized toroidal plasma Gennady Plyushchev, Ambrogio Fasoli, Ivo Furno, Benoit Labit, Paolo Ricci, Christian Theiler An important question, related to burning plasma physics, is how fast ions interact with low frequency plasma turbulence. To address this question on TORPEX a miniaturized Li-6 ion source with relatively low ion energy (100eV-1keV) is used. The source is mounted on a 2D poloidally moving system inside the vacuum vessel. The energy and current density profile of the ion beam are completely characterized using a 2D movable gridded energy analyzer. The generation and measurements of the beam is carried out in hydrogen plasma at two plasma regions: in the main plasma region, where the plasma is produced by ECH and density fluctuations are dominated by coherent modes (interchange) and in the source free region with turbulent structures (blobs), which carries similarities to tokamaks SOLs. The changes of fast ion beam properties in these two scenarios with respect to propagation in the vacuum are investigated for different fast ion beam energies. [Preview Abstract] |
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UP6.00100: Instabilities, blobs and transport in TORPEX simple magnetized toroidal plasmas A. Fasoli, I. Furno, D. Iraji, B. Labit, G. Plyushchev, P. Ricci, C. Theiler, A. Diallo, S. Muller, M. Podesta, F. Poli Basic properties of fluctuations, turbulence and related transport are investigated in the basic device TORPEX via high-resolution measurements of plasma parameters and fields, obtained using fixed probe arrays, conditionally sampled data from movable probes and a fast framing optical camera. Drift or interchange instabilities dominate the fluctuation spectra, depending on the vertical magnetic field. The influence of the instability nature on the nonlinear development into plasma blobs is studied, along with the fluctuations statistics. Blobs are observed to generate electric fields and related flow patterns self-consistently, to cause particle transport, and to influence the plasma angular momentum. Two confinement regimes, akin to tokamak L and H mode, are theoretically predicted. Investigations are under way to identify these regimes in terms of the same observables in experiments and simulations. Other ongoing research lines, such as control of turbulent structures using in-vessel limiters, and the effect of turbulence on supra-thermal ions, will be discussed. [Preview Abstract] |
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UP6.00101: Modeling Alfven and Whistler Waves Generation by Rotating Magnetic Field Source X. Shao, A. Karavavev, A.S. Sharma, K. Papadopoulos, N. Gumerov, G. Joyce, A. Gigliotti, W. Gekelman Recent experiments by Gigliotti et al. 2008 and Karavaev et al. 2008 demonstrated excitation of Alfven and whistler waves, respectively, by Rotating Magnetic Fields (RMF) created by a phased orthogonal loop antenna. This paper presents a combination of computations along with experiments that emphasize the RMF properties for generating MHD and whistler waves. For RMF rotating frequencies in the whistler wave frequency range, the electrons quickly come to a co-rotation with the RMF, generating a differential azimuthal current. For rotating frequencies below the ion cyclotron frequency wave, the electron and ion motion decouple within the ion skin-depth near the antenna and co-rotates with the RMF outside the ion skin depth. In order to understand the RMF and plasma interaction and the resultant radiation in different frequency regimes, we developed a 3D code to simulate experimental configurations. The simulation help us understand the general characteristics of impedance matching, energy coupling and far field radiation pattern from an RMF antenna in plasmas. The dependence of the induced magnetic field on RMF frequency, and plasma parameters, as well as space applications of RMF antennas are discussed. This work was sponsored by ONR MURI Grant 5-28828. [Preview Abstract] |
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UP6.00102: Generation of Whistler Wave by a Rotating Magnetic Field Source A. Karavaev, K. Papadopoulos, X. Shao, A.S. Sharma, A. Gigliotti, W. Gekelman, P. Pribyl, S. Vincena The interaction of Rotating Magnetic Fields (RMF) with plasmas is a fundamental plasma physics problem with implications to fusion, space propulsion, control of energetic population in the radiation belts. In this paper we report recent experiments on the generation of whistler waves with a new type RMF-based antenna. The experiments were conducted on UCLA's Large Plasma Device (LAPD). The RMF is created using poly-phased loop antennas. A number of parameter combinations, e.g. plasma density, background magnetic field, and driving current, were used. It was found that RMF created by a two loop antenna drives significant currents along the ambient magnetic field. The measured amplitude of induced wave field was proportional to the square-root of the plasma density. The spatial decay rate for the wave perturbation across the background magnetic field scales with the plasma skin depth. We also present analytic and simulation results that account for the experimental results; in particular, the scaling of the induced magnetic field as a function of the RMF and plasma parameters. Applications of RMF as an efficient radiation source of plasma waves in space plasmas will be discussed. This work was sponsored by ONR MURI Grant 5-28828. [Preview Abstract] |
