Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session BP9: Poster Session I: Magnetic Reconnection I; Plasma Astrophysics Theory; Laboratory Plasma Astrophysics; MST and Other Reversed Field Pinches; Laser and Beam Driven Acceleration |
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Room: Hall A |
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BP9.00001: MAGNETIC RECONNECTION I |
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BP9.00002: Large scale electron acceleration by parallel electric fields during magnetic reconnection J. Egedal, A. Le, W. Daughton Magnetic reconnection is an ubiquitous phenomenon in plasmas. It permits an explosive release of energy through changes in the magnetic field line topology. In the Earth's magnetotail, reconnection energizes electrons up to hundreds of keV and solar flares events can channel up to 50\% of the magnetic energy into the electrons. Electron energization is also fundamentally important toastrophysical applications, where X-rays generated by relativistic electrons provide a unique window into the extreme environments. Here we show that during reconnection powerful energization of electrons by $E_{\parallel}$ can occur over spatial scales which hugely exceed what previously thought possible. Thus, our results are contrary to a fundamental assumption that a hot plasma -- a highly conducting medium for electrical current -- cannot support any significant $E_{\parallel}$ over length scales large compared to the small electron inertial length $d_e=c/\omega_{pe}$. In our model $E_{\parallel}$ is supported by non-thermal and strongly anisotropic features in the electron distributions not permitted in standard fluid formulations, but routinely observed by spacecraft in the Earth's magnetosphere. This allows for electron energization in spatial regions that excide the regular de scale electron diffusion region by at least three orders of magnitude. [Preview Abstract] |
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BP9.00003: Unmagnetized Electron Jets in Reconnection with a Guide Field A. Le, J. Egedal, W. Daughton Jets of super-Alfvenic outflowing electrons form in kinetic simulations of anti-parallel reconnection and have been observed in Earth's magnetosphere. A model describes how gradients in the electron pressure tensor, which obeys equations of state that have been verified using kinetic simulations of reconnection in both 2D and 3D, drive the jets. Following the model, the magnitude of the current depends on the upstream electron beta [1,2]. Here, to study the electron layer in the presence of a guide magnetic field, PIC simulation runs are carried out with a range of guide fields. With initial guide fields up to 15\% of the upstream reconnecting field, collimated electron outflow jets are driven in the central region by the electron pressure anisotropy as in the model originally based on an anti-parallel geometry. The Hall currents, however, become asymmetric and expel the guide field from the region around the electron layer. \\[1ex] [1] Le A, Egedal J, Daughton W, Drake JF, Fox W, and Katz N, Geophys. Res. Lett. 37, L03106 (2010).\\[1ex] [2] Ng J, Egedal J, Le A, Daughton W, and Chen L-J, Phys. Rev. Lett. 106, 065002 (2011). [Preview Abstract] |
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BP9.00004: Reconnection experiments with 3D magnetic nulls A. Vrublevskis, J. Egedal, A. Le, P. Montag Three-dimensional effects have been crucial in explaining experiments at the Versatile Toroidal Facility (VTF) even in nominal axisymmetric plasmas with a non-vanishing toroidal field [1]. In general, depending on the topological and geometric structure of the magnetic field, a rich collection of magnetic reconnection scenarios is possible in three dimensions. The new adjustable set of coils in VTF allows exploring reconnection in 2D and 3D geometries including configurations with magnetic null points. We present results of a numerical and experimental investigation of magnetic field topologies attainable in VTF.\\[1ex] [1] Katz, N. et al., (2010) Phys. Rev. Lett. 104, 255004. [Preview Abstract] |
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BP9.00005: A Two-Fluid Reconnection Code, Implementing the Electron Pressure Tensor With New Anisotropic Equations of State O. Ohia, J. Egedal, A. Le, S. Lukin, W. Daughton Collisionless magnetic reconnection plays an important role in space and laboratory plasmas. Comparing two-fluid and kinetic particle reconnection simulations, it is found that the structure surrounding the electron diffusion region and the electron current layer differ vastly [1]. Recently, a new fluid closure has been obtained for electrons that relate parallel and perpendicular pressures to the density and magnetic field [2]. The closure has been confirmed in fully kinetic simulations and is obtained using an adiabatic solution of the Vlasov equation, which includes the dynamics of electrons trapped in parallel electric fields [3]. Using the HiFi framework (by S Lukin), a two-fluid code is developed that implements the new approximation for the electron pressure tensor in guide-field reconnection. The results of the fluid simulation are compared to a kinetic particle simulation with a similar setup.\\[1ex] [1] Daughton W, et al., Phys. Plasmas 13, 072101 (2006).\\[0ex] [2] Le A, et al., Phys. Rev. Lett. 102, 085001 (2009).\\[0ex] [3] Egedal J, et al., J. Geophys. Res. 113 (2008). [Preview Abstract] |
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BP9.00006: A Design Study of a New Magnetic Reconnection Experiment, MITPX P. Montag, J. Egedal, M. Porkolab, A. Le, A. Vrublevskis, O. Ohia A new model for effective heating of electrons during reconnection is now gaining support from spacecraft observations, theoretical considerations and kinetic simulations. The key ingredient in the model is the physics of trapped electrons whose dynamics causes the electron pressure tensor to be strongly anisotropic [1]. The heating mechanism becomes highly efficient for geometries with low upstream electron pressure; conditions relevant to the magnetotail. We propose a new Magnetic Interaction Toroidal Plasma Experiment (MITPX) that will be optimized for the study of kinetic reconnection including the dynamics of trapped electrons and pressure anisotropy. \\[1ex] [1] Le A, et al., (2009) Phys. Rev. Lett. 102, 085001. [Preview Abstract] |
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BP9.00007: Effects of Ion Anisotropy in reconnection exhausts J. N, J. Egedal, A. Le, H. Karimabadi, W. Daughton Strong ion anisotropy has been observed in the reconnection exhausts of large-scale kinetic simulations, and this can be described by models considering the motion of single particles in one-dimensional fields [1, 2]. The anisotropy supports the formation of an elongated current sheet in the exhaust, in which most of the current is carried by the ions, extending the electron current sheet observed close to the x line. In addition, by considering momentum balance across the reconnection exhaust, we show the details of how the magnetic tension is balanced by the combination of fluid inertia and anisotropy, and how the anisotropy has a feedback effect on the reconnection process.\\[1ex] [1] S. W. H. Cowley, P. Shull Planet. Space Sci., 31, 235 (1983)\\[0ex] [2] J. F. Drake et. al. J. Geophys. Res., 114, A05111 (2009) [Preview Abstract] |
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BP9.00008: Interactions between magnetic flux ropes Walter Gekelman, Bart Van Compernolle Magnetic flux ropes are structures, which consist of magnetic field lines with pitch varying with radius across the rope, a result of currents flowing along the field. They exist in abundance in the solar corona and have been observed by satellites near the earth. They often exist in pairs or triplets. Flux ropes have been produced and the interaction and reconnection between two ropes has been studied in detail in the LAPD device at UCLA. In this experiment the space-time behavior of three kink unstable flux ropes in a background magnetoplasma are studied. The ropes twist about each other and themselves as well as collide with one another. The ropes are line tied on one end and are free to move at the other end, where they are observed to rotate around the background magnetic field. The quasi-separatrix layers (regions in which magnetic field line reconnection occurs and neighboring field lines diverge) associated with the ropes are derived from the 3D data and their motion and topology is studied. LIF measurements indicate significant ion heating in the ropes. Swept Langmuir probe measurements indicate large density and temperature changes. These give rise to large amplitude drift waves. [Preview Abstract] |
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BP9.00009: Quasi-separatrix layers and three-dimensional reconnection diagnostics for line-tied tearing modes Andrew S. Richardson, John M. Finn In three-dimensional magnetic configurations for a plasma in which no closed field line or magnetic null exists, no magnetic reconnection can occur, by the strictest definition of reconnection. A finitely long pinch with line-tied boundary conditions, in which all the magnetic field lines start at one end of the system and proceed to the opposite end, is an example of such a system. Nevertheless, for a long system of this type, the physical behavior in resistive magnetohydrodynamics (MHD) essentially involves reconnection. This has been explained in terms comparing the geometric and tearing widths. The concept of a quasi-separatrix layer was developed for such systems. We study a model for a line-tied system in which the corresponding periodic system has an unstable tearing mode. We analyze this system in terms of two magnetic field line diagnostics, the squashing factor and the electrostatic potential difference, which has been used in kinematic reconnection studies. We discuss the physical and geometric significance of these two diagnostics and compare them in the context of discerning tearing-like behavior in line-tied modes. [Preview Abstract] |
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BP9.00010: Magnetic reconnection studies with wire ablation plasmas John Greenly, Charles Seyler, Xuan Zhao We have found that arrays of fine wires driven by pulsed current (1 MA, 300 ns pulse length) can be used to study magnetic reconnection. A pair of parallel wires produce an X-point magnetic topology between them. The wires continuously evolve plasma from their expanding surfaces and when the driving voltage reverses, the current in the surface plasma reverses and it is repelled from the wire cores, producing a reconnection flow. These aluminum plasmas at a temperature of 30 - 100 eV and 10$^{18}$ /cm$^{3}$ density radiate strongly in soft X-rays, and XUV imaging shows the development of the X-point magnetic null into an extended current sheet. These experimental results will be presented. [Preview Abstract] |
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BP9.00011: Simulation Study of Magnetic Reconnection Using Wire Ablation Plasma Xuan Zhao, John Greenly, Charles Seyler The results of simulations of 2D X-point magnetic reconnection using the PERSEUS [1,2] code are presented for 2-wire and 4-wire configurations. It is shown that ablated wires begin a rapid plasma expansion phase after the reversal of driving voltage, which is followed by magnetic reconnection characterized by inflow and super-Alfv\'{e}nic outflow. The quantitative details of the 2D simulation results are presented, including the density, temperature and magnetic field distributions and the results are compared to the experiments on the~COBRA 1 MA pulser. Comparison between the magnetohydrodynamics (MHD) and extended-MHD models are also shown~for same pulsed power loads. \\[4pt] [1] M. R. Martin, Ph.D. thesis, Cornell University, 2010 \newline [2] C. E. Seyler, et al., Physics of Plasmas 18, 012703, 2011 [Preview Abstract] |
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BP9.00012: The kinetic structure of collisionless slow shocks and reconnection exhausts Yi-Hsin Liu, James Drake, Marc Swisdak, William Daughton, Hui Li A 2-D Riemann problem is designed to study the development and dynamics of the slow shocks that are thought to form at the boundaries of reconnection exhausts. Particle-in-Cell (PIC) simulations are carried out with different propagation angles with respect to the upstream magnetic field. When the angle is sufficiently oblique, the simulations reveal a large firehose-sense ($P_\|>P_\bot$) temperature anisotropy in the downstream region, accompanied by a transition from a coplanar slow shock to a non-coplanar rotational mode. In the transition region the firehose stability parameter $\varepsilon=1-\mu_0(P_\|-P_\perp)/ B^2$ tends to plateau at 0.25. An explanation for the critical value 0.25 is proposed by examining the Anisotropic MHD equations. The anisotropy value of 0.25 is significant because it is closely related to the degeneracy point of the slow and intermediate modes, and corresponds to the lower bound of the transition point in a compound slow shock(SS)/rotational discontinuity(RD) wave. This work implies that it is a pair of compound SS/RD waves that bounds the reconnection outflow, instead of a pair of switch-off slow shocks as in Petschek's model. In large-scale PIC reconnection simulations, the signature of $\varepsilon=0.25$ at the downstream exhausts is also identified. Its implication for in-situ observations of earth's magnetotail is discussed. [Preview Abstract] |
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BP9.00013: Kinetic Simulations of Current-Sheet Formation and Reconnection at a Magnetic X Line Carrie Black, S.K. Antiochos, M. Hesse, J.T. Karpen, C.R. DeVore, M.M. Kuznetsova, S. Zenitani The integration of kinetic effects into macroscopic numerical models is currently of great interest to the plasma physics community, particularly in the context of magnetic reconnection. We are examining the formation and reconnection of current sheets in a simple, two-dimensional X-line configuration using high-resolution particle-in-cell (PIC) simulations. The initial potential magnetic field is perturbed by thermal pressure introduced into the particle distribution far from the X line. The relaxation of this added stress leads to the development of a current sheet, which reconnects for imposed stress of sufficient strength. We compare the evolution and final state of our PIC simulations with magnetohydrodynamic simulations assuming both uniform and localized resistivities, and with force-free magnetic-field equilibria in which the amount of reconnection across the X line can be constrained to be zero (ideal evolution) or optimal (minimum final magnetic energy). We will discuss implications of our results for reconnection onset and cessation at kinetic scales in dynamically formed current sheets, such as those occurring in the terrestrial magnetotail and solar corona. [Preview Abstract] |
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BP9.00014: Compressible gyrofluid simulations of collisionless reconnection F.L. Waelbroeck, D. Grasso, E. Tassi, L. Comisso Ion temperature plays an important role in collisionless magnetic reconnection, where it can both raise the stability threshold and, once the mode is unstable, accelerate its growth. We have investigated magnetic reconnection with a recently constructed noncanonical Hamiltonian formulation of a four-field electromagnetic gyrofluid model. The new model extends previous Hamiltonian models in two ways: (1) It retains the effect of ion compressibility, enabling the description of sound waves and drift Kelvin-Helmholtz instabilities, and (2) It accounts for the role of magnetic curvature, enabling the description of geodesic acoustic modes (GAM) and ballooning modes. We find that in order for the Poisson bracket to satisfy the Jacobi identity, we must halve the coefficient of the curvature term in the parallel momentum equations. We present the resulting Casimir invariants and show them to be associated to four Lagrangian invariants advected by distinct velocity fields. Examination of the invariants helps to understand the changes in the reconnection dynamics as a function of the plasma beta and the ratio of species temperatures. [Preview Abstract] |
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BP9.00015: Slow shock formation and its structure with sub-Alfvenic shear flow in magnetic reconnection Zhi-Wei Ma Slow shock formation and its structures associated with magnetic reconnection are investigated in the presence of sub-Alfv\'enic shear flow based on compressible resistive MHD model and compressible Hall MHD model. It is found for the first time that one or two pairs of the slow shocks are formed in the inflow region away from the reconnection separatrices in the compressible resistive MHD. The distributions of the slow shocks largely depend on the plasma beta and the shear flow velocity. One pair of the slow shocks is formed for the case $\beta =0.2$ and two pairs of the low shocks are generated for the case $\beta =1.0$with the shear flow velocity around the range from 0.6V$_{A}$ to 0.9 V$_{A}$. In the case of the high plasma beta ($\beta =5.0)$, there is no slow shock formed outside the reconnection separatrices. In the compressible Hall MHD, the slow shocks are gradually evolved into wave trains as increase of the ion inertial length $d_i $. [Preview Abstract] |