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UP6.00103: On Possibility of Fizeau Interferometry In Plasma$^{\ast }$ D.L. Brower, W.X. Ding, V.V. Mirnov The phase velocity of electromagnetic waves propagating through a substance depends on whether they propagate in a moving or stationary medium (Fizeau effect). In the case of a high frequency electromagnetic wave, the plasma dielectric response is dominated by the electrons so that the velocity of the medium is associated with the electron flow velocity. The Fizeau measurement of this characteristic can be linked to the electron current in plasma [1]. In the case of cold unmagnetized electrons, the plasma refraction index N$^{2 }$=1-- $\omega _{pe}^{2}$/$\omega ^{2}$. Due to specific $\omega ^{-2}$ scaling, the phase velocity turns out to be insensitive to the electron flow velocity. Any deviation from this scaling may result in wave vector dependence on the electron flow velocity that can make the Fizeau effect measurable. We evaluate the phase difference caused by the Fizeau effect and relate it to the experimental high-resolution, vertically viewing far-infrared polarimeter-interferometer system currently used on the Madison symmetric torus (MST) reversed field pinch (RFP). The calculations include the effect of motion of the plasma-vacuum interface, corrections caused by electron gyrorotation, and the influence of the finite electron temperature on the wave dispersion. [1] D. L. Brower, W. X. Ding, B. H. Deng, M. A. Mahdavi, V. V. Mirnov, S. C. Prager, Rev. of Sci. Inst. \textbf{75 }(10) 3399 (2004). $^{\ast }$The work was supported by the U.S. D.O.E. and N.S.F. [Preview Abstract] |
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UP6.00104: The Asymmetric Current Mirror Probe J.E. Maggs, T.A. Carter The difference in floating potential, between two closely spaced probe tips, is often used as a measure of the electric field in plasmas. This technique assumes a thermal Maxwellian distribution for plasma electrons and is unreliable in the presence of a fast non-Maxwellian electron tail. The influence of fast-tail electrons on floating potential measurements can be mitigated by using emissive probes or probes of unbalanced collection area. These probes have floating potentials that are closer to plasma potential than the floating potential of a standard Langmuir probe. An example of an unbalanced area probe is the ball-pen probe [Schrittwieser, et al., Rom. Journ. Phys., Vol. 50, 2005], in which the ion collection area is substantially larger than the electron collection area. The asymmetric current mirror probe achieves the effect of unbalanced collection area electronically, by amplification of the current drawn to the ion tip. Comparisons of radial profiles of floating potential in the LAPD at UCLA, measured using a Langmuir probe, ball-pen probe and asymmetric mirror probe are presented. The effects of using differences in floating potential, measured by these various probes, to determine the electric field is discussed. [Preview Abstract] |
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UP6.00105: Ion Temperature Measurements in Ultracold Neutral Plasmas Jose Castro, Hong Gao, Thomas Killian Plasma ion temperatures are measured through fluorescence spectroscopy of Ultracold Neutral Plasmas. Ultracold Neutral Plasmas (UNP's) are created by photoionizing laser-cooled strontium atoms in a magneto-optical trap (MOT). Spatially-resolved fluorescence imaging of the strontium ions produces a spectrum that is Doppler-broadened due to the thermal ion velocity and shifted due to the ion expansion velocity. The fluorescence excitation beam is spatially narrowed into a sheet, allowing for localized analysis of ion temperatures within a volume of the plasma with small density variation. Using this technique, measurements of the ion temperature are shown to display characteristics of plasmas with strong coupling. Disordered induced heating is shown to match with theory in a wide range of experimental parameters; kinetic energy oscillations of the ions are demonstrated as well. Finally measurements of ion temperature are shown for very long expansion times ($\sim $70 $\mu $s) after plasma formation displaying further heating of the ions and adiabatic cooling. [Preview Abstract] |
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UP6.00106: Measurement of two-point two-velocity correlation functions in the ion phase-space Fred Skiff Using two single frequency tunable lasers we generalize the technique used previously to measure two-point correlation functions to allow two different selected velocities as well. A steady-state singly-ionized Argon plasma with density 10$^{9}$ cm$^{-3}$, electron temperature of 2 eV, and ion temperature of 0.1 eV in a uniformly magnetized plasma cylinder is produced using a CW radio-frequency source. The plasma column is 10 cm in diameter and the main chamber is 200 cm in length. The measurements concern the low-frequency electrostatic fluctuations that occur naturally near the electron drift frequency driven by the radial electron temperature gradient. Two independent LIF systems, with detection systems on movable carriages, are scanned using computer-controlled stepper motors. One system involves a Argon-ion pumped single-frequency dye laser at 611nm with detection at 461 nm. The second system uses a single-frequency tunable diode laser at 668nm with detection at 443nm. By looking at cross-correlation between the two detection systems it is a measurement of $<$f(x,v,t)f(x',v',t')$>$ is realized. We will describe the tests and validations used to rule out instrumental effects on the measurement and compare the results to previous measurements of $<$f(x,v,t)f(x',v,t')$>$ made using a single laser beam. [Preview Abstract] |
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UP6.00107: Whistler Turbulence: Particle-in-Cell Simulations S. Peter Gary, Shinji Saito, Hui Li Two dimensional electromagnetic particle-in-cell simulations in a magnetized, homogeneous, collisionless electron-proton plasma demonstrate the forward cascade of whistler turbulence. The simulations represent decaying turbulence, in which an initial, narrowband spectrum of fluctuations at $kc/\omega_e \simeq$ 0.1 cascades toward increased damping at $kc/\omega_e \simeq$ 1.0, where $c/\omega_e$ is the electron inertial length. The turbulence displays magnetic energy spectra that are relatively steep functions of wavenumber and are anisotropic with more energy in directions relatively perpendicular to the background magnetic field ${\bf B}_o = \hat {\bf x} B_o$ than at the same wavenumbers parallel to ${\bf B}_o$. In the weak turbulence regime, the simulations demonstrate that the cascading fluctuations have the following properties: 1) Magnetic spectra become more anisotropic with increasing fluctuation energy; 2) the wavevector dependence of the three magnetic energy ratios, $|\delta B_j|^2/|\delta {\bf B}|^2$ with $j = x, y, z$, show good agreement with linear dispersion theory for whistler fluctuations; 3) the magnetic compressibility summed over the cascading modes satisfies 0.3 $< |\delta B_x|^2/|\delta {\bf B}|^2 <$ 0.6; and 4) the turbulence heats electrons in directions both parallel and perpendicular to ${\bf B}_o$, with stronger heating in the parallel direction. [Preview Abstract] |
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UP6.00108: Modeling radiation from relativistic collisionless shocks Shriharsha Pothapragada, Sarah Reynolds, Mikhail Medvedev Observed radiation from Gamma Ray Bursts (GRB) exhibits extremely rapid variation of spectral and temporal characteristics. The emission is thought to be produced at relativistic shocks associated with a GRB explosion. Our work uses the detailed theory of jitter radiation from relativistic collisionless shocks mediated by the Weibel instability, which produces small scale (of order a skin depth), high magnitude magnetic fields to model this variability. We employ relativistic shock kinematics to derive the light curves and the underlying spectral evolution. Our analysis has been extended to obtain complete light curves and correlations in the spectral parameters. We have developed a code that emulates the random source activity enabling us to extend our previous modelling of individual subpulses to a typical full duration burst. The distinct tracking of the photon flux and the low energy spectral index $\alpha$ -- a salient feature of our theory -- has indeed been observed in a number of bursts . We also discuss how plasma parameters of the shock and ejecta may be deduced. [Preview Abstract] |
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UP6.00109: Effects of light mass doping on ion velocity distribution functions in an electric double layer Ioana A. Biloiu, Earl E. Scime The effect of a lighter mass ion specie on the Ar and Xe ion velocity distribution functions (ivdfs) in the expansion region of a helicon plasma source has been investigated for those conditions under which an electric double layer (EDL) is known to form, i.e., pressure less than 2 mTorr, source magnetic field strength of 700 G, and expansion chamber magnetic field strength of 20 G. 4 cm upstream from the helicon source-expansion chamber junction, the Ar ivdf, as measured with laser-induced-fluorescence, is bimodal - comprised of a slow, nearly stationary ion population created by local ionization and a fast ion population created by the acceleration through the EDL. With the addition of He gas, the axial flow speed of the fast ion group increases from 5.7 to 7.6 km/s as the He/(He+Ar) mixing ratio increases from 0 to 30{\%}. The slow ion population ivdf includes a long tail that stretches over a velocity range of $\sim $ 5 km/s. Both the fast and slow ion group population densities, decrease as the mixing ratio increases. The slow population almost completely disappears at a ratio of 30{\%}He/70{\%}Ar. Similar effects were observed for Xe plasmas for which the lighter mass ion was Ar. Although no Xe ion beam was observed, addition of Ar led to an increase in the speed of the background ion population from 1.3 km/s in pure Xe to 2.3 km/s for an 87.5{\%} Ar/(Ar+Xe) ratio. [Preview Abstract] |
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UP6.00110: Maxwellianization of electron distribution functions by convective instabilities in presheaths S.D. Baalrud, C.C. Hegna, J.D. Callen Langmuir's paradox is a measurement of anomalous electron scattering where a Maxwellian electron velocity distribution function was measured much closer to a boundary than the electron collision length in a stable plasma; here one should expect truncation corresponding to the sheath energy. In this paper we theoretically analyze the presheath region that is present in Langmuir paradox-relevant plasmas ($T_e \gg T_i$). It is shown that the ion-acoustic instability is present throughout the presheath causing convective amplification of thermal fluctuations. A collision operator for the plasma kinetic equation including instabilities in a finite space-time domain is derived [1] which shows that electron scattering can be dominated by wave-particle interactions in the presheath. The modified collision operator satisfies the Boltzmann $\mathcal{H}$-theorem, so the only equilibrium is a Maxwellian which is achieved at a rate depending on collisionality. Wave-particle scattering shrinks the electron collision length to within a few cm for these discharges suggesting that one should expect a Maxwellian at the location of previously reported measurements. [1] S.D. Baalrud, J.D. Callen, C.C. Hegna, UW-CPTC 08-4, June 2008 (sub. to Phys. Plasmas). [Preview Abstract] |