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BP9.00016: Self-Generation of Turbulence in Collisionless Magnetic Reconnection William Daughton, Vadim Roytershteyn, Homa Karimabadi Magnetic reconnection releases energy explosively in space, laboratory and astrophysical plasmas. In large 2D collisionless kinetic simulations, the nonlinear development features both ion and electron kinetic-scale features, with highly extended electron current layers which can trigger the formation of secondary magnetic islands. However, the influence of realistic 3D dynamics remains poorly understood. Using petascale kinetic simulations for guide field geometries, we show that the 3D evolution is dominated by the formation and interaction of magnetic flux ropes. In contrast to previous theories, the majority of flux ropes are produced by secondary instabilities within the electron current layers. New flux ropes spontaneously appear within these electron layers, leading to a turbulent evolution.\footnote{Daughton et al, Nature Physics {\bf 7}, 539, 2011} The turbulence is highly inhomogeneous and features anisotropic structures across multiple scales, including electron-scale current sheets that continually reform and breakup into filaments, along with flux ropes generated at these scales and quickly growing well above ion scales. To better understand the parametric dependencies, a range of guide fields are considered as well as asymmetric current layers of relevance to the magnetopause. [Preview Abstract] |
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BP9.00017: Effect of Inflow Density on Magnetic Reconnection: Particle-in-Cell Simulations Pin Wu We perform a systematic study of the effect of inflow density on reconnection diffusion regions using a 2.5-D particle-in-cell (PIC) code. The diffusion region structures are analyzed at times when all simulations have reconnected the same amount of magnetic flux. We find that reducing the inflow density from 1 to 1/100th of the current sheet density dramatically increases the diffusion region physical size and the reconnection rate. The width of the diffusion region scales with the upstream ion inertial length systematically. Consistent with the presence of counter-streaming inflowing ion beams near the x-line, the ion meandering width in the diffusion region also scales with the ion inertial length. The aspect ratio of the ion diffusion region remains a constant, independent of the inflow density. The quadrupole Hall magnetic field is reduced. The upstream magnetic field deviates from its asymptotic value by $\sim 50\%$ at the lowest simulated inflow density. The downstream ion outflow velocity scales linearly with the upstream Alfv\'en speed with a multiplication factor $\sim 0.4 <1$. When applied to magnetic reconnection in the Earth's magnetotail, this factor of 0.4 is a possible explanation as to why bulk flow velocities in the magnetotail are typically on the order of 500 km/s, while the Alfv\'en speeds of inflowing plasmas can exceed 2000 km/s. [Preview Abstract] |
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BP9.00018: Variational approach to collisionless magnetic reconnection Makoto Hirota, Philip J. Morrison In collisionless regimes, magnetic reconnection may be accelerated by the mesoscopic effects which play the role of the singular perturbation to the ideal MHD model. Several authors have recently performed noncanonical Hamiltonian formulations of such extended MHD models, for which the dynamical systems approach is expected to provide further understandings of linear and nonlinear reconnection processes. This work focuses on the effect of electron inertia and develops the variational principle for a 2D fluid model including it. By introducing the displacement field of ideal plasma motion, the perturbation expansion around equilibrium state leads to a 2nd-order potential energy ($\delta W$). The linear growth rate of the reconnecting mode can be reproduced in the same manner as the MHD energy principle. Moreover, the recent perturbation method for deriving 3rd-order potential energy [M. Hirota, J. Plasma Phys. at press 2011] applies to this dynamical system, which can predict the early nonlinear phase of the reconnecting mode accompanied by a relaxation of the equilibrium state and mode-mode couplings. [Preview Abstract] |
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BP9.00019: Self-Organization of Reconnecting Plasmas to a Marginally Collisionless State Ellen Zweibel, Shinsuke Imada It was suggested by Uzdensky (2007) and Cassak et al. (2008) that stellar coronal loops heated by magnetic reconnection naturally maintain themselves in a marginally collisionless state. The mechanism is based on the observation, reported in many calculations and experiments, that collisionless reconnection is faster than collisional reconnection. The mechanism operates as follows: increasing the heating rate increases the conductive heat flux to the dense lower atmosphere, driving an evaporative flow. The resulting increase in coronal density increases the collisionality, reducinging the heating rate. Likewise, reducing the heating rate decreases the collisionality. We are testing this scenario using a time dependent, 1D model for a coronal loop in thermal contact with a colder, denser plasma. We include a collisionality dependent heating rate. heat transport by thermal conduction, radiative cooling, and enthalpy flux. We will discuss the conditions under which the plasma self organizes, and the stability of the resulting state. References: Cassak, P.A., Mullan, D.J, \& Shay, M.A. ApJ 676, L69 (2008) Uzdensky, D. ApJ 671, 2139 (2007) [Preview Abstract] |
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BP9.00020: Reconnecting Modes in Weakly Collisional Plasmas* P. Buratti, B. Coppi Drift-tearing modes in ohmically heated toroidal plasmas are observed usually to propagate in the direction opposite to the equilibrium plasma current (the electron direction), with an associated rotation frequency of the order of the electron diamagnetic frequency. On the other hand, after subtraction of the Doppler shift associated with the radial equilibrium electric field, magnetic islands that rotate in the ion direction have been found in neutral-beam-heated plasmas. The same kind of Doppler-shift analysis should be applied to plasmas without beam heating, in order to account for their spontaneous rotation, but this effect, which could revert the sign of mode rotation in the $E_r=0$frame, has been ignored so far. Two different types of reconnecting modes were considered to explain the experimental results on reconnection in high-temperature plasmas, one related to the drift-tearing mode that propagates in the electron diamagnetic velocity direction and another that propagates in the ion direction. In the first case, a substantial reduction of the ratio between parallel and perpendicular thermal conductivities associated with a pre-excited mode (e.g. due to the electron temperature gradient) is required. In the second case a pre-excited mode driven by the combined effects of the plasma pressure gradient and the magnetic field curvature and the presence of a high-energy particle population are also required. *Sponsored in part by the U.S. DOE. [Preview Abstract] |
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BP9.00021: PLASMA ASTROPHYSICS THEORY |
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BP9.00022: Tridimensional Plasma Spirals and High Frequency Quasi Periodic Oscillations Around Black Holes* P. Rebusco, B. Coppi, M. Bursa A theoretical interpretation based on a novel kind of disc plasma modes [1] is proposed for High-Frequency Quasi-Periodic Oscillations (HFQPOs) in low mass X- ray binaries [2]. Tridimensional, tightly wound spirals are considered that co-rotate with the plasma disc in the vicinity of a black hole. These modes can be excited, from an axisymmetric disc embedded in a ``seed'' vertical magnetic field, by the combined effects of the differential rotation and the vertical gradients of the plasma density and temperature. Considering the electron temperature gradient is a clear oversimplification of the gradients that electron distributions can have in the highly non-thermal regimes from which HFQPOs emerge [3]. The tridimensional spiral modes considered are localized radially over relatively narrow widths [1] and have frequencies that are multiples of the local plasma rotation frequency. The higher toroidal number $m_\phi$ modes are considered to decay into $m_\phi=2$ and $m_\phi=3$ modes, explaining the observed twin peak HFQPOs with the 3:2 ratio. Large variations in the collisional mean free path, corresponding to local compression and rarefaction, are associated with the considered spirals. These variations can lead to different emission characteristics. *Sponsored in part by the U.S. DOE. [1] B. Coppi, A\&A 321, 504 (2009). [2] P. Rebusco, New Astronomy Review 855, 51 (2008). [3] B. Coppi, Phys. Plasmas 032901, 18 (2011). [Preview Abstract] |
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BP9.00023: Solitary Ring Pairs and Non-Thermal Regimes in Plasmas Connected with Black Holes* Bruno Coppi The two-dimensional plasma and field configurations that can be associated with compact objects such as black holes are described, (in the limit where assuming a scalar pressure can be justified), by two characteristic non-linear equations: i) one that connects the plasma density profile to that of the relevant magnetic surfaces [1] and is called the ``master equation'': ii) the other, the ``vertical equilibrium equation,'' connects the plasma pressure to the density and the magnetic surfaces and is closely related to the G-S equation for magnetically confined laboratory plasmas. Two kinds of solutions are found that consist of: i) a periodic sequence of plasma rings; ii) solitary pairs of rings. Experimental observations support the presence of rings around collapsed objects. Tridimensional configuration are found in the linear approximation [2] as consisting of trailing spirals. Observations of High Frequency Quasi-Periodic oscillations implies that they originate from 3-dimentional structures. The existing theory is extended to involve non-thermal particle distributions in order to comply with relevant experimental observations. *Sponsored in part by the U.S. DOE.\\[4pt] [1] B. Coppi, \textit{Phys. Plasmas} {\bf 032901}, 18 (2011).\\[0pt] [2] B. Coppi, \textit{A\&A} {\bf 321}, 504 (2009). [Preview Abstract] |
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BP9.00024: Structure and Scale of Cosmic Ray Modified Shocks Vassili Rozanov, Mikhail Malkov, Patrick Diamond, Roald Sagdeev Astrophysical shocks, diffusively accelerating cosmic rays (CR) ought to develop CR precursors. The length of the precursor $L_{p}$ is believed to be set by the ratio of the CR mean free path $\lambda$ to the shock speed, $L_{p}\sim c\lambda/V_{sh}\sim cr_{g}/V_{sh}$, which is independent of the CR pressure $P_{c}$. However, the X-ray observations of supernova remnant shocks suggest that the precursor scale may be significantly shorter than $L_{p}$ which would question the above estimate unless the magnetic field is strongly amplified and the gyroradius $r_{g}$ is strongly reduced. We argue that while the CR pressure builds up ahead of the shock, the acceleration enters into a strongly nonlinear phase in which an acoustic instability, driven by the CR pressure gradient, dominates other instabilities (for $\beta < 1$ ). In this regime the precursor steepens into a strongly nonlinear front whose size scales with \emph{the CR pressure }as $L_{f}\sim L_{p}\cdot\left(L_{s}/L_{p}\right)^{2}\left(P_{c}/P_{g}\right)^{2 }$, where $L_{s}$ is the scale of the developed acoustic turbulence, and $P_{c}/P_{g}$ is the ratio of CR to gas pressure. Since $L_{s}\ll L_{p}$, the precursor scale reduction may be strong in the case of even a moderate gas heating by the CRs through the acoustic and (possibly also) the other instabilities driven by the CRs. [Preview Abstract] |
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BP9.00025: Probing Nearby CR Accelerators and ISM Turbulence with Milagro and IceCube Hot Spots Luke Drury, Mikhail Malkov, Patrick Diamond, Roald Sagdeev Acceleration of cosmic rays (CR) in supernova remnant shocks should result in an almost isotropic CR spectrum. Yet the MILAGRO TeV observatory and now IceCube discovered a sharp $\sim$10 deg arrival anisotropy. We suggest a mechanism for producing a narrow CR beam which operates en route to the observer. The key assumption is that CRs are scattered by anisotropic Alfven waves formed in a turbulent cascade across the local field direction. The strongest pitch-angle scattering occurs for particles moving almost precisely along the field line. The enhanced scattering results in a narrow particle excess. The width, the excess and the maximum momentum of the beam are calculated from a systematic transport theory depending on a scale L which can be associated with the longest Alfven wave, efficiently scattering the beam. The best match to all the three characteristics of the beam is achieved at L$\sim$1 pc. The distance to a possible source of the beam is estimated to be within a few 100pc. Possible approaches to determination of the scale L from the characteristics of the source are discussed. Alternative scenarios of drawing the beam from the galactic CR background are considered. The beam related large scale anisotropic CR component is found to be energy independent which is also consistent with the observations. [Preview Abstract] |
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BP9.00026: Alfven Wave Collisions: The Fundamental Building Block of Plasma Turbulence Gregory Howes, Kevin Nielson, Frederick Skiff, Craig Kletzing Alfv\'en waves play a central role in the dynamics of magnetized plasma turbulence. Theoretical studies suggest that the nonlinear interactions that constitute the turbulence occur only between Alfv\'en waves traveling in opposite directions along the magnetic field. Therefore it is these interactions, often referred to simply as ``collisions'' between counter-propagating Alfv\'en waves, that form the fundamental building blocks of plasma turbulence. Today's modern theories of anisotropic magnetized plasma turbulence have been developed based on this intuitive concept of counter-propagating Alfv\'en wave collisions. We describe here a fundamental study of the properties of these Alfv\'en wave collisions both in the MHD and kinetic Alfv\'en wave regimes, employing both asymptotic analytical solutions of the nonlinear interaction between counter-propagating Alfv\'en waves and supporting gyrokinetic numerical simulations. Intuition from these studies is exploited to support planned laboratory experiments to measure the nonlinear evolution of Alfv\'en wave collisions on the Large Plasma Device (LAPD) at UCLA. [Preview Abstract] |
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BP9.00027: Gyrokinetic Study of Nonlinearly Interacting Pairs of Alfv\'en Waves Kevin Nielson, Gregory Howes Gyrokinetic simulations of Alfv\'en waves play a primary role in turbulent energy transport in magnetized plasmas. MHD theory predicts that the transfer of energy from large to small scales results only from ``collisions'' between Alfv\'en waves propagating oppositely along the mean magnetic field. Within MHD theory, simple predictions can be made about the rate of energy transfer versus amplitude, scale, and wave polarization. Physical plasmas, however, particularly those found in the solar wind, are not always well described by MHD theory. There are, therefore, more complicated effects that may be expected, arising from compressibility and kinetic effects, such as the dispersion of shear Alfv\'en waves. In this work, we model simple systems consisting of individual collisions between pairs of parent Alfv\'en wave modes. By comparing the MHD prediction in the weak interaction limit to simulations performed with the gyrokinetics code \texttt{AstroGK}, we are able to examine the limits of validity of the MHD theory of nonlinear interaction and the nature of nonlinear interactions in regions outside this simple limit. [Preview Abstract] |
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BP9.00028: Hybrid Simulation of Propagating Magnetospheric Alfven Wave in the Earth Polar Area Galina Dudnikov, Lyudmila Vshivkova One of the important problems in space physics is the acceleration of electrons in the aurora region. Recent work indicated that electrons are accelerated by the magnetospheric shear Alfven wave which can potentially travel along the Earth's magnetic field lines and can produce aurora. This paper presents the new hybrid code for simulation of the electron acceleration in the polar area of the Earth magnetic field due to propagation of shear Alfven waves on open magnetic field lines. An ion component of plasma is described by a standard set of the MHD equations and electrons are accounted for via the Vlasov equation. To solve the Vlasov equation a particle-in-cell method is used. The series of 2D simulation are presented with emphasis on electron distribution function evolution. [Preview Abstract] |