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UP6.00111: Low Velocity Ion Stopping in Dense and Multicomponent Plasmas Bekbolat Tashev, Claude Deutsch, Patrice Fromy We focus attention on low velocity ion slowing down (LIVSD) in dense and multicomponent plasmas of ICF and astrophysical interest as well. The target is treated in a dielectric formalism with classical electrons neutralizing binary ionic mixtures (BIM) of any relative proportion. We consider first and mostly charge symmetric BIM such as deuterium-tritium of current fusion interest, proton-heliumlike iron in the solar interior or proton-helium ions considered in planetology as well as other mixtures of relevance to the heavy ion production of warm dense matter at Bragg peak conditions. We single out ion projectile velocities (so-called critical) for which target electron LIVSD matches the sum of target ions one. Proton stopping in the white dwarf crust (carbon-helium BIM)is also considered. [Preview Abstract] |
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UP6.00112: Determining electron temperature for small spherical probes from network analyzer measurements of complex impedance David N. Walker, Richard F. Fernsler, David D. Blackwell, William E. Amatucci In earlier work,\footnote{Walker, D.N., R.F. Fernsler, D.D. Blackwell, W.E. Amatucci, S.J. Messer, \textit{Phys of Plasmas}, $13$, 032108 (2006)} using a network analyzer, we have shown the existence of collisionless resistance (CR) in the sheath of a spherical probe when driven by a small rf signal. As shown in that paper the CR depends on the plasma density gradient at a given location. Because of this there is a cutoff in the CR which is proportional to the applied bias level and which will occur at the plasma frequency at the surface of the probe, $r = r_{0}$. We show that, in the frequency regime \textit{$\omega $}$_{pi}$\textit{$<<\omega <<\omega $}$_{pe}(r_{0})$, the complex impedance measurements made with a network analyzer can be used to determine electron temperature. We present an overview of the theory used along with comparisons to data sets made using three small spherical probes of different sizes. The numerical algorithm requires only a solution of the Poisson equation to determine the approximate sheath dimensions and integrals to determine approximate plasma and sheath inductances. We compare the results of the temperature measurements to those made by conventional Langmuir probe sweeps. [Preview Abstract] |
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UP6.00113: Localized momentum sources in a transport code as a model for biased-rings in a Helicon plasma T. Marine, M. Breyfogle, A.S. Ware, M. Gilmore, C. Watts, E. Schuster In the HELCAT experiment, biased concentric rings are used as control elements for the radial electric field profile. By varying the bias voltages, the local {\bf E}$\times${\bf B} flow can be modified. Here, we investigate modeling the biased rings as localized momentum sources in a 1-D transport code. The effect will be identical to a source of {\bf E}$\times${\bf B} flow in the limit of zero $\beta$ (i.e., when diamagnetic flows are negligible). By varying the momentum sources a sheared radial electric field can be generated that can suppress turbulent particle and heat transport. The results of modeling typical HELCAT experimental equilibria will be presented along with an investigation of the density dependence of ion-electron thermal coupling. We will also test the impact of different numerical models for the momentum sources and compare the results with experimental measurements of the radial electric field in the HELCAT experiment. [Preview Abstract] |
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UP6.00114: Laser-plasma expansion into a preformed magnetized plasma C. Niemann, C. Constantin, D. Schaeffer, E. Everson, A. Zylstra, N. Kugland, P. Pribyl, W. Gekelman, A. Collette, S. Tripathi, S. Vincena The expansion of an energetic laser-produced plasma across the magnetic field (0.1-1 kG) in a large (1 m x 17 m) magnetized plasma (He, 4x10$^{12}$ cm$^{-3}$, 5 eV) is studied by means of magnetic pickup coils and fast shutter photography. The bulk blow-off velocity of the laser-plasma is initially larger than the Alfv\'{e}n velocity. The laser plasma radiates large amplitude Alfv\'{e}n waves. We will present measurements of the compression and amplification of the magnetic field at the edge of the diamagnetic cavity. [Preview Abstract] |
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UP6.00115: DIVERTORS, EDGE PHYSICS AND FUELLING |
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UP6.00116: Divertor Configurations which Optimize Helium Pumping James Strachan Helium accumulation in DT plasmas is often presumed to be one limitation to the fusion power production. The core helium density has an unavoidable central source and a confinement time which tends to be long as is consistent with the required energy confinement times. Any pumping of the helium can only act to reduce the helium recycling. Within that constraint, however, it is still valuable to efficiently pump helium. Helium pumping can be aided by optimal placement of the helium pump in the divertor. The pump should be on the SOL side of the separatrix displaced into the region where the current of impurity particles enters into the divertor and initially strike the target. A numerical example will be given of helium pumping by the ITER divertor. A factor-of-two reduction in core helium densities is possible by optimal pump placement. One difficulty is the need for low temperatures along the targets to prevent their erosion. On ITER, recycled DT near the strike points is hoped to cool this region. The angle between the separatrix and the target is such that recycled neutrals cause ionization, excitation, and dissociation power losses along the target. The ITER solution constrains the choice of pump locations. Alternatively, the strike point cooling can be achieved by local DT (or low Z impurity) injection at the strike point. [Preview Abstract] |
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UP6.00117: New physics basis for SOL width Mike Kotschenreuther, Prashant Valanju, Swadesh Mahajan Existing empirical projection for SOL power width (projected from the divertor plate to the plasma midplane) have a very large uncertainty. Hence, the adequacy of required divertor operation on projected high power density devices (FDF, ST-CTF, Fusion Reactors, etc.) is highly uncertain. A simple general physical principle is presented which greatly reduces the uncertainty -- transport in the near-SOL cannot substantially differ from the immediately adjacent pedestal. This is motivated by a diverse class of experimental data and theoretical estimates, and unifies apparently different projection approaches. We quantitatively formulate this and test against several lines of experimental data with good agreement. Our predictions for ITER are is reasonable agreement with the 2007 ITER physics basis, and generally support the narrower range of projections for next generation devices. SOL widths are necessarily narrow if there is a good H-mode pedestal, i.e., good core confinement and challenging divertor operation are intrinsically intertwined. [Preview Abstract] |
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UP6.00118: Analysis of Geometric Modifications to Optimize High-Power Tokamak Divertors M.V. Umansky, R.H. Bulmer, R.H. Cohen, I. Joseph, T.D. Rognlien, D.D. Ryutov Next generation tokamak experiments and fusion reactors will have to accommodate divertor power flux an order of magnitude higher than in the present day experiments, as measured by the characteristic parameter P$_{LCFS}$/R$_{maj}$. Due to the engineering and materials constraints designing a divertor for such environment poses a challenging and presently unresolved task. In the present study the MHD code Corsica and edge transport code UEDGE are used for quantitative assessment of performance of high-power tokamak divertors. A multi-parametric study is conducted where a range of options is explored: varying x-point flux expansion using regular or snowflake-like divertor configuration [1], choice of divertor leg length and shape, shape of target plates, options for the radiating impurity, and assumptions for the anomalous transport. Varying the parameters we analyze the trends to optimize the peak power flux density on the target plate in a high-power divertor. \newline [1] D.D. Ryutov Physics of Plasmas 14, 064502 [Preview Abstract] |
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UP6.00119: Control of edge localized modes through toroidally asymmetric scrape-off layer current perturbations I. Joseph, R.H. Cohen, D.D. Ryutov, M.V. Umansky, X.Q. Xu Resonant magnetic perturbations (RMPs) can suppress the edge-localized MHD instabilities (ELMs) that limit the divertor target lifetime of H-mode tokamak fusion reactors. However, efficiently driving RMPs requires placing current carrying conductors as close as possible to the plasma, and engineering issues complicate the design of in-vessel components. We suggest driving the needed current through the scrape-off layer (SOL) plasma itself: current densities as large as J$_{sat}$=en$_{i}$c$_{s}$ can be driven by several means, including biasing the target to potentials of order T$_{e}$. Biasing in a toroidally varying fashion generates an asymmetric current density parallel to field lines in the SOL and a magnetic perturbation that is naturally aligned with field lines near the separatrix. Analytic estimates indicate that the resonant harmonics are larger than the $\delta $B/B $>$ 10$^{-4}$ criterion required for experimental ELM control. Magnetic island structure inside the separatrix will be investigated using the BOUT code. [Preview Abstract] |
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UP6.00120: The dynamics of coherent scrape-off layer structures in a snowflake divertor D.D. Ryutov, R.H. Cohen, I. Joseph, T.D. Rognlien, M.V. Umansky A characteristic feature of a snowflake divertor is the quadratic dependence of the poloidal magnetic field strength vs the distance from the field null. Compared to a standard X-point divertor, where the magnetic field dependence over distance is linear, this leads to significant changes in the geometry of flux tubes passing in the vicinity of the null. In particular, squeezing of flux tubes by the magnetic shear becomes stronger; the field line mapping from the midplane to the divertor plate indicates much higher poloidal velocities of plasma filaments near the divertor plates. Thus, significant changes are expected in the dynamics of coherent structures (sometimes called ``blobs'') in the scrape-off layer. An analysis of the dynamical effects associated with curvature drive, divertor boundary conditions, and strong magnetic shearing is presented. Regimes of enhanced blob transport are identified. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
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UP6.00121: Coupled GEM-XGC Simulations of Edge Pedestal Plasmas Scott Parker, Weigang Wan, Yang Chen Global GEM gyrokinetic turbulence simulations of the edge pedestal are performed assuming closed flux surfaces and using numerical profiles obtained from the XGC neoclassical calculation\footnote{Y. Chen and S. Parker, Phys. Plasmas, 15 055905 (2008)}. The plasma profiles used in GEM are output from an XGC simulation of L and H-mode DIII-D plasmas. XGC's magnetic geometry includes the separatrix and the magnetic X-point. Only the plasma profiles inside the separatrix are output to GEM. Simulations show the interesting result that in the electrostatic limit the anomalous diffusion agrees with what is used in the XGC simulation to accurately predict the pedestal profile build up. However, electromagnetic turbulence simulations produce too much transport which would causes a pedestal crash in the XGC calculation. Work is under way to couple GEM and XGC under the EFFIS End-to-end Grame for Fusion Integrated Simulation. [Preview Abstract] |