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BP9.00029: Electron pitch-angle scattering by magnetic waves A.N. Simakov, J. Daligault, S.P. Gary, D. Lemons, K. Liu, D. Winske Fluxes of relativistic electrons are trapped in the earth's radiation belts and exhausted by loss-cone pitch-angle scattering through interaction with various magnetospheric plasma waves. The high temporal variability of the fluxes is poorly understood and routinely modeled using quasi-linear pitch-angle diffusion theory, which is strictly only applicable for rather low ratios $\epsilon$ of the wave energy to the earth's magnetic field energy. Here, we present a novel electron pitch-angle scattering theory valid for arbitrary $\epsilon$. We concentrate on the simplest case of electromagnetic ion cyclotron (EMIC) waves, approximated with a set of time-independent transverse magnetic fluctuations, and obtain a general integro-differential evolution equation for a pitch-angle distribution $f$. If $f$ evolves weakly on the correlation time scales, the equation reduces to a Fokker-Planck diffusion equation with a {\it time-dependent} diffusion coefficient $D$. Quasi-linear theory is recovered as a first-order truncation of the asymptotic expansion in $\epsilon$ of electron equations of motion and breaks down for $\epsilon \geq 10^{-4}$ [1]. In particular, $D$ changes scaling around this point from $D \propto \epsilon$ to $D \propto \sqrt{\epsilon}$ and is found to be 16 times smaller that the quasi-linear result for $\epsilon = 10^{-2}$ at time $t=30$ electron gyroperiods. [1] K. Liu {\it et al.}, J. Geophys. Res. {\bf 115} A04204 (2010). [Preview Abstract] |
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BP9.00030: Particle acceleration via the Kelvin-Helmholtz instability Eduardo Paulo Alves, Thomas Grismayer, Ricardo Fonseca, Luis Silva Collisionless plasma instabilities are a critical ingredient to understand the acceleration of high-energy particles in extreme astrophysical scenarios such as active galactic nuclei and gamma ray bursters. Since these extreme scenarios are usually associated with strain and~rapid variability of the ejecta, it is likely that strong velocity shears are present, triggering the collisionless Kelvin-Helmholtz instability (KHI). We show that particles may accelerate to high energies by scattering in the evolving electric and magnetic fields of the KHI. Moreover,~we present the relativistic two-fluid model of the KHI and perform a detailed comparison with PIC simulations results, namely growth-rates and length-scales of the instability. We analyze the dependence of the KHI on the gradient length of the shear, observing lower~growth-rates for longer gradient-lengths. We further study the particle energy spectra generated by the instability, where we find evidence of non-thermal particle acceleration. [Preview Abstract] |
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BP9.00031: Comparison of 2-D PIC simulations with 2-D quasilinear calculations of ion-election energy transfer from instabilities driven by counter-streaming ion beams Jaehong Park, Eric G. Blackman, Xianglong Kong, Chuang Ren, Z.-M Sheng Whether an efficient collisonless energy equilibration mechanism exists for an ion-electron plasma, with E$_{i}>$E$_{e}$, is important for understanding collisionless astrophysical shocks and for constraining models of radiatively inefficient accretion flows. Instabilities driven by counter-streaming ion beams is one candidate mechanism. Previously, we compared 2-D PIC simulations of the resulting instabilities with a quasilinear calculation for two separate 1-D limits corresponding to the filamentation and two-stream instabilities, and found qualitative agreement (Park et al., Phys.Plasmas 17, 022901,2010). Here we generalize the quasilinear calculation to include the fully 2-D oblique modes. Compared to PIC simulations, this quasilinear calculation is more computationally efficient and facilitates studying the problem with the actual ion/electron mass ratio. In addition, by comparing the PIC results with the quasilinear predictions, we can assess the relative importance of various nonlinear processes. This work was supported by NSF under Grant PHY-0903797, by DOE under Grant DE-FG02-06ER54879, and by the National Natural Science Foundation of China under Grant No. 10828509. [Preview Abstract] |
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BP9.00032: UHECR Acceleration at Filaments of Cosmological Structure Formation Roald Sagdeev, Mikhail Malkov, Patrick Diamond A mechanism of particle acceleration to $\sim10^{21}eV$ is suggested. It operates in accretion flows around thin DM filaments of cosmic structure formation. The magnetic field is compressed by the flow to become nearly parallel to the filament. Initially, particles $\mathbf{E}\times\mathbf{B}$ drift towards the filament in the azimuthal electric field $\mathbf{E}$. Upon approaching the filament, the particle \emph{drift} changes to a nearly \emph{circular} rotation around the filament, i.e. along the motion electric field. In this ``betatron'' acceleration regime the electrodynamic limit on the particle energy $cp_{max}=eBR$ in an accelerator with the orbit radius $R$ and magnetic field $B$, is reached very rapidly. As soon as $p$ exceeds $p_{max}$, the particle slings out of the filament to the region of a weak (uncompressed) magnetic field and the acceleration is terminated. The mechanism is a re-acceleration that operates on particles with the required initial energy. Particle pre- acceleration is likely to occur in structure formation shocks. Such shocks are efficient proton accelerators to a firm upper limit $\sim10^{19.5}eV$ placed by the catastrophic photo-pion losses. The suggested mechanism, being explosive in its betatron phase, has a potential to overcome the losses and boost protons to $\sim10^{21}eV$. [Preview Abstract] |
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BP9.00033: LABORATORY PLASMA ASTROPHYSICS |
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BP9.00034: The Eagle Nebula on NIF Jave Kane, Amy Cooper, Bruce Remington, Dmitri Ryutov, Vladimir Smalyuk, Marc Pound In one of the eight Science on NIF campaigns, dynamics of molecular clouds such as the Eagle Nebula will be studied in scaled laboratory astrophysics experiments, focusing on new hydrodynamic stabilities of ablation fronts induced by strong directionality of a sustained radiation drive, and on the formation of cometary structures as a model for the famous Eagle Pillars. The NIF Radiation Transport Platform will be adapted to drive a foam target stood off several mm from the halfraum to simulate a molecular cloud illuminated by a distant O-type star, with the drive collimated by an aperture. Pulses of length 20-100 ns generating effective radiation temperatures of 100 eV are being sought. Design of the experiment, theory of the directional radiation instabilities, and supporting astrophysical modeling will be presented. [Preview Abstract] |
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BP9.00035: Measuring Magnetic Fields Using Protons for Characterizing Laser-Driven Collisionless Shocks* N.L. Kugland, H.-S. Park, L. Gargate, C. Plechaty, R. Presura, J.S. Ross, D.D. Ryutov, A. Spitkovsky, B.A. Remington We present progress in the use of proton deflectometry and radiography to study collisionless shocks in experiments at the OMEGA {\&} OMEGA EP laser facilities. Collisionless shocks are important for astrophysical phenomena such as supernova remnants and ultra high-energy cosmic ray acceleration. The shocks will be created by two laser-ablated counter-streaming supersonic CH$_{2}$ plasmas with v = 1000 km/s and n$_{i}$ = 10$^{17}$ -- 10$^{18}$ cm$^{-3}$, sufficient that the ion-ion collisional mean free path is larger than the mm-scale system. Hydrodynamic simulations predict possible regular grad(n) x grad(T) magnetic fields; particle-in-cell simulations predict filaments and turbulent magnetic fields B$_{t}$ that change direction with a correlation length L. Turbulent fields blur the proton beam; for example, 5 MeV protons blur $\approx $ 2 mrad for B$_{t}$ = 20 kG and L = 100 $\mu $m. The particle-in-cell code LSP has been used to generate synthetic proton images and explore the sensitivity of proton diagnostics to the signatures of collisionless shock development. *Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
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BP9.00036: Inter-relation of radiative and transport properties of HED plasmas with small-scale magnetic turbulence B. Keenan, M.V. Medvedev Instabilities and dissipation mechanisms for relativistic beams are of central importance to laboratory plasmas, where they can affect the efficiency of the wake field acceleration and defeat the ignitor scheme. Kinetic streaming instabilities are also dominant processes in astrophysical plasmas, e.g., in relativistic collisionless shocks. It has earlier been proposed that radiation emitted by relativistic electrons, called jitter radiation, during the field generation and its subsequent self-similar evolution and self-organization can deliver wealth of information about the internal structure of ``Weibel turbulence.'' The small-scale fields simultaneously affect the particle transport via pitch-angle diffusion and the radiation production and its spectra. Both effects are related and can be used to diagnose the plasma state. Indeed, the radiation pattern is intimately related to the particle orbits and, thus, to the transport properties of the turbulence. We study such a relation between transport and radiation in sub-Larmor-scale turbulence numerical simulations and analysis. Our results will improve the radiative diagnostic technique of lab and astro HED plasmas. [Preview Abstract] |
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BP9.00037: Spectroscopic Signature of Radiative Shocks Michel Busquet Radiative Shocks (RS) are shock strong enough that x-ray emitted by the compressed region launch a radiative precursor wave in the uncompressed region. RS can be found in SuperNovae atmospheres, accretion shocks, as well as in laboratory ( $>$ 100 J laser,..) created strong shock. We use state-of-the art opacities and emissivities, to analyze spectral x-ray emission of strong shocks, and to study signatures of the onset of radiative precursor. Departure from a pure Marshak wave is found. Tentative of analysis of experiment on a laser driven radiative shock [1] will be presented. \\[4pt] [1] M. Busquet, F. Thais, E. Audit, M. Gonz\'{a}lez, J. Appl. Phys. 107 (2010) 083302 and references therein; ibid, arXiv:1005.1745vl [Preview Abstract] |
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BP9.00038: Comparison of Sub- and Super-Alfv\'{e}nic Laser-Plasma Explosions through Low-Density, Magnetized Helium and Hydrogen Plasmas E.T. Everson, D. Schaeffer, M. Lauter, G. Rennenkampff, A.S. Bondarenko, C.G. Constantin, C. Niemann Recent experiments performed at the University of California at Los Angeles (UCLA) utilized the Large Plasma Device (LAPD) and the Phoenix Laser to drive sub- and super-Alfv\'{e}nic laser-plasma explosions through the uniform, magnetized background plasma of the LAPD. The $30$ J, $5$ ns FWHM Phoenix laser ablated a graphite target to produce a debris plasma that is allowed to expand $>50$ cm and shock the low-density ($1-5\times10^{12}$ cm$^{-3}$), magnetized ($275-600$ G) Helium (or Hydrogen) plasma of the LAPD. An array of seven 3-axis b-dot probes were used to measure the magnetic field compression, expulsion, and fast-diffusion of the diamagnetic cavity formed by the laser-plasma expansion as well as the quasi-parellal launched waves. The diamagnetic cavity structure and influence is studied for various background plasma species (Helium and Hydrogen), magnetic fields, and densities. [Preview Abstract] |
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BP9.00039: Simulations of Magnetic Field Generation in Laser-Produced Blast Waves D. Lamb, M. Fatenejad, G. Gregori, F. Miniati, H.-S. Park, B. Remington, A. Ravasio, M. Koenig, C.D. Murphy Magnetic fields are ubiquitous in the Universe. The origin of these fields and process by which they are amplified are not fully understood, although amplification is thought to involve turbulence. Experiments being conducted at medium-scale laser facilities (such as the LULI laser the Janus laser) can investigate the self-generation of magnetic fields under conditions that resemble astrophysical shocks. In these experiments, two 527~nm, 1.5~ns long laser beams are focused onto a 500~$\mu$m diameter graphite rod producing an explosion and asymmetric blast wave into a Helium filled chamber. A variety of diagnostics measure the velocity, electron density, and show that a large scale magnetic field is produced. We report preliminary hydrodynamic and MHD simulations using FLASH of a simplified version of the experiment. The results provide insights into the origin and generation of the magnetic field. [Preview Abstract] |
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BP9.00040: Development of a Compact RF Pre-Ionization System for an MHD-Driven Jet Experiment Vernon H. Chaplin, Paul M. Bellan, Hannah V. Willett We are studying MHD-driven jets relevant to spheromak formation and to magnetically threaded accretion disks in astrophysics. At present, the jet density and velocity in our experiment are constrained by the requirement that the initial neutral gas density be high enough to achieve plasma breakdown in the applied electric field. This constraint could be overcome by puffing pre-ionized plasma into the chamber instead of neutral gas. We are investigating pre-ionization with a pair of 13.56 MHz class D RF power amplifiers capable of outputting over 3 kW pulsed power each. One RF source is tuned to output a high voltage and initiate breakdown, while the other is tuned to maximize power transfer and sustain the pre-ionized plasma. Helicon waves may be used to efficiently couple RF power to the plasma. The RF amplifiers are mounted on printed circuit boards and powered by AA batteries, allowing them to float at the high voltage of the center electrode of the jet experiment. Characterization of the RF source behavior and spectroscopic measurements of the pre-ionized plasma properties will be presented. [Preview Abstract] |
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BP9.00041: Experiment to Study Alfven Wave Pulses in Plasma Loops Mark Kendall, Paul Bellan Arched plasma-filled twisted magnetic flux tubes are generated at Caltech using pulsed power techniques [1]. The structure and time evolution of these flux tubes exhibit similarities with solar coronal loops, spheromaks, and astrophysical jets. We are now developing a method to excite propagating torsional Alfven wave modes by superposing a $\sim$10kA, $\sim$100ns current pulse upon the $\sim$50kA, 10$\mu$s main discharge current that flows along the $\sim$20cm long, 2cm diameter arched flux tube. To achieve this high power short pulse, a magnetic pulse compression technique based on saturable reactors is employed. A low power prototype has been successfully tested, and design and construction of a full-power device is nearing completion. The final stage of the device utilizes a coaxial water-filled transmission line with ultra-low inductance to attain the final timescale. The water system is additionally de-gassed to reduce bubble formation which otherwise facilitates electrical breakdown between the conductors. The pulse device will be used to investigate interactions between Alfven waves and the larger-scale loop evolution; one goal will be to capture the motion of the propagating wave using high-speed photography capable of resolving the Alfven timescale. \\[4pt] [1] J.F. Hansen, S.K.P. Tripathi, P.M. Bellan, \textit{Phys. Plasmas} \textbf{11}, 6 (2004) [Preview Abstract] |
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BP9.00042: Overview of recent results from the Princeton MRI experiment Erik Spence, Austin Roach, Eric Edlund, Hantao Ji, Christophe Gissinger The magnetorotational instability (MRI) is believed to generate the turbulence in accretion disks needed to explain observationally inferred accretion rates. The Princeton MRI experiment is a Taylor-Couette device used to generate conditions under which the MRI should be unstable, namely a radially decreasing azimuthal velocity profile in a vertical magnetic field. The velocity field of the working fluid, GaInSn, is measured using an ultrasonic Doppler velocimetry system. Though an ideal-Couette profile can almost be attained, through the ability to modify the end-cap ring speeds of the experiment, residual Ekman circulation remains. This secondary circulation moves in the same radial direction as the flow expected from the MRI, complicating the instability's identification. Comparison of radial flows in MRI-stable and MRI-unstable regimes is used to search for the instability's signature. Three dimensional numerical simulations are also compared to experimental data to determine proximity to instability. [Preview Abstract] |