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UP6.00122: Nonlinear Full-f Edge Gyrokinetic Turbulence Simulations X.Q. Xu, A.M. Dimits, M.V. Umansky TEMPEST is a nonlinear full-f 5D electrostatic gyrokinetic code for simulations of neoclassical and turbulent transport for tokamak plasmas. Given an initial density perturbation, 4D TEMPEST simulations show that the kinetic GAM exists in the edge in the form of outgoing waves [1], its radial scale is set by plasma profiles, and the ion temperature inhomogeneity is necessary for GAM radial propagation. From an initial Maxwellian distribution with uniform poloidal profiles on flux surfaces, the 5D TEMPEST simulations in a flux coordinates with Boltzmann electron model in a circular geometry show the development of neoclassical equilibrium, the generation of the neoclassical electric field due to neoclassical polarization, and followed by a growth of instability due to the spatial gradients. 5D TEMPEST simulations of kinetic GAM turbulent generation, radial propagation, and its impact on transport will be reported. \newline [1] X. Q. Xu, Phys. Rev. E., 78 (2008). [Preview Abstract] |
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UP6.00123: Neoclassical orbit calculations with a full-f code for tokamak edge plasmas T.D. Rognlien, R.H. Cohen, M. Dorr, J. Hittinger, X.Q. Xu, P. Collela, D. Martin Ion distribution function modifications are considered for the case of neoclassical orbit widths comparable to plasma radial-gradient scale-lengths. Implementation of proper boundary conditions at divertor plates in the continuum TEMPEST code, including the effect of drifts in determining the direction of total flow, enables such calculations in single-null divertor geometry, with and without an electrostatic potential. The resultant poloidal asymmetries in densities, temperatures, and flows are discussed. For long-time simulations, a slow numerical instability develops, even in simplified (circular) geometry with no endloss, which aids identification of the mixed treatment of parallel and radial convection terms as the cause. The new Edge Simulation Laboratory code, expected to be operational, has algorithmic refinements that should address the instability. We will present any available results from the new code on this problem as well as geodesic acoustic mode tests. [Preview Abstract] |
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UP6.00124: A velocity-dependent anomalous radial transport model for (2-D, 2-V) kinetic transport codes Kowsik Bodi, Sergei Krasheninnikov, Ron Cohen, Tom Rognlien Plasma turbulence constitutes a significant part of radial plasma transport in magnetically confined plasmas. This turbulent transport is modeled in the form of anomalous convection and diffusion coefficients in fluid transport codes. There is a need to model the same in continuum kinetic edge codes [such as the (2-D, 2-V) transport version of TEMPEST, NEO, and the code being developed by the Edge Simulation Laboratory] with non-Maxwellian distributions. We present an anomalous transport model with velocity-dependent convection and diffusion coefficients leading to a diagonal transport matrix similar to that used in contemporary fluid transport models (e.g., UEDGE). Also presented are results of simulations corresponding to radial transport due to long-wavelength ExB turbulence using a velocity-independent diffusion coefficient. A BGK collision model is used to enable comparison with fluid transport codes. [Preview Abstract] |
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UP6.00125: Fully Electromagnetic Nonlinear Gyrokinetic Equations for Tokamak Edge Turbulence Z. Zhang, H. Nobu, T.S. Hahm, Lu Wang An energy conserving set of the fully electromagnetic nonlinear gyrokinetic Vlasov equation and Maxwell's equations, which is applicable to both L-mode turbulence with large amplitude and H-mode turbulence in the presence of high $\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}}\over {E}} \times \mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}}\over {B}} $ shear has been derived. In gyrokinetic Maxwell's equation, the particle charge density and current have been explicitly evaluated via pull-back transformation from the gyrocenter distribution function. Our generalized ordering takes$\rho _i <<\rho _{i\theta } \sim L_E \sim L_p $ as typically observed in the H-mode edge. We take $k_\bot \rho _i \sim 1$for generality, and keep the relative fluctuation amplitudes $e\delta \phi /T_i \sim \delta B/B<1$ up to the second order. [Preview Abstract] |
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UP6.00126: Numerical Simulation of SOL-Divertor Plasma Transport for Gas Puffing Operation in the KSTAR Tokamak Hyunsun Han, Ki Min Kim, Sang Hee Hong A two-dimensional numerical simulation is conducted to investigate the neutral gas puffing effect on the SOL(Scrape-off Layer)-divertor plasma in the KSTAR(Korea Superconducting Tokamak Advanced Research) tokamak. In this simulation, the location of gas puffing is assumed to be in the outer mid-plane of the tokamak while the pumping position is located at a fixed point near the outer divertor in the private flux region. The injection time, period and rate of gas puffing are considered as control parameters in the plasma-neutral transport code developed on the basis of the Braginskii's formulation for plasma and the diffusion model for neutrals. Under the KSTAR baseline operation mode, anomalous particle and energy transport coefficients are modulated to reproduce a situation of high energy flux dumped on the divertor target, like an ELM event. As results of the simulation, the plasma characteristics in a computing domain and the heat flux profiles on the divertor plate are represented for some proposed puffing scenarios. For a gas puffing scenario just before or after the first ELM burst occurs, the peak heat flux on the divertor target appears to be higher than that for no gas puffing case. This means that not only the neutral quantity but the gas injection time is an important factor for the gas puffing to control the heat load on the divertor plate. [Preview Abstract] |
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UP6.00127: Flows and electric fields near the edge of a tokamak plasma Frederick Hinton The expressions for neoclassical parallel flows are modified from their standard forms by orbit effects in plasmas with steep electric field profiles, such as near the edge of a tokamak plasma in an improved confinement mode. The diamagnetic and electric field-induced contributions to the bounce-averaged parallel flows are affected differently. This leads to modifications of the standard neoclassical expressions for poloidal and toroidal flows. These flows can change rapidly during a transition to an improved confinement mode. The electric field which develops during such a transition is partly determined by neoclassical polarization, which itself is modified by orbit effects in the steep electric field profile. By using a distribution function which is a function of particle constants of motion, plus a correction due to collisional effects, these modifications are estimated analytically. [Preview Abstract] |
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UP6.00128: Overview of MARFE along Improved confinement mode in ohmic and LHCD plasmas with graphite limiters on HT-7 tokamak M. Asif MARFE phenomena along Improved confinement mode in ohmic and LHCD Experiments with new graphite limiters on the HT-7 tokamak are summarized. The best correlation has been found between the total input (ohmic + LHCD) power and the product of the edge line average density and Z$_{eff}$. Studies show that the critical density of MARFE onset is observed in the region of ${Z_{eff}}^{1/2}f_{GW} =0.9\sim 1.2$, for ohmic and ${Z_{eff}}^{1/2}f_{GW} =0.6\sim 0.9$ for LHCD Plasmas, where$f_{GW} =\frac{\bar {n}_e }{n_{GW} }$, (Here $\bar {n}_e $ is the maximum line average electron density and $n_{GW} $ is the Greenwald density). Improved confinement mode induced by a MARFE is observed, and it is maintained for about 90 ms for ohmic and about 65 ms for LHCD plasmas. MARFE cools the plasma edge, and the electron density profile is observed to become more narrow and peaked. [Preview Abstract] |
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UP6.00129: Predictive Simulation of Profile Modification by Hydrogenic Pellet Injection into Tokamak Plasma Ki Min Kim, Hyunsun Han, Sang Hee Hong Pellet injection is a useful method for fueling and plasma profile control in the advanced tokamak operation. In this numerical work, profile modifications by hydrogenic pellet injection into tokamak plasmas have been simulated with a 1.5D core transport code. A neutral gas shielding (NGS) model is coupled to the transport code to calculate the pellet ablation rate during pellet passing through the background plasma. At the same time, a model of pellet drift caused by the variation of the toroidal magnetic field is taken into account. Simulation results present the plasma pressure profiles modified by the pellet injection in the H-mode operation, and the diverse characteristics of pellet ablations are compared according to background plasma property, pellet parameter and injecting location. The pellet injection from the high-filed side (HFS) predicts the deeper penetration of pellet materials into the core plasmas compared with the one from the low-field side (LFS) injection because of the pellet movement in the direction of major radius after ablation. Based on the simulation results, a pellet pace making scenario using the hydrogenic pellet injection method is proposed for ELM mitigation in the ELMy H-mode discharge, and tested under the KSTAR tokamak baseline operation conditions. [Preview Abstract] |
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UP6.00130: Performance of the Ignitor Pellet Injector A. Frattolillo, S. Migliori, S. Podda, F. Bombarda, L.R. Baylor, J.B.O. Caughman, S.K. Combs, C. Foust, D. Fehling, J.M. McJill, S. Meitner, G. Roveta ENEA and ORNL have built a four barrel, two-stage pneumatic injector for the Ignitor experiment featuring two innovative concepts: (i) an optimal shaping of the propellant pressure pulse to improve pellet acceleration, and (ii) the use of fast closing ($<$ 10 ms) valves to drastically reduce the expansion volumes of the propellant gas removal system. The injector is designed to deliver pellets of different sizes with velocities up to 4 km/s, capable of penetrating near the center of the plasma column when injected from the low field side in Ignitor burning plasmas ($n_0\cong 10^{21} \rm m^{-3}$, $T_0 \cong 11 $ keV). The ENEA sub-system, which includes the two-stage guns and pulse-shaping valves, the gas removal system, with associated controls and diagnostics, and the ORNL sub-system, consisting of the cryostat and pellet diagnostics, with related control and data acquisition system, have been assembled in Oak Ridge. Pellet speeds of 2 km/s have been achieved, despite the unfavorable configuration adopted in order to carry out some preliminary tests immediately after assembling the system, a very promising result. A second experimental campaign is planned for the 2008 Fall, when all four diagnostic channels should be complete. [Preview Abstract] |