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BP9.00043: Observation of a spiral instability in the Princeton MRI Experiment A.H. Roach, E.J. Spence, C. Gissinger, E.M. Edlund, P. Sloboda, H. Ji The Princeton MRI Experiment is a modified Taylor-Couette device with a GaInSn working fluid used for the study of rotating MHD flows. An Ultrasound Doppler Velocimetry (UDV) system is used to measure the velocity field. It has revealed an instability causing large-amplitude velocity fluctuations when an axial magnetic field is applied to both hydrodynamically stable and hydrodynamically unstable background flow states with the split axial endcaps rotating differentially. The azimuthal velocity has a characteristic spiral mode structure at saturation, with an azimuthal mode number m=1. This instability appears in a region of parameter space distinct from that where the magnetorotational instability is expected to be present. Nonlinear 3D simulations have shown an instability of the Shercliff layer that forms at the split endcaps when a magnetic field is applied, and the resultant azimuthal flow patterns are largely consistent with experimental observations. Work is ongoing to measure the Shercliff layer in the experiment, and to clarify in simulations whether the shear of the azimuthal velocity in the Shercliff layer or the associated poloidal recirculation is the principle free energy source for the instability. Experimental measurements and possible mechanisms for the instability will be presented. This work was supported by DOE contract DE-AC02-09CH11466. [Preview Abstract] |
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BP9.00044: In search of a subcritical transition to turbulence in rotating hydrodynamic flows E.M. Edlund, A.H. Roach, P. Sloboda, E.J. Spence, H. Ji Angular momentum transport in stellar accretion disks is likely governed by the magneto-rotational instability (MRI), an MHD instability which is active even with a very weak magnetic field. In addition to providing a direct path for angular momentum transport, the saturated state of the MRI is also a source of turbulence. Colder, proto-planetary accretion disks should not be subject to the MRI and a hydrodynamic path to turbulence is needed to enhance the frictional forces and the transport of angular momentum. Hydrodynamic experiments in Taylor-Couette devices with controlled boundary conditions have shown that quasi-Keplerian flows are stable with very low levels of fluctuations. Yet there remains the possibility that these prior studies either have not accessed a nonlinear or subcritical transition to turbulence. We report here on recent studies in the Hydrodynamic Turbulence eXperiment (HTX), an order unity aspect ratio Taylor-Couette device at the Princeton Plasma Physics Laboratory, where quasi-Keplerian flows at Reynolds numbers of order $10^6$ are probed with active perturbations to search for a subcritical transition. The role and regulation of secondary circulation in these experiments will be discussed. [Preview Abstract] |
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BP9.00045: Global MHD simulations of the magneto-rotational instability in complex magnetic topologies Bertrand Lefebvre, Amitava Bhattacharjee, Fatima Ebrahimi, Kai Germaschewski The velocity shear-driven magnetorotational instability (MRI) is believed to contribute to turbulence and momentum transport in astrophysical accretion disks. We conduct global three-dimensional MHD simulations of the MRI in a cylindrical geometry. Our focus is on the effects of complex magnetic topologies with both poloidal and toroidal magnetic fields. Previous theoretical ideal MHD studies of laboratory plasmas in the presence of sheared flows have shown that the continuum MHD modes are destabilized by the flows, and overstability is generic. We find that a toroidal magnetic field component transforms the MRI from a purely growing low-frequency mode to an overstable one, and that the fastest growing modes are non-axisymmetric. This modifies the nonlinear evolution compared to cases with simpler topologies. We compare simulations from the NIMROD and MRC codes, present linear and nonlinear results and their implications for astrophysical plasmas as well as the MPCX. [Preview Abstract] |
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BP9.00046: Momentum transport and magnetic field generation by magnetorotational instability in the Hall-MHD regime Fatima Ebrahimi, A. Bhattacharjee, C.B. Forest, B. Lefebvre The two-fluid Hall effect can be important in astrophysical plasmas, such as weakly ionized disks, as well as in laboratory rotating plasmas. We show that the flow-driven Magnetorotational Instability (MRI) and its contribution to the nonlinear dynamics of these plasmas are affected by the Hall term. First it is shown that momentum transport and the saturated level of Maxwell and Reynolds stresses strongly depend on the magnetic Prandtl number and the direction of magnetic field in the Hall-MHD regime. Second, the possibility of magnetic field generation and its role on the MRI saturation are investigated in both MHD and Hall regimes. In an earlier study, we have shown that a large-scale magnetic field is generated through the correlated product of velocity and magnetic field fluctuations (the alpha effect) and causes the MRI mode to saturate. Here, the correlation of current and magnetic field fluctuations (the Hall effect) as a means to generate magnetic field will also be examined. Compressible nonlinear computations are performed in the configuration of the Madison Plasma Couette flow Experiment (MPCX) using the extended MHD code NIMROD. The stresses and dynamo terms are also analytically calculated in the quasilinear regime. Supported by NSF grant PHY0962244. [Preview Abstract] |
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BP9.00047: Plasma Dynamo Experiments David Weisberg, Cami Collins, Noam Katz, John Wallace, Ivan Khalzov, Jonathan Jara-Almonte, Cary Forest The Madison Plasma Dynamo Experiment (MPDX) is under construction to explore the self-excitation processes of a range of astrophysical dynamos. Numerical simulations of von K\'arm\'an flow have shown that a two-vortex flow can produce a dynamo when the magnetic Reynolds number is sufficiently high, which, for a plasma, requires a large, hot, flowing and unmagnetized plasma. This poster discusses experimental plans for von K\'arm\'an flow in MPDX as well as prototype experiments on the Plasma Couette Experiment (PCX). The PCX is a cylindrical plasma experiment currently being used to optimize a multi-cusp magnetic confinement scheme for experiments on the magnetorotational instability. It also provides a platform for prototyping two types of plasma sources (electron cyclotron heating and LaB6 cathode) as well as an ExB stirring mechanism, diagnostics, and future MPDX dynamo scenarios. This poster will review recent findings from PCX involving the fabrication and operation of a new LaB6 electron source and its use in driving Dean flow. While currently attainable densities ($n_e \approx 10^{17}$ m$^{-3}$, using electron cyclotron heating) require Hall MHD in calculating the plasma response to various flow profiles, the new LaB6 electron source may allow high enough densities to place the plasma in a purely MHD regime. Work supported by NSF. [Preview Abstract] |
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BP9.00048: Construction Status of the Madison Plasma Dynamo Experiment John Wallace, Mike Clark, Cami Collins, Noam Katz, Dave Weisberg, Cary Forest Construction of the Madison Plasma Dynamo Experiment (MPDX) is partially complete. This facility will be utilized to create large, un-magnetized, fast flowing, hot plasma for investigating magnetic field self-generation and flow driven MHD instabilities. A 3 meter diameter spherical vacuum chamber lined with a series of high strength samarium cobalt magnets will provide plasma confinement. The plasma will be stirred from the magnetized edge using electrodes to produce JxB flows. Plasma sources will include lanthanum hexaboride cathodes and electron cyclotron heating. This poster will describe the current status of the design and construction of the facility including laboratory infrastructure, cast aluminum vacuum chamber, magnets, stirring electrodes, sources and diagnostics. Construction is being funded by the NSF Major Research Instrumentation program. [Preview Abstract] |
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BP9.00049: Numerical modeling of the Parker instability in a rotating plasma Ivan Khalzov, Ben Brown, Noam Katz, Cary Forest We study numerically the analogue of the Parker (magnetic buoyancy) instability in a rotating plasma screw pinch confined in a bounded cylinder. The goal of the study is to show the possibility of reaching the Parker instability for the plasma parameters achievable in the Madison Plasma Couette Experiment (MPCX). Simulations are performed using the extended magnetohydrodynamic (MHD) code NIMROD for an isothermal compressible plasma model. Both linear and nonlinear regimes of the instability are studied, and the obtained results are compared with analytic results for a slab geometry. It is shown that the effect of plasma rotation in a cylindrical geometry is two-fold: first, centrifugal acceleration acts as analogue of gravity and provides the equilibrium density stratification; second, the presence of Coriolis force results in increase of critical gradient of magnetic field required for the onset of instability. [Preview Abstract] |
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BP9.00050: Viscous Coupling of Momentum from a Magnetized to an Unmagnetized Plasma Noam Katz, Cami Collins, Ivan Khalzov, Ben Brown, Cary Forest In order to drive rotation in the Plasma Couette Experiment (PCX), we must understand how momentum couples from the magnetized edge into the unmagnetized core. PCX uses rings of alternating-polarity permanent magnets to provide edge confinement, resulting in a well-contained, cylindrical and unmagnetized plasma. The rotation is driven at the edge with a J$\times$B torque, and viscous coupling is then required to achieve rotation in the bulk plasma. The plasma viscosity has not been well-measured in previous experiments, but PCX provides an excellent setup for its measurement. I will discuss progress towards explaining and optimizing the PCX velocity profile. This problem involves an anisotropic viscosity tensor, neutral drag, and strong gradients in the magnetic field. [Preview Abstract] |
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BP9.00051: Rotation Profile Measurements in the Plasma Couette Experiment C. Collins, N. Katz, D. Weisberg, J. Wallace, M. Clark, C.B. Forest The goal of the Plasma Couette Experiment (PCX) is to create a differentially rotating plasma with a parameter range (Re$\sim $500, Rm$\sim $200, Pm$\sim $0.1-20) relevant for studying a host of astrophysically motivated processes, including the magnetorotational instability, a mechanism that may account for outward transport of angular momentum in accretion disks. In PCX, plasma is produced by 5 kW of 2.45 GHz electron cyclotron heating power and confined at the edge by a cylindrical, axisymmetric ring cusp magnetic field. To generate rotation, hot filaments are installed between the magnets and biased with respect to cold anodes to drive JxB torque. Taylor-Couette type flow profiles can be generated through biased filament arrays on the inner and outer boundaries. Helium flow speeds of 5 km/s at the edge have been show to viscously couple inward to the bulk, unmagnetized region. Mach probe measurements of the resulting azimuthal velocity profiles will be presented. The velocity profiles in the bulk are determined by viscosity and ion-neutral drag. The dependence of rotation on density, neutral density, and temperature will be discussed. [Preview Abstract] |
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BP9.00052: Overview of recend developments in the Madison Dynamo Experiment Elliot Kaplan, Mike Clark, Kian Rahbarnia, Mark Nornberg, Zane Taylor, Alex Rasmus, Cary Forest, Erik Spence Generation of large scale, organized magnetic fields from MHD processes, {\it i.e} dynamo, has been a long studied phenomenon in astrophysical and laboratory systems. The Madison Dynamo Experiment drives a two vortex flow suggested by numerical simulations to dynamo at a modest Magnetic Reynolds number. Since the beginning of the experiment, sets of baffles have been added to modify the properties of the flow in order to make dynamo conditions more favorable. An equatorial baffle breaks up the largest scale turbulent eddies, and a set of six steering baffles modifies the pitch of the flow to maximize magnetic field generation. The expected and realized effects of these baffles upon the induced field are presented. Furthermore, a new diagnostic has been added to measure, directly and locally, the turbulence driven emfs in the experiment. These local measurements corroborate the emfs inferred from global magnetic measurements. [Preview Abstract] |
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BP9.00053: Construction of vanes for helical pitch flow control in the Madison Dynamo Experiment M.M. Clark, N.Z. Taylor, E.J. Kaplan, A.M. Rasmus, K. Rahbarnia, M.D. Nornberg, J.P. Wallace, C.B. Forest The Madison Dynamo Experiment (MDE) comprises a 1 m diameter spherical chamber that contains liquid sodium flowing under the influence of two counter rotating impellers and vanes close to the vessel wall. MDE seeks to observe a magnetic field grow at the expense of kinetic energy in the liquid sodium flow. The most recent upgrade has been the addition of three vanes symmetrically located around each impeller to control the helical pitch of the flow and thus minimize the critical velocity at which the dynamo onset occurs. Each of the new vane/shaft assemblies can be rotated anywhere in a full circle about the shaft axis and then fixed to the desired position. The design and construction of the so called rotatable vanes will be discussed and illustrated today. [Preview Abstract] |
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BP9.00054: A Visco-Elastic Instability Analog of Magnetorotational Instability Don Huynh, Stanislav Boldyrev We report in further detail a proposed visco-elastic instability that is analogous to the magnetorotational instability. Numerical simulations of a Couette-Taylor flow of a polymer fluid in a narrow gap between two rotating concentric cylinders with a Kepelerian-like velocity profile, where the angular velocity decreases radially outward while the specific angular momentum increases radially outward, shows a visco-elastic instability that cannot possibly be the inertial Rayleigh instability and the purely elastic instability under these considered parameters. It is proposed that this observed instability is analogous to the magnetorotational instability which plays a fundamental role in astrophysical Keplerian accretion disks. [Preview Abstract] |
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BP9.00055: Turbulence and selective decay in the SSX plasma wind tunnel Tim Gray, Michael Brown, Dan Dandurand, Mike Fisher, Ken Flanagan, Darren Weinhold, V Lukin A helical, relaxed plasma state has been observed in a long cylindrical volume. The cylinder has dimensions $L = 1$~m and $R = 0.08$~m. The cylinder is long enough so that the predicted minimum energy state is a close approximation to the infinite cylinder solution. The plasma is injected at $v \ge 50$~km/s by a coaxial magnetized plasma gun located at one end of the cylindrical volume. Typical plasma parameters are $T_i = 25$~eV, $n_e \ge 10^{15}$~cm$^{-3}$, and $B = 0.25$~T. The relaxed state is rapidly attained in 1--2 axial Alfv\'{e}n times after initiation of the plasma. Magnetic data is favorably compared with an analytical model. Magnetic data exhibits broadband fluctuations of the measured axial modes during the formation period. The broadband activity rapidly decays as the energy condenses into the lowest energy mode, which is in agreement to the minimum energy eigenstate of $\nabla \times \vec{B} = \lambda \vec{B}$. While the global structure roughly corresponds to the minimum energy eigenstate for the wind tunnel geometry, the plasma is high beta ($\beta = 0.5$) and does not have a flat $\lambda$ profile. Merging with plasma plumes injected from both ends of the cylinder will be compared to the non-merging plasmas. [Preview Abstract] |
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BP9.00056: Magnetic Structures and Bursty Events in a Current-Carrying Arcade D. Craig, C. Adams, M. Cartolano, M. McMillan We report on new magnetic field observations of the current-carrying arcade produced in the Wheaton Impulsive Reconnection Experiment (WIRX). The experiment is composed of two parallel electrodes linked by a magnetic arcade that is generated by a coil surrounding the electrodes. Fast imaging diagnostics are used to follow rapid changes in the plasma emission. A newly completed array of internal magnetic probes is used to connect these emission features with magnetic structures. In general, regions of higher emission correspond to regions of more intense current density. Under some conditions, bursty events appear which may correspond to reconnection events. ICCD camera images suggest a spontaneous emission of plasma from the arcade during these events. Correlation analysis of magnetic data imply a rapid redistribution of current density and a propagating magnetic disturbance associated with this rapid change. At large discharge current, both emission diagnostics and magnetic probe arrays indicate that the plasma grows to fill a larger volume. Studies are ongoing to identify candidate locations for three dimensional magnetic reconnection in these plasmas. Work supported by U.S.D.O.E. grant DE-FG02-08ER55002. [Preview Abstract] |
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BP9.00057: Plasma Jog Experiments on MRX in Collaboration with MMS team Masaaki Yamada, Jongsoo Yoo, Tim Tharp, Hantao Ji, Eric Lawrence In the Magnetic Reconnection Experiment (MRX), a multi-probe mock-up system is utilized to investigate the fine structure of the diffusion region of the reconnection layer and to identify data signatures which indicate the nearby presence of a reconnection neutral sheet. The reconnection layer is swept through the probe system in controlled speeds of 0.01-0.2 of the Alfv\'{e}n velocity. This situation is very similar to the space measurements in which the current sheet moves with respect to satellites as expected in the Magnetosphere Multi-scale Satellite (MMS) cluster configuration [1]. The main objectives of the proposed joint research are (1) to compare basic properties of the reconnection regions in the neutral sheet of space and laboratory plasmas, (2) to study their roles in the process of magnetic reconnection, and (3) to measure fine scale profiles of the thin electron diffusion layer. A series of the first results from the experimental campaign are presented. \\[4pt] [1] J. Burch, et al., AGU 2005 Fall Meeting. [Preview Abstract] |
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BP9.00058: Observation of the potential well at the neutral sheet and its effects on ion dynamics during magnetic reconnection in MRX Jongsoo Yoo, Masaaki Yamada, Hantao Ji, Seth Dorfman, Eric Lawrence, Timothy Tharp, Clayton Myers Radial floating potential profiles have been obtained through the use of floating potential probes and Langmuir probes. Similar to those seen in numerical simulations and space observations, potential wells develop across the diffusion region during ``Pull'' reconnection in the Magnetic Reconnection Experiment (MRX). The potential well becomes deeper and broader further downstream. The magnitude of the potential drop is 10- 15V at the center and 15-35V downstream. A downstream-pointing electric field of 300-700 V/m is also measured. The magnitude of this potential well is related to the dip in the sum of magnetic pressure and electron pressure, indicating ions are heated by the in-plane electric field. In-plane ion flow profiles are measured by Mach probes to verify ion energy gain from in-plane electric field. [Preview Abstract] |
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BP9.00059: Systematic Study of the Effects of Guide Field on Reconnection Layer Dynamics in MRX Tim Tharp, Masaaki Yamada, Hantao Ji Past results from collisionless simulation and laboratory experiments agree that guide field acts to reduce the reconnection rate, though the precise physical mechanism for this is not apparent. Here, we perform a systematic study of guide field effects on collisionless reconnection in a laboratory plasma. An external guide field has been applied to reconnecting plasmas in MRX. Reconnection rate is observed to decrease with guide field as expected. The quadrupole field, a signature of two-fluid reconnection, is readily identifiable in $B_g=0$ plasmas, and a morphing of this structure is observed as guide field is incrementally applied. The change is largely due to a strong paramagnetic effect, similar to that seen in $O$-point configurations like the RFP, but in this case produced by poloidal currents flowing around the reconnection x-point. An overall anti-symmetric structure in the toroidal field can still be seen with total guide fields as large as $B_g \sim B_0 = 200$ Gauss. These observations are directly compared with numerical simulations performed by the group at UNH, and will be discussed in the context of space and fusion reconnecting plasmas. [Preview Abstract] |
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BP9.00060: Hall Reconnection in Partially Ionized Plasmas in the Magnetic Reconnection Experiment Eric Lawrence, Hantao Ji, Masaaki Yamada, Jongsoo Yoo In many space and astrophysical plasmas, such as the solar chromosphere and protoplanetary disks, the degree of ionization can be quite low; often 1\% or less. In addition, magnetic reconnection is thought to be a fundamental process in these plasmas. The presence of a large neutral atom population has at least two effects relevant to magnetic reconnection. First, electron-neutral collisions enhance resistive dissipation. Second, strong ion-neutral collisions increase effective ion inertia. This may increase the length scales on which fast Hall reconnection is predicted to occur. By using high gas fill pressures in the Magnetic Reconnection Experiment (MRX), we can study reconnection in partially or weakly ionized plasmas ($n_n/n_e = 1--200$). A newly constructed magnetic probe array allows us to make magnetic measurements of the reconnection region with high spatial resolution and large spatial extent. This will allow us to diagnose, for example, the structure of the Hall quadrupole field in these conditions. Langmuir and spectroscopic diagnostics will also provide insight into how neutrals affect the reconnection process. These results will also be discussed in the context of ongoing theoretical work. [Preview Abstract] |
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BP9.00061: Equilibrium and Stability of Solar-Relevant Magnetized Arc Discharges C.E. Myers, M. Yamada, E.E. Lawrence, H. Ji, R.M. Kulsrud, S. Dorfman, J. Yoo, T.D. Tharp The equilibrium and stability properties of partial toroidal arc discharges are studied in the laboratory. These discharges, which are produced between two electrodes in the Magnetic Reconnection Experiment (MRX), have an arched magnetic flux rope topology that is similar to structures in the solar corona. Using internal magnetic probes and a fast framing camera, the discharge equilibria are found to expand and contract in response to radially-directed $\mathbf{J}\times\mathbf{B}$ forces. Preliminary indications of eruptive phenomena are also observed. With regard to stability, the effect of the electrode boundary conditions on the external kink instability is studied in detail. In particular, it is found that changing the boundary conditions at the anode from fixed to free changes both the observed mode structure and the measured stabilization criteria. The ongoing development of enhanced diagnostics will also be discussed. [Preview Abstract] |
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BP9.00062: Phase Diagram for Magnetic Reconnection in Heliophysical, Astrophysical and Laboratory Plasmas and Opportunities for A Next Generation Magnetic Reconnection Experiment H. Ji, M. Yamada, S. Prager, W. Daughton, V. Roytershteyn Recent progress in understanding the physics of magnetic reconnection is conveniently summarized in terms of a phase diagram\footnote{H. Ji \& W. Daughton, submitted to Phys. Plasmas (2011).} which organizes the essential dynamics for a wide variety of applications in heliophysics, laboratory and astrophysics. The two key dimensionless parameters are the Lundquist number and the macrosopic system size in units of the ion sound gyroradius. In addition to the conventional single X-line collisional and collisionless phases, multiple X-line reconnection phases arise due to the presence of the plasmoid instabilities in both collisional and collisionless current sheets. In particular, there exists a unique phase termed \lq\lq multiple X-line hybrid phase" where a hierarchy of collisional islands or plasmoids is terminated by a collisionless current sheet, resulting in a rapid coupling between the macroscopic and kinetic scales and a mixture of collisional and collisionless dynamics. The new phases involving multiple X-lines and collisionless physics may be important for the emerging applications of magnetic reconnection to accelerate charged particles beyond their thermal speeds. A large number of heliophysical and astrophysical plasmas are surveyed and grouped in the phase diagram. Scientific opportunities for a next generation reconnection experiment to explore these new phases, guided by large-scale computation, are discussed in detail. [Preview Abstract] |
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BP9.00063: Laboratory Magnetospheric Plasma Studies in LDX and CTX M. Mauel, M. Davis, D. Garnier, T.M. Roberts, M. Worstell, J. Kesner During the past decade, results from the CTX and LDX laboratory dipole plasma experiments have advanced our understanding of magnetized plasma dynamics and shown the influence of magnetic geometry on turbulent transport and high-beta stability. The CTX and LDX devices operate over a wide range of plasma parameters, allow detailed observations spanning global to small spatial scales, and show dynamics relevant to space weather models. Results include slow and fast plasma convection, centrifugal interchange instability and plasma rotation effects, energetic particle and complex wave-particle dynamics, rapid dipolarization in high-beta plasma, intermittent bursty plasma flows, and fascinating plasma turbulence and transport phenomenon. The unique capabilities of these dipole experiments represent remarkable opportunities for the development and validation of models that help understand turbulent transport in fusion devices and also the magnetospheric dynamics of space weather. We describe upcoming experiments to investigate (i) turbulence control with electrostatic feedback, (ii) whole-plasma imaging of turbulent dynamics, and (iii) nonlinear gyrokinetic simulations of bounded driven dipole plasma. [Preview Abstract] |
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BP9.00064: Estimation of the Polytropic Index of the Thermal Electron Population in the Levitated Dipole Experiment (LDX) M.S. Davis, D.T. Garnier, M.E. Mauel, J.L. Ellsworth, J. Kesner, P.C. Michael, P. Woskov LDX studies plasma confined by the field of a magnetic dipole. The plasma profiles are distinctly different when the superconducting dipole is mechanically supported and when it is magnetically levitated. When supported, profiles are determined by particle losses to the supports, leading to flat density profiles. By contrast, when the dipole is levitated, parallel losses are eliminated, a thermal electron population is formed and turbulent radial ExB driven transport creates ``stationary'' profiles. Interferometry shows that the density profile is centrally peaked during levitation. Equilibrium calculations with the measured density profile and edge temperature imply a centrally peaked temperature profile. The condition for marginal MHD stability, $\delta(pV^{\gamma})=0$, relates the polytropic index of the turbulent transport, $\gamma$, to the peakedness of the pressure profile. We estimate the value of $\gamma$ by developing the magnetic reconstructions and examining both soft X-ray and visible spectra. [Preview Abstract] |
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BP9.00065: Exciting quasi-coherent interchange modes in a dipole-confined plasma M.W. Worstell, M.E. Mauel, T.M. Roberts Research in the Collisionless Terella Experiment (CTX) studies intense, nonlinear interchange dynamics of magnetized plasma confined by a dipole magnetic field. When operating at lower densities fast-electon interchange instabilities appear in quasiperiodic coherent bursts.\footnote{B. Levitt, D. Maslovsky, and M.E. Mauel, {\it Phys. Plasmas} {\bf9} 2507 (2002)} When the neutral pressure is increased, the interchange instabilities transition to a turbulent state consisting of chaotic time- varying modes with broad global mode structures.\footnote{B.A. Grierson, M.W. Worstell, M.E. Mauel, {\it Phys. Plasmas} {\bf16} 055902 (2009)} Transient bursts of radial transport are a consequence of the random dynamics of the large scale convective structures. In this presentation, we describe experiments that modify the spectrum, coherence, and intensity of the interchange fluctuations through the application of electrostatic bias that drives plasma convection. In particular, when static bias is applied to a turbulent plasma in the high-density regime, a quasi- coherent mode appears, which alters the turbulent spectrum. The turbulent spectra and the characteristics of the quasicoherent mode vary as the strength and azimuthal structure of the driven convection change. [Preview Abstract] |
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BP9.00066: Observation and Measurement of Turbulent Radial Transport in a Dipole T.M. Roberts, M.E. Mauel, A.M. Senter, D.P. Singh, M.W. Worstell, A.Z. Qin We present results from measurements of turbulent radial in a mechanically supported dipole. A new diagnostic composed of 16 floating potential probes and 15 ion saturation probes alternately placed on a 90 degree arc. By considering the difference in floating potential between two tips, we measure the local electric field. This measurement gives us a measure of the radial particle flux. We propose an active feedback system which attempts to suppress this turbulent radial transport via applying radial electric fields from a equatorial biasing array, as determined by the reference input from the 31-tip array. We also present results from a previously attempted method for observing bulk plasma motion through the imaging of mono- disperse dust particles. By tracking the particle trajectories, we can get measurements of acceleration, and therefore electric fields, in the interior of our plasma. In combining these diagnostics we hope to achieve a better understanding of turbulent transport in a dipole. [Preview Abstract] |
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BP9.00067: MST AND OTHER REVERSED FIELD PINCHES |
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BP9.00068: 3-D soft-X ray imaging diagnostics for the study of MHD mode dynamics in RELAX Akio Sanpei, Sadao Masamune, Kensuke Oki, Daisuke Fukahori, Kazuaki Deguchi, Seiya Nakaki, Haruhiko Himura, Satoshi Ohdachi, Nobuhiro Nishino, Takumi Onchi In a low-$A$ RFP machine RELAX ($R$ = 0.51 m/$a$ = 0.25 m ($A$ = 2)), a quasi-periodic transition to quasi-single helicity (QSH) state has been observed in shallow-reversal discharge regions. During the QSH state, the fluctuation power is concentrated to the dominant $m$ = 1/$n$ = 4 mode. We have applied a SXR pin-hole camera and a ICCD camera to take tangential soft-X ray (SXR) images (snapshots) of the RFP during the QSH state, identifying characteristic helical SXR structures suggesting hot or dense helical core. As a next step, we have been developing a SXR imaging diagnostic system for 3-D structural studies. The system uses multiple SXR cameras together with high-speed cameras to take time-evolution of SXR images from tangential and vertical directions simultaneously for the study of dynamic structures of 3-D SXR emissivity, through which we expect to discuss 3-D dynamics of MHD instabilities associated with the QSH state in RELAX. Initial results will be reported, together with some discussion on 3-D reconstruction techniques. [Preview Abstract] |
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BP9.00069: Thermal conductivity effects on resistive g-mode stability of the RFP Jan Scheffel, Ahmed Mirza Tearing modes presently dominate fluctuations in the reversed-field pinch (RFP). Using current profile control techniques, the tearing modes can be removed experimentally. Pressure driven resistive g-modes remain for all equilibria, however, according to classical theory. In the tokamak these modes can be eliminated by curvature effects. Resistive g-modes may cause modest global energy confinement and severly limit the reactor potential of the RFP. Work by Bruno et al, where the energy equation has been supplemented by heat conduction terms, appear to show that heat conduction smoothens pressure gradient effects and stabilises resistive g-modes at low beta. On the other hand, fully numerical studies including heat conduction effects as well as experimental work identify resistive g-mode activity. In this work, we present a detailed computational analysis of linear resistive g-mode stability with and without heat conductivity effects. Both traditional delta prime analysis and a fully resistive code, based on the novel Generalized Weighted Residual Method (GWRM), are used. [Preview Abstract] |
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BP9.00070: Overview of the RFX-mod fusion science program Maria Ester Puiatti, Piero Martin RFX-mod is a 2 MA reversed field pinch device (major radius R=2 m, minor radius a=0.457 m) equipped with a system of 192 feedback controlled active coils. This paper describes the recent results of the RFX fusion science program. The 2011 experimental campaign has been dedicated to exploration of RFP confinement in the current range above 1 MA, with particular attention to the optimization of MHD active control and electromagnetic boundary, and to the control of first wall properties and of density profiles. RFX-mod has been operated also as a tokamak, aiming at exploiting and optimizing operation at $q_{edge} \approx $2 with active control of MHD instabilities and resistive wall modes in particular. Results on properties of $q_{edge} \approx $2 tokamak discharges will be presented. [Preview Abstract] |
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BP9.00071: Benchmark of Orbit and NEMATO codes on magnetic topology reconstruction in RFPs G. Ciaccio, M. Veranda, D. Bonfiglio, S. Cappello, G. Spizzo, L. Chac{\'o}n, R.B. White \textsc{Orbit} is a Hamiltonian guiding center code which describes test-particle motion in an electromagnetic field.\footnote{R. B. White and M. S. Chance, Phys. Fluids B \textbf{27} (1984) 2455} In the limit $\rho_{\parallel} \rightarrow 0$, $\rho_{\parallel} = v_{\parallel}/B$ it can be used to trace the magnetic field topology, in a way in all respects similar to simplectic codes. \textsc{NEMATO}\footnote{J. M. Finn, L. Chac\'on, Phys. of Plasma \textbf{12} (2005) 054503} is a field-line tracing code, implemented to integrate solenoidal flows for incompressible fluid dynamics, with automatic volume preservation. In a practical application, the two codes have been used to study the structure of the $q = 0$ island chain which characterizes the RFP edge and its behavior as a function of the reversal parameter $F=B_ {\phi}(a)/\langle B_{\phi} \rangle$. As input for both codes we used the snapshot of a 3D nonlinear MHD visco-resistive simulation (SpeCyl code). The first benchmarking test employs a Hamiltonian (single-mode) magnetic field configuration. Both codes successfully yield field lines which follow flux surfaces in both the $m = 1$ and $m = 0$ cases. The comparison between the codes has been successfully extended to a chaotic magnetic field configuration, including many modes. [Preview Abstract] |
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BP9.00072: ITG turbulence and role of the impurities in the RFP I. Predebon, L. Carraro, S.C. Guo, F. Sattin, S.F. Liu ITG modes have been found to be rarely unstable in reversed-field pinch pure-hydrogen plasmas, close to marginality only in correspondence to the transport barriers arising during single helicity states. We revisit this topic considering more realistic multi-species plasmas, to understand the possible excitement of ITG or impurity-drift instabilities. Furthermore, the back-reaction of microturbulence on the impurity transport is investigated. We present linear and nonlinear gyrokinetic simulations with the codes GS2 and HD7 (integral eigenmode equation solver), alongside with a comparison with gyrofluid 2-species nonlinear simulations. [Preview Abstract] |
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BP9.00073: Kinetic Stabilization of the Resistive Wall Mode in Reversed Field Pinch plasmas Zhirui Wang, Shichong Guo, Yueqiang Liu It is known that, in tokamak, the kinetic effect may change the Resistive Wall Mode (RWM) stability condition and even stabilize the mode in the presence of little or no plasma rotation. In RFPs, previous studies based on MagnetoHydroDynamics pointed out that the stabilization of RWM requires the plasma rotation being in the range of Alfv\'{e}n frequency (around 20{\%} of Alfven velocity). In this work, we present the new results obtained by hybrid code Mars-K, associated with the latest developed potential energy analysis module. It is found that the full stabilization of RWM due to the parallel ion Landau damping (resonance between thermal passing particle and RWM) is revealed in high beta region, where the critical velocity required for the stabilization is predicted to be decreased to the ion acoustic velocity range (e.g. for n$=-6$ mode, around 3{\%} of Alfv\'{e}n velocity is required). Finally, the impact of kinetic effect on the fluid part of plasma potential energy has been be clarified, which can help to further understand the difference between perturbative approach and self-consistent approach in RWM study. [Preview Abstract] |
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BP9.00074: Physics and engineering design of KTX Reverse Field Pinch Wenzhe Mao KTX is a large RFPinch device which is now proposed in the USTC. The maximum plasma current is 1 MA with discharge duration longer than 100 ms. The aspect ratio of KTX is 3.5(R/a=1.4/0.4 m). High temperature plasma will be achieved in KTX with T$_{e}\sim $1 KeV and n$_{e}\sim $2x10$^{19}$ m$^{-3}$. Stainless steel is used as the inner chamber wall for its mechanical strength and the better plasma-wall interaction properties. Particularly, it is also suitable for the intended lithium wall experiment. The thickness of the stainless steel is 6mm corresponding to a time constant of 2ms. In order to guarantee a high plasma quality, a second passive copper shell with a thickness of 1.5mm (t$\sim $20ms) will be attached tightly to, but insulated from, the vacuum vessel to stabilize fast-growing MHD modes, especially in the current ramp-up phase. Moreover this modular shell structure will simplify the planned active MHD control system. The details of the air core Ohmic-heating winding, the toroidal field (TF) winding and the equilibrium field winding are provided. Calculations have been done to minimize the stray field and design an equilibrium state in KTX. Special attention has also been paid to the design of the TF coils to reduce the ripple field and to obtain good diagnostic accessibility. Based on the energy balance principle and the alpha model, the KTX discharge has been simulated. The model also gives the requirements for the different power supply systems. [Preview Abstract] |
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BP9.00075: Electrostatic Mode Locking and Mode Suppression in RFPs and Tokamaks Richard Nebel, John Finn It is possible to lock and amplify m=1 modes from the boundary in an RFP by using electrostatic fields. Furthermore, it is possible to do this without any magnetic field lines penetrating the boundary (i.e. the normal component of the magnetic field vanishes at the boundary). These can result in single-helicity or quasi-single-helicity states which have good flux surfaces. A key to forming these states is to drive the primary unstable RFP mode to large amplitude. For the unstable modes, perturbations from the boundary amplify into the interior. These same ideas can be applied to suppressing modes as well, such as the secondary m=1 modes in RFPs or edge modes in Tokamaks. We derive the required phasings to implement this scheme. We also present a conceptual feedback control scheme for suppressing instabilities. [Preview Abstract] |
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BP9.00076: Control of ideal and resistive magnetohydrodynamic instabilities in reversed field pinches with a resistive wall by sensing three components of B Karl Sassenberg, Andrew S. Richardson, Dylan P. Brennan, John Finn Numerical studies are presented of magnetohydrodynamic instability control through sensing and proportional feedback in Reversed Field Pinches (RFPs) of all three components of the helical magnetic field perturbation {\boldmath $\tilde{B}$}, specifically three control parameters for three measurements. In particular, investigations of the stability of m=1 modes with sensing on the interior of the resistive wall are shown. Furthermore, the effect on mode stability with respect to the three applied control parameters compared to previous work (Phys. Plasmas vol. 17, p. 112511 (2010)) which sensed only two components of {\boldmath $\tilde{B}$} at the wall is discussed. Here the third parameter is applied to the parallel field component. This could potentially lead to improved performance in current day experiments through routine access to the favored quasi-single-helicity states. While this study has focused on RFPs, it may also be possible to achieve comparable performance improvements in future tokamak experiments. [Preview Abstract] |
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BP9.00077: Overview of MST Research J.S. Sarff MST progress in advancing the RFP for (1) fusion plasma confinement with minimal external magnetization, (2) toroidal confinement physics, and (3) basic plasma physics is summarized. New tools and diagnostics are accessing physics barely studied in the RFP. Several diagnostic advances are important for ITER/burning plasma. A 1 MW neutral beam injector operates routinely for fast ion, heating, and transport investigations. Energetic ions are also created spontaneously by tearing mode reconnection, reminiscent of astrophysical plasmas. Classical confinement of impurity ions is measured in reduced-tearing plasmas. Fast ion slowing-down is also classical. Alfven-eigenmode-like activity occurs with NBI, but apparently not TAE. Stellarator-like helical structure appears in the core of high current plasmas, with improved confinement characteristics. FIR interferometry, Thomson scattering, and HIBP diagnostics are beginning to explore microturbulence scales, an opportunity to exploit the RFP's high beta and strong magnetic shear parameter space. A programmable power supply for the toroidal field flexibly explores scenarios from advanced inductive profile control to low current tokamak operation. A 1 MW 5.5 GHz source for electron Bernstein wave injection is nearly complete to investigate heating and current drive in over-dense plasmas. Supported by DOE and NSF. [Preview Abstract] |
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BP9.00078: Pellet fueling of axisymmetric and non-axisymmetric MST plasmas K.J. Caspary, B.E. Chapman, A.F. Almagri, J.K. Anderson, D.J. Den Hartog, J.A. Goetz, J. Ko, S. Kumar, S.T. Limbach, S.P. Oliva, E. Parke, J.A. Reusch, J.S. Sarff, F. Ebrahimi, D.L. Brower, W.X. Ding, L. Lin, S.K. Combs, C.R. Foust Deuterium pellet injection into toroidally axisymmetric MST plasmas with a broadband reduction in magnetic tearing fluctuations and improved confinement has resulted in a total $\beta $ of 26{\%} with a pressure gradient that exceeds the Mercier criterion. The density limit has been exceeded by 50{\%} in 200kA discharges and by 20{\%} in 500kA discharges, with the latter case having a density exceeding 7.5e19 m$^{-3}$. Simulations in toroidal geometry with NIMROD reveal that the plasma is linearly unstable to pressure-driven tearing and interchange modes. Pellets have also been injected into a new class of plasmas in which toroidal axisymmetry is broken by a 3D helical structure in the core. This structure emerges when the innermost-resonant tearing mode grows to large amplitude and dominates the mode spectrum. Pellet injection during growth of this mode can trigger a rapid change in that mode's growth rate. Pellet fueling after the mode has saturated leads to substantial density gradients. Supported by USDoE. [Preview Abstract] |
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BP9.00079: Suppression of Core-Resonanat Mode by Neutral Beam Injection in MST J.K. Anderson, D. Liu, D.J. Den Hartog, C.B. Forest, V.V. Mirnov, M.D. Nornberg, J.S. Sarff, J. Waksman, G. Fiksel , V.I. Davydenko, P. Deichuli, A.A. Ivanov, N. Stupishin The reversed field pinch is characterized by a monotonically decreasing safety factor profile which allows multiple resonant m=1 modes. The amplitude of the innermost mode is substantially reduced during NBI experiments, where up to 1 MW of 25kV H atoms are injected and well-confined near the magnetic axis. The position and toroidal harmonic of the innermost mode can be controlled by imposing boundary conditions on the toroidal magnetic field. In cases where the n=5 mode is resonant, a rapid and robust reduction in mode amplitude (up to 40{\%}) is observed during NBI. In deeper reversed plasmas (q(0) $<$ 0.2), a substantial reduction of the n=6 mode amplitude is observed. Only the innermost mode is affected in either case. Several mechanisms could be responsible for the local change in stability; one candidate is FLR effects of fast ions in the vicinity of an island. A methodical scan of the position of the innermost rational surface (while holding approximately fixed the localized fast ion population) is used to investigate the plausibility of fast ion stabilization in MST. Work supported by USDOE. [Preview Abstract] |
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BP9.00080: Time-resolved measurements of the energetic ion distribution in MST S. Eilerman, A.F. Almagri, J.K. Anderson, D.J. Den Hartog, S.T.A. Kumar, D. Liu, R.M. Magee, M.D. Nornberg, J. Waksman, V.V. Belykh, S.V. Polosatkin, J. Titus, G. Fiksel Magnetic reconnection in MST generates a non-Maxwellian and anisotropic population of energetic ions. Neutral particle analysis shows a fast deuterium ion tail out to the diagnostic limit of 5 keV, and the measured neutron flux indicates that ions with higher energies must exist. A recently installed Advanced Neutral Particle Analyzer (ANPA) is capable of simultaneously measuring hydrogen and deuterium ions with energies up to 30 keV. Hydrogen beam ions are observed up to the nominal beam injection energies, and deuterium ion energies up to 25 keV are observed after magnetic reconnection events. ANPA signal levels are dependent on the background neutral density n$_0$, calculated from D$_{\alpha}$ emission and NENE Monte Carlo computations. Neutron flux measurements, which are less sensitive to n$_0$, are used in conjunction with the ANPA signals to constrain the time behavior of the fast ion distribution. The current ANPA viewport primarily samples ions with high v$_{\bot}$/v; a tangential viewport will be installed to sample ions with high v$_{\parallel}$/v, and Fokker-Planck modeling will be used to reconstruct the energetic ion velocity space. Work supported by USDOE. [Preview Abstract] |
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BP9.00081: Neutral Beam Heating Of Reversed Field Pinch Plasmas in MST J. Waksman, J.K. Anderson, D. Liu, M.D. Nornberg, G. Fiksel, V.I. Davydenko, P. Deiculi, A.A. Ivanov, N. Stupishin Neutral beams are a powerful source of particle heating in tokamak plasmas, but only recently have studies of NBI heating begun in RFP devices. Previous work has shown that while thermal energy and particle transport are strong due to radial magnetic fluctuations, NBI-born fast ions are well confined ($tau_{fi} \gg tau_{e}$). This motivated the installation of a 1 MW, 20 ms tangential neutral beam injection system on MST. The NBI has no measurable effect on the central electron temperature in standard MST plasmas. In improved confinement plasmas, the magnetic fluctuations are transiently reduced, leading to a five-fold increase in thermal confinement. These periods are characterized (without NBI) by an increase in $T_{e}$ from 400 to 800 eV in $\sim5$ ms. In this case, significant NBI heating of nearly 150 eV was measured in the plasma core. To model these data, a 1-D temperature profile evolution model was developed. This model uses ensembled data from non-NBI enhanced confinement discharges to solve for a consistent set of $P_{ohmic}$ and $X_{e}$ profiles versus time. Assuming no additional change in thermal conductivity due to NBI, the measured increase in $T_{e}$ is consistent with classical slowing and heating of the MST plasma. Work supported by the USDOE. [Preview Abstract] |
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BP9.00082: Ion energization during magnetic reconnection in the RFP D.J. Den Hartog, R.M. Magee, S.T.A. Kumar, A.F. Almagri, B.E. Chapman, G. Fiksel, V.V. Mirnov, M.D. Nornberg, E.D. Mezonlin, J.B. Titus In the MST RFP, ions are strongly heated to several times \textit{Te} during impulsive magnetic reconnection events. Three new experimental observations may help distinguish among theoretical explanations. First, spatially localized spectroscopic measurements of impurity C$^{+6}$ ions reveal that the thermal heating is anisotropic, with perpendicular \textit{Ti} always increasing more than parallel \textit{Ti}. Second, measurements of neutral particle energy spectra and neutron flux show the generation of a high-energy tail on the distribution function of the majority ions during reconnection events. Fast ion density is typically a few percent of thermal ion density, and the fast ions have a power-law energy spectrum. The fast ion acceleration mechanism may be distinct from the thermal heating mechanism, although both exhibit characteristics that are clearly dependent on plasma density. Third, spectroscopic measurements of various impurity ions (Al$^{+1}$, Al$^{+2}$, O$^{+1}$, O$^{+2}$, O$^{+3}$ and N$^{+2})$ have been made in the MST edge plasma to investigate the charge and mass dependence of impurity ion heating. [Preview Abstract] |
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BP9.00083: Hollow profile of effective ionic charge in the MST RFP S.T.A. Kumar, D.J. Den Hartog, B.E. Chapman, S. Eilerman, G. Fiksel, M. Nornberg, E. Parke, J. Reusch, D. Craig, L. Lin Densities of various impurity ions ($B^{+5}$, $C^{+6}$, $O^{+8}$, $Al^{+11}$, $Al^{+13}$) are measured in the MST reversed field pinch using a fast, active charge-exchange recombination spectroscopy diagnostic with high spatial and temporal resolution. Measurements are made in improved-confinement deuterium plasmas with $I_p \sim$550 kA, core $n_e \sim1.2\times10^{19}\ \rm{m}^{-3}$, core $T_e \sim$1.5-2 keV and core $T_i \sim$ 1.2 keV. It is found that radial profiles are hollow for these impurity ions. These impurity ion densities are used in combination with a collisional radiative model (with measured radial profiles of electron temperature, electron density and neutral deuterium density) to calculate densities of all other charge states of these impurities in the plasma. The effective ionic charge, $Z_{\rm eff}$, estimated from the impurity densities is $\sim$2 near the magnetic axis and up to $\sim$ 5 at mid radius. Analysis using collisional transport theory shows that the hollow profile could be explained by temperature screening of impurities due to the ion temperature gradient. [Preview Abstract] |
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BP9.00084: Momentum transport studies in the MST RFP M.D. Nornberg, A.F. Almagri, J.K. Anderson, D.J. Den Hartog, D. Liu, J.S. Sarff, J. Waksman, W.X. Ding, L. Lin The self-organization process that shapes the current density profile in the standard RFP plasma simultaneously gives rise to large turbulent stresses that shape the parallel flow profile of the plasma. While outward momentum transport and profile flattening are evident during sawtooth crashes, these stresses could also play a roll in the spontaneous plasma spin-up observed between crashes. Several different experiments have been performed on MST to quantify these effects using a range of diagnostics to measure the flow profile and correlated magnetic, velocity, and density fluctuations. External momentum sources are also employed to manipulate the plasma flow. Biased electrodes are used to drive a pulsed torque at the plasma edge to perform transient momentum transport experiments in standard and improved-confinement plasmas. A recently added 1 MW, tangentially-oriented neutral beam injector provides a new tool to deposit momentum in the core region of plasma. Co-current injection is used to spin up the plasma to measure momentum confinement while counter-current injection is used to balance the spontaneous acceleration to facilitate core stress measurements. [Preview Abstract] |
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BP9.00085: Biased electrode expperiments on MST A.F. Almagri, C.B. Forset, S.T.A. Kumar, J. Laufenberg, M.D. Nornberg, J.S. Sarff, A.H. Seltzman, J.C. Triana, J. Wallace Biased electrodes have been used on MST to study momentum transport. In earlier experiments a single electrode in the outer plasma region increased core toroidal flow from 20 km/s to 45 km/s. Following a fast turn off the toroidal flow relaxes to pre-bias values within 2.5 ms. This is an anomalously fast decay of the flow, similar to the particle and energy confinement times and is related to stochastic transport. A set of three biased electrodes is planed for concurrent use in many experiments. Experiments to explore the limit to the toroidal flow that can be driven with up to three probes. Experiments where the three electrodes are configured to produce a localized sheared-flow in the outer region of the plasma will be Performed in standard and improved confinement plasmas to investigate flow shear effects and momentum transport. A compact dipole created by a spherical NdFeB permanent magnet (0.88 T, 1.5 inches) inserted in the outer plasma while plasma flowing around the magnet at speeds up to 60 km/s to simulate Solar winds in a laboratory setting. Results and observations from these experiments will be reported. [Preview Abstract] |
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BP9.00086: Deep Insertion Probe Measurements on MST J.C. Triana, A.F. Almagri, D.J. Holly, J.R. King, K.J. McCollam, J.S. Sarff, C.R. Sovinec Recent measurements and numerical simulations expose the importance of multiple fluctuation-induced forces and stresses in the self-organization processes of the RFP. Probe measurements in the region $\frac{r}{a} > 0.8$ show that the MHD and Hall dynamo terms $( \langle \tilde{\bf{v}} \times \tilde{\bf{b}} \rangle _ \Vert \mbox{and} \langle \tilde{\bf{j}} \times \tilde{\bf{b}} \rangle _\Vert )$ are both large, but with opposite trends in their radial profiles. Two-fluid NIMROD simulations predict complex radial profiles for these quantities, where one dominates the other in different regions. Fluctuation measurements deeper in the plasma would be valuable, and a magnetic probe for measuring $\langle \tilde{\bf{j}} \times \tilde{\bf{b}} \rangle _\Vert$ is first in development. Aided by MST's new programmable toroidal field power supply, RFP plasmas are reliably produced at low plasma current, allowing probe insertion to $\frac{r}{a} > 0.6$. The plasma parameters (e.g., Lundquist number) are closer to simulation values, making direct comparison with simulation more straightforward. Pseudo-spectral analysis will be used to measure the radial profile of the tearing mode structure, to compare with predictions from NIMROD and DEBS (single-fluid MHD). [Preview Abstract] |
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BP9.00087: Bifurcation to 3D Helical Magnetic Equilibrium in an Axisymmetric Toroidal Device D.L. Brower, W.F. Bergerson, W.X. Ding, L. Lin, B.E. Chapman, J.S. Sarff, F. Auriemma, P. Zanca, P. Innocente, R. Lorenzini, E. Martines, M. Momo, D. Terranova We report the first direct measurement of the internal magnetic field structure associated with a 3D helical equilibrium generated spontaneously in the core of an axisymmetric, magnetically-confined, toroidal plasma. Magnetohydrodynamic equilibrium bifurcation occurs in MST RFP plasmas when the innermost resonant magnetic perturbation grows to large amplitude, reaching up to 8{\%} of the mean field strength. Evolution of the magnetic topology is determined by measuring the Faraday effect, revealing that as the perturbation grows, toroidal symmetry is broken, and a helical equilibrium is established. Computational reconstruction of the magnetic field and electron density profiles based on a 3D topology agrees well with experimental data, providing a better fit than reconstructions based on a standard 2D cylindrical topology. These helical plasmas sometimes exhibit an improvement in electron particle confinement and increased temperature. [Preview Abstract] |
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BP9.00088: Kinetic Stress as a Flow Driver in the MST Reversed Field Pinch W.X. Ding, D.L. Brower, L. Lin, W.F. Bergerson, A. Almagri, D.J. Den Hartog, J.A. Reusch, J.S. Sarff Self-generated or intrinsic parallel flows are routinely observed in the MST RFP where flow parallel to equilibrium magnetic field reverses sign at mid-radius. In the absence of external torque, the intrinsic flow may arise from residual stresses. Kinetic stress, the correlated product of parallel pressure and radial magnetic field fluctuations, has been measured by using a high-speed polarimetry-interferometry diagnostic (for both radial magnetic field and density fluctuations). Away from the sawtooth crash, it is found that the measured kinetic stress has the finite amplitude comparable to the change of flow in the core. This indicates that kinetic stress plays an important role in self-generated flow in high-temperature RFP plasmas. Work supported by US DOE and NSF. [Preview Abstract] |
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BP9.00089: Internal Measurements of Density and Magnetic Fluctuations in MST-RFP L. Lin, W.X. Ding, D.L. Brower, A.F. Almagri, J.K. Anderson, B.E. Chapman, J.J. Koliner, D. Liu, M.D. Nornberg, J.S. Sarff, J. Waksman Internal density and magnetic fluctuations in two types of MST plasmas are measured with a high-speed laser-based interferometry and polarimetry diagnostic. First, we present the first-ever measurement of internal density and magnetic fluctuations associated with fast particle instabilities in a reversed field pinch. The measurements are performed in plasmas with a 1 MW tangential neutral beam, where a fast-particle-induced mode is observed. Profiles of the mode amplitude and phase are resolved by correlating internal measurements with edge magnetic fluctuations. The radial profile of density fluctuation peaks near the core, where fast ions reside. This structure is different from the global tearing mode, which peaks near the edge where the density gradient is large. Second, core density and magnetic fluctuations in high-performance plasmas assisted with pulsed poloidal current drive (PPCD) are measured. It is found that magnetic-fluctuation-induced transport has been significantly reduced, consistent with the improved confinement. [Preview Abstract] |
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BP9.00090: Electron Thermal Transport due to Magnetic Diffusion in the MST RFP J.A. Reusch, J.K. Anderson, D.J. Den Hartog, C.B. Forest, C.P. Kasten, D.D. Schnack, H.D. Stephens Comparison of measurements made in the MST RFP to the results from extensive nonlinear resistive MHD simulations has provided two key observations. First, trapped particles reduce electron thermal diffusion; inclusion of this effect is required for quantitative agreement of simulation to measurement. Second, the structure and evolution of long-wavelength temperature fluctuations measured in MST shows remarkable qualitative similarity to fluctuations appearing in a finite-pressure simulation. These simulations were run at parameters matching those of 400~kA discharges in MST ($S\approx 4\times10^6$). In a zero $\beta$ simulation, the measured $\chi_e$ is compared to the thermal diffusion due to parallel losses along diffusing magnetic field lines, $\chi_{st}=v_\parallel D_{mag}$. Agreement is only found if the reduction in $\chi_{st}$ due to trapped particles is taken into account. In a second simulation, the pressure field was evolved self consistently assuming Ohmic heating and anisotropic thermal conduction. Fluctuations in the simulated temperature are very similar in character and time evolution to temperature fluctuations measured in MST. This includes $m=1$, $n=6$ fluctuations that flatten the temperature profile as well as $m=1$, $n=5$ fluctuations that generate hot island structures near the core shortly after sawtooth crashes. This work supported by the US DOE and NSF. [Preview Abstract] |
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BP9.00091: MST's Programmable Power Supplies: Results and Plans D.J. Holly, B.E. Chapman, K.J. McCollam, J.C. Morin, M.A. Thomas Programmable power supplies for the toroidal field (BT PPS, recently commissioned) and poloidal field (BP PPS, now being designed) will maximize MST's inductive capabilities. The BT PPS gives unprecedented control of MST's toroidal field, and has already produced several key results. For example, it has provided routine reduction of the dominant magnetic fluctuations via inductive current profile control, and it has allowed the first production of tokamak plasmas in MST. These results have been somewhat limited, however, by the present passive control of the poloidal field. The planned BP PPS will augment tremendously MST's inductive control flexibility, and it will maximize MST's plasma current and pulse length. This supply will use IGBT H-bridge modules similar to those in the BT PPS. Groups of three 900-V modules in series will be combined to drive about +/- 80 kA at +/- 2.5 kV into the existing poloidal field transformer, connected at 10:1. The series-triplet topology allows control using seven-level Pulse Width modulation, which provides reduced ripple, noise, and IGBT switching losses. [Preview Abstract] |
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BP9.00092: Electron Bernstein Wave Studies in MST Andrew Seltzman, Jay Anderson, Cary Forest, Paul Nonn, Jason Kauffold, Allan O'Conner, Stephanie Diem The overdense plasma in an RFP prevents electromagnetic waves from propagating past the edge. However use of the electron Bernstein wave (EBW) has the potential to heat and drive current in the plasma. MHD simulations have demonstrated that resistive tearing mode stability is very sensitive to gradients in the edge current density profile, allowing EBW current drive to be potentially stabilizing. The development of the new equipment includes a 5.5GHz klystron driven by a novel switchmode power supply. In preparation for the commissioning of a 1MW heating system which will evaluate the potential use of EBW for current profile control, several experiments of EBW coupling to the MST plasma have been performed. Due to the steep edge density gradient in the RFP, it is possible to efficiently couple to the EBW. The EBW is strongly damped at the electron cyclotron resonance where it couples to the electron gyromotion and alters the electron distribution. Either Fisch-Boozer or Ohkawa current drive mechanisms can be activated to drive off axis current in the plasma. Preliminary experiments will be performed to verify high power coupling and understand heating via observed x-ray emission when compared to Fokker-Plank modeling in CQL3D. Work supported by USDOE. [Preview Abstract] |
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BP9.00093: RF Current Drive and Heating Experiments on MST M.A. Thomas, J.K. Anderson, D.R. Burke, C.B. Forest, J.A. Goetz, E.R. Hendries, M.C. Kaufman, A.H. Seltzman, S.J. Diem, R.W. Harvey Two rf schemes are being studied on the MST reversed field pinch for possible use in current profile control experiments. MHD modeling has shown that externally driven off-axis parallel current can improve stability of the dominant core tearing modes. Coupling experiments at the 100 kW level with lower hybrid (LH) and electron Bernstein waves (EBW) both show soft x-ray emission consistent with rf heating of electrons, and a small driven current in the LH case. Computational work in both cases suggests that, for sufficiently low energetic electron diffusivity, between 2 and 5 MW should drive enough current for mode stabilization. A 1 MW EBW system is under construction, with a compact antenna allowing variable polarization. A decision on higher power LH development will follow tests of a repaired antenna. Status and results of power and coupling tests will be presented. [Preview Abstract] |
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BP9.00094: Magnetic Relaxation with Oscillating Field Current Drive on MST D.R. Stone, A.F. Almagri, G. Fiksel, K.J. McCollam, J.S. Sarff, D.L. Brower, W.X. Ding, L. Lin In oscillating field current drive (OFCD), poloidal and toroidal ac magnetic fields with the same frequency but different phases are inductively applied to the plasma to drive dc plasma current through magnetic relaxation. Measurements of the dynamo mechanisms associated with magnetic relaxation are conducted during OFCD for a variety of phases both to better understand the phase-dependent relaxation dynamics and to aid in optimizing OFCD performance. The fluctuation-induced dynamo $\left\langle {\tilde {v}_e \times \tilde {b}} \right\rangle _{\vert \vert } $ and its constituent Hall dynamo $\frac{\left\langle {\tilde {j}\times \tilde {b}} \right\rangle _{\vert \vert } }{ne}$ are measured in the edge using insertable probes. The fluctuation-induced magnetic helicity flux $<\tilde {\phi }\tilde {B}_r >$ is also measured. All three are enhanced during OFCD by a factor of two relative to standard RFP operation and, as expected, the induced transport of helicity is in the inward radial direction. Probes used include a secondary-emission capacitive probe that was developed to measure electric fields and tested by comparison to Langmuir probe measurements. Measurement of the Hall $\frac{<\tilde {j}\times \tilde {b}>}{ne}$ dynamo in the core using far-infrared interferometry-polarimetry is in progress as well. This work is supported by the US DOE. [Preview Abstract] |
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BP9.00095: MHD phenomena with AC loop voltages in RFP plasmas K.J. McCollam, A.F. Almagri, D.J. Holly, J.S. Sarff, D.R. Stone, J.C. Triana The plasma's MHD response is an important aspect of experiments with applied AC loop voltages. For example, when oscillating-field current drive (OFCD), a type of helicity injection entailing phased AC poloidal and toroidal loop voltages, is applied to RFPs in the MST device with an empirically optimum phase of $\sim\pi/8$ between the two voltages, there is a decrease in magnetic-fluctuation amplitudes. By contrast, for $\pi/2$, which is the phase of maximum helicity injection, additional bursts of magnetic fluctuations are induced, which internal measurements suggest are a linear MHD tearing response to the applied fields. Meanwhile, the AC loop voltages can entrain the normally quasiperiodic background sawtooth cycle in the RFP, triggering these discrete relaxation events to occur only at characteristic times within the OFCD cycle. This effect may involve criteria on the core safety factor and is investigated by equilibrium reconstructions of experiments in which AC fields of different frequencies and amplitudes are applied with a new programmable power supply. Finally, using internal probes, we plan to study the radial penetration of broadband AC fields from the switching of the solid-state programmable supply for possible effects on relaxation and current-profile control. [Preview Abstract] |
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BP9.00096: Validity of atomic models for motional Stark effect diagnostic at low magnetic fields J. Ko, D.J. Den Hartog, K.J. Caspary, E.A. Den Hartog The motional Stark effect diagnostic for the MST reversed field pinch (RFP) deals with Stark spectra generated under low magnetic fields (0.1 to 0.6 T). Therefore, its analysis, in principle, should rely on an atomic model that includes spin-orbit coupling, Zeeman effects, and non-statistical populations of upper states in diagnostic-neutral-beam excitation. Currently, however, no atomic model has been validated to be reliable for these low-field MSE spectra. A recent collisional radiative model [O Marchuk et al, J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 011002] confirms that the observed Stark multiplets deviate from a statistical population, but calculation was done only at high fields ($>$ 1 T). In this work, an operating window of MST RFP plasmas will be explored where a direct spectrum fit (that is, with no atomic model involved) can produce reasonable magnetic field measurements. The Stark intensities as well as the inferred magnetic fields obtained this way will be compared with those from the existing atomic models. [Preview Abstract] |
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BP9.00097: Effects of boronization on reversed field pinch plasmas J.A. Goetz, J. Ko, D.J. Den Hartog, S.T. Limbach, P.J. Weix First successful gaseous boronization during a series of pulsed discharges is reported. Sublimation of o-carborane $(C_{2}B_{10}H_{12})$ combined with pulsed discharge plasmas with a repetition rate of 1 Hz produces a hard boron-containing coating for reversed field pinch (RFP) plasmas in MST. X-ray photoelectron spectroscopy with Ar ion beam etching for aluminum and silicon coupons installed at the plasma boundary shows about 60\% boron concentration in the deposited layer. Both profilometer and scanning electron microscope (SEM) analysis of the silicon coupons imply that the thickness of the B/C coating is about 80 nm. Ellipsometry calibrated with the SEM results yields a refractive index of 2.6 for the film. This high refractive index implies that the coating is hard and has a well-ordered morphology. A reduction in wall recycling has consistently been observed after all boronization sessions. Comparison of the x-ray spectra in standard RFP plasmas before and after boronization indicates a slight decrease in the effective ionic charge. A similar comparison will also be presented for improved confinement RFP plasmas. [Preview Abstract] |
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BP9.00098: High-frequency Electron Temperature Fluctuations in MST E. Parke, D.J. Den Hartog High-frequency electrostatic fluctuations are known to contribute to transport in tokamaks. As confinement improves in RFPs, there is increased motivation to measure electron temperature fluctuations at frequencies above the tearing mode frequencies to determine the importance of electrostatic transport. Because the RFP plasma is overdense, Te cannot be measured using electron cyclotron emission. Instead, on MST we have developed a Thomson scattering diagnostic capable of measurements at high effective frequencies. A large ensemble of two-time-point Te measurements has been collected from many MST discharges. Time separations of the measurements vary from 1.25 to 5 microseconds. This should enable resolution of Te fluctuations over a wide range of frequencies. Analysis of the fluctuation spectrum is underway. This work is supported by the U. S. DOE and NSF. [Preview Abstract] |
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BP9.00099: ABSTRACT WITHDRAWN |
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BP9.00100: Heavy Ion Beam Probe sample volume characteristics in the MST RFP and effects on single and multipoint measurements P.J. Fimognari, D.R. Demers, P.W. Terry Heavy ion beam probe operation in the MST reversed field pinch entails a highly three dimensional beam trajectory. Modeling suggests that the sample volumes are complex functions of the strong magnetic shear in the RFP (which rotates the fan angle of the detected ions), and interaction of the beam with physical structures in the diagnostic beamlines. The orientation, shape, and size of the sample volumes vary across the plasma radius. An electrostatic analyzer with three apertures acquires data from two sample locations simultaneously. Sample volume features affecting single point data influence measurements of fluctuations in density and potential, and those affecting multiple point data influence the inference of electric field and wavenumber. Traditional methods of HIBP data analysis are at times insufficient for this system and, therefore, alternatives are being developed. Additionally, methods to mitigate the effects through changes in diagnostic operating conditions are considered. [Preview Abstract] |
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BP9.00101: Electron Temperature Measurement on MST Using SXR Brightness Meghan McGarry, Paolo Franz, Daniel Den Hartog, John Goetz A new soft x-ray (SXR) tomography diagnostic is being constructed and commissioned on the Madison Symmetric Torus (MST). The diagnostic measures electron temperature using the two-color technique, which takes the ratio of chord-averaged bremsstrahlung brightness in two different spectral bands. Initial measurements in the core plasma using a prototype detector array show an electron temperature around 350eV for a 400 kA standard MST plasma, which is consistent with Thomson scattering temperature measurements. Four detector arrays are under construction and will provide complete SXR tomographic measurements of emissivity. The two-color technique will also be applied to this reconstructed emissivity to create a two-dimensional temperature map. These two temperature measurements will be used in conjunction with SXR topology to study the relationship between long-wavelength magnetic fluctuations and electron temperature evolution. [Preview Abstract] |
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BP9.00102: Initial measurements from the new radial x-ray spectrometer on MST J.D. Lee, A.F. Almagri, B.E. Chapman, J.S. Sarff, D.J. Clayton, R.W. Harvey X-ray spectra in the MST are used to investigate the transport of energetic electrons and to estimate the effective ionic charge, $Z_{\mathrm{eff}}$. The x-ray diagnostic consists of six Amptek XR-100CR detectors, each of which can be placed on any of 17 ports covering $\frac{r}{a}$ values from 0.87 inboard to 0.84 outboard. The detectors are connected to Cremat Gaussian shaping amplifiers with a shaping time of $500\,\mathrm{ns}$. The shaping amplifier output is digitized, and a new code is used to identify the times and amplitudes of the pulses. With this configuration, in the best case, an x-ray spectrum can be generated for time periods of one millisecond or less. Measurements have been taken in quasi-single helicity plasmas over MST's entire range of plasma currents. Work has begun on modeling $D_{r}$ and $Z_{\mathrm{eff}}$ radial profiles using the CQL3D code constrained by measured x-ray spectra. [Preview Abstract] |
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BP9.00103: LASER AND BEAM DRIVEN ACCELERATION |
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BP9.00104: Quasi-matched propagation of an ultrashort and intense laser pulse in a plasma channel Carlo Benedetti, Carl Schroeder, Eric Esarey, Wim Leemans The propagation of an ultrashort and relativistically-intense laser pulse in a preformed parabolic plasma channel is investigated. The nonlinear paraxial wave equation is solved both analytically and numerically. Numerical solutions are obtained using the 2D cylindrical, envelope, ponderomotive, hybrid PIC/fluid code INF{\&}RNO, recently developed at LBNL. For an arbitrary laser pulse profile with a given power for each longitudinal slice (less then the critical power for self-focusing), we determine the laser intensity distribution ensuring matched propagation in the channel, neglecting non-paraxial effects (self-steepening, red-shifting, etc.). Similarly, in the case of a Gaussian pulse profile, we determine the optimal channel depth yielding a quasi-matched laser propagation, including the plasma density modification induced by the laser-pulse. The analytical results obtained for both cases in the weakly-relativistic intensity regime are presented and validated through comparison with numerical simulations. [Preview Abstract] |
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BP9.00105: Coupling laser pulses and low-emittance lepton bunches into 10 GeV laser plasma stages David Bruhwiler, Estelle Cormier-Michel, Ben Cowan, John Cary, Cameron Geddes, Min Chen, Eric Esarey The ponderomotive laser-envelope algorithm of the parallel VORPAL framework [1] is used to simulate 10 GeV scale laser plasma accelerator (LPA) stages in the quasilinear regime. We generalize previous work on coupling intense ultra-short laser pulses into a plasma channel with minimal betatron oscillations [2]. Low-emittance, externally injected electron and positron bunches are considered, using the quasistatic ``space charge'' approach of accelerator tracking codes to reduce numerical noise and the associated artificial emittance growth by orders of magnitude, which also enables correct cancellation of transverse forces due to beam self-fields. \\[4pt] [1] Cowan, Bruhwiler, Cormier-Michel et al., J. Comput. Phys. (2011).\\[0pt] [2] Dimitrov, Giacone, Bruhwiler et al., Phys. Plasmas (2007). [Preview Abstract] |
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BP9.00106: Low noise particle in cell simulations of laser plasma accelerator stages Estelle Cormier-Michel, D.L. Bruhwiler, B.M. Cowan, J.R. Cary, C.G.R. Geddes, E. Esarey, C.B. Schroeder, W.P. Leemans Because of their ultra-high accelerating gradient, laser plasma accelerators (LPA) are contemplated for the next generation of high-energy colliders and light sources. The upcoming BELLA project will explore acceleration of electron bunches to 10 GeV in a meter long plasma. Particle-in-cell (PIC) of experiments get more challenging as applications require lower energy spread and emittance beams, and particle noise artificially increases those quantities. We show that calculating the beam self-fields using a static Poisson solve in the beam frame dramatically reduces noise, allowing for more accurate simulation of the beam evolution. In particular, this method gets correct cancellation of the beam transverse fields, eliminating artificial self-forces usually present in the PIC algorithm. This method is used to simulate high efficiency BELLA relevant LPA stages, where methods such as plasma tapering or high-order laser modes are explored. [Preview Abstract] |
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BP9.00107: Betatron radiation calculation and application for electron beams accelerated in laser plasma accelerators M. Chen, C.G.R. Geddes, G.R. Plateau, E. Esarey, C.B. Schroeder, S.S. Bulanov, C. Benedetti, W.P. Leemans Due to the unique trajectory character of the electrons accelerated in a laser wakefield accelerator (small strength parameter K and small betatron oscillation number N$_{\beta })$, the radiation from these electrons cannot be simply estimated by the usual asymptotic formulas which is used in wiggler radiation devices. A new parallel code named ``Virtual Detector for Synchrotron Radiation'' (VDSR) has been made and used for radiation calculations in laser plasma accelerators. Differences between VDSR calculations and the asymptotic formula are shown. Radiation characteristics of accelerated electron beams from different injection schemes (self-injection and colliding pulse injection) are compared. Comparison of radiation calculations with data are also used to infer electron beam parameters which cannot be obtained from other typical diagnostic methods in experiments, such as beam bunch size and emittance. [Preview Abstract] |
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BP9.00108: Precise charge measurement for laser plasma accelerators Kei Nakamura, Anthony Gonsalves, Chen Lin, Thomas Sokollik, Satomi Shiraishi, Jeroen van Tilborg, Alan Smith, Dave Rodgers, Rick Donahue, Warren Byrne, Wim Leemans A comprehensive study of charge diagnostics was conducted to verify their validity for measuring electron beams produced by laser plasma accelerators (LPAs). The electron energy dependence of a scintillating screen (Lanex Fast) was studied with sub-nanosecond electron beams ranging from 106 MeV to 1522 MeV at the Lawrence Berkeley National Laboratory Advanced Light Source (ALS) synchrotron booster accelerator. Using an integrating current transformer as a calibration reference, the sensitivity of the Lanex Fast was found to decrease by 1{\%} per 100 MeV increase of the energy. By using electron beams from LPA, cross calibrations of the charge were carried out with an integrating current transformer, scintillating screen (Lanex from Kodak), and activation based measurement. The diagnostics agreed within $\sim $8{\%}, showing that they all can provide accurate charge measurements for LPAs provided necessary cares. [Preview Abstract] |
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BP9.00109: Recent OSIRIS simulation results of LWFA using the Lorentz boosted frame method P. Yu, W. Lu, F.S. Tsung, W.B. Mori, J. Vieira, R.A. Fonseca, J.L. Martins, L.O. Silva Simulation of the Laser Wakefield Accelerator (LWFA) in an Lorentz boosted frame, in which the ratio of the plasma length and laser pulse length decreases, provides the potential for significant speed-up to the conventional simulation in the laboratory frame. We present results on using the boosted frame technique to study the self, and external injection of the trailing bunch, and laser guiding in the nonlinear LWFA regimes. Modeling self-injection is challenging due to reduce particle statistics in high gamma frames. Parameter scans, and the preliminary results utilizing the high gamma boosted frames will be presented. Numerical issues encountered in the high gamma simulation will also be discussed. [Preview Abstract] |
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BP9.00110: Full-scale EM-PIC modeling: new developments in the OSIRIS framework Ricardo Fonseca, Paulo Abreu, Frederico Fi\'uza, Joana Martins, Jorge Vieira, Luis Silva, Frank Tsung, Viktor Decyk, Warren Mori The complexity of the phenomena involved in several relevant plasma physics scenarios, where highly nonlinear and kinetic processes dominate, makes purely theoretical descriptions impossible. Further understanding of these scenarios requires detailed numerical modeling, but fully relativistic particle-in-cell codes such as OSIRIS [1] are computationally intensive. We report on the new developments in the OSIRIS framework focusing on performance optimization, new physics models and deployment on new hardware paradigms. We will discuss our implementation of shared memory parallelism, and improvements to the dynamic load balance algorithm for improved scalability of strongly unbalanced physical problem in systems of $\sim $ 0.25 M cores. We will also present our new energy conserving EM-PIC implementation. Finally, we will present our work on deploying the EM-PIC algorithm on state of the art, large scale parallel GPGPU architectures [2]. \\[4pt] [1] R. A. Fonseca et al., LNCS 2331, 342, (2002) \\[0pt] [2] V. K. Decyk, T. V. Singh; Comput. Phys. Commun. 182, 641-648 (2011) [Preview Abstract] |
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BP9.00111: Ion motion in the proton driven plasma wakefield accelerator Jorge Vieira, Nelson Lopes, Carlos Russo, Ricardo Fonseca, Warren Mori, Luis Silva The proton driven plasma wakefield accelerator (PDPWFA) is a novel plasma based accelerator which uses proton bunches to excite large amplitude wakefields. A proposal for a proof-of-principle experiment using the SPS LHC proton bunch at CERN is currently being prepared. The length of the SPS proton bunch is much longer than the plasma wavelength for the typical plasma densities being considered. Thus, in a PDPWFA proof-of-principle experiment, the long proton bunch is self-modulated through the transverse modulation instability, which enhances the amplitude of the accelerating gradients. In this work we explore the role of the ion dynamics in the PDPWFA. We show that the ion motion is driven by a ponderomotive-like force associated with the radial plasma wakefields. Multi-dimensional particle-in-cell simulations confirm the analytical model, and reveal that the ion motion leads to the saturation of the self-modulation instability, thus limiting the accelerating gradients. We show that the ion motion can be avoived by suitably adjusting the plasma density, and by using heavier ion plasmas. [Preview Abstract] |
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BP9.00112: Plasma heating by ultra-short laser pulses creates waveguides suitable for guiding J.M. Dias, Nuno Lemos, J. Berardo, N. Lopes, G. Figueira, F. Fiuza, R.C. Issac, D.A. Jaroszynski, L.O. Silva Several important applications using ultra-short laser pulses require laser guiding over distances greater than the Rayleigh length. Nowadays the most promising guiding schemes are based on thermally driven laser-induced plasma expansion. Until now it was thought that laser pulses with 100s of ps were needed to heat the plasma through inverse Bremsstrahlung. Nevertheless ultra-short intense laser pulses can heat the plasma through the ionization mechanism allowing the generation of plasma channels. This work presents an experimental study using $\sim $60fs and $\sim $400fs laser pulses to characterize the time evolution of expanding plasma columns created with different gases. Simulations show that the dominant effect, which contributes for the initial plasma temperature for plasmas created by ultra-short laser pulses, is associated to the ionization process. Also circular polarized light can contribute for a higher initial plasma temperature. [Preview Abstract] |
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BP9.00113: Numerical simulations towards reaching high transformer ratio in the nonlinear regime of the PWFA Yun Fang, Warren Mori, Chengkun Huang, Weiming An, Patric Muggli We have previously shown initial simulation results from the Quickpic particle in cell (PIC) code that uses the quasi-static approximation indicating that a transformer ratio larger than two can be achieved with a train of one to three electron bunches driving the PWFA interaction into the weakly