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UP6.00131: First results of a new high resolution divertor IR camera at JET S. Devaux, T. Eich, G. Arnoux, W. Fundamenski, A. Alonso A new high resolution divertor infrared camera has been recently installed observing the JET divertor targets. This camera combines high spatial resolution of 1.7mm for the outboard divertor leg and around 5mm for the inboard leg with comparable high data frame rate of 35-85$\mu$s and recording length up to 40s. Due to the high sensitivity of the detector sufficient signal dynamic can be recorded on 10$\mu$s only. Hence, the camera resolves the fast evolution of the footprint of ELM filaments on the divertor targets. This latter evolution is observed for a large subset of the overall database, in particular in type-I ELMy H-Modes with q95 values of around 3 and fields of 2.5MA/2.5T and above. Analysis of the radial/poloidal distribution allows for a detailed investigation of the toroidal quasi-mode-numbers (QMN) on the divertor targets on the mentioned time scales. Analysis of the spatial structure of the ELM filaments observed by the IR camera complemented by pre-ELM magnetic equilibrium reconstructions allows estimation of the QMN of type-I ELMs. These latter derived values are compared with results from e.g. ASDEX Upgrade as well as with results on the toroidal structure of type-I ELMs based on first wall heat flux measurements in JET in this contribution. [Preview Abstract] |
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UP6.00132: Study of effects of non-thermal particles on kinetic H-mode pedestal evolution A.Y. Pankin, G. Bateman, F.D. Halpern, A.H. Kritz, T. Rafiq, C.S. Chang, S. Ku, G. Park, D.C. McCune Effects of non-thermal particles on the evolution of the plasma edge in tokamaks are investigated using the self-consistent kinetic XGC0 code. The beam geometry package from the NTCC NUBEAM module has recently been implemented in the XGC0 code. The NTCC Plasma State module is used to interface the kinetic XGC0 code and NUBEAM module. Neutrals in the beam geometry package are started at the injector plate with random angles and subsequently tracked to the tokamak plasma edge. Once the neutrals enter the plasma, their behavior is governed by a model for neutrals in the XGC0 code and the dynamic evolution of the plasma edge is computed in self-consistent simulations. These simulations include the formation of sheared velocity flows and the effects of $\textbf{E} \times \textbf{B}$ flow shear, turbulence transport suppression, and formation of the H-mode pedestal, up to the triggering of an ELM crash. The dependence of pedestal parameters on the additional torque that is introduced by the neutral beam and non-thermal particles is investigated. Neutral beam effects on triggering ELM crashes are also studied. [Preview Abstract] |
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UP6.00133: MHD-consistent Kinetic XGC0 study of 3D RMP effect on edge pedestal transport Gunyoung Park, H. Strauss, C.S. Chang, S. Ku, J-K Park Experiments have shown that the resonant magnetic perturbations (RMPs) applied by an external coil array can control edge localized modes (ELMs) in an H-mode pedestal. The externally applied 3D magnetic field perturbations could be significantly modified by the plasma response, which include RMP screening, RMP amplification, and convective cell formation. In this work, kinetic XGC0 simulation of RMP transport is performed using an MHD-evaluated 3D RMP perturbation in the plasma, either the ideal MHD response from the IPEC code or the resistive MHD response from the M3D code. In the case of the resistive MHD response, plasma rotation profile (as well as the density and temperature profiles) in XGC0 is coupled with the RMP penetration in M3D for a more self-consistent screening of the external RMPs and the convective cell formation in a real geometry edge plasma. Difference in the kinetic pedestal behaviors between the ideal and resistive MHD RMP responses will be reported. Experimental validation will be performed in DIII-D and NSTX plasmas. [Preview Abstract] |
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UP6.00134: Development of a Coupled Kinetic Plasma - Neutral Transport Code D.P. Stotler, C.S. Chang, G. Park Monte Carlo neutral transport codes have been run in conjunction with fluid plasma transport codes for more than a decade. The logical next step is to couple a Monte Carlo neutral transport package to a kinetic plasma transport code. The XGC neoclassical particle transport does just this with a built-in, rudimentary Monte Carlo neutral transport routine. A primary objective of the Center for Plasma Edge Science project is the replacement of this routine with a more general routine based on the DEGAS 2 Monte Carlo neutral transport code. As was done by XGC's neutral routine, the DEGAS 2 neutrals collide off of a fluid plasma background with its moments computed from the kinetic XGC ions. The resulting neutral density, flow velocity and temperature profiles are passed back to XGC. XGC's ions and electrons collide off of this background using the same ionization and charge exchange rates employed in the neutral transport calculation. We describe an approach to the coupling that ensures overall conservation of particles, momentum, and energy so as to avoid an accumulation of spurious sources that could compromise the accuracy of the simulation. [Preview Abstract] |
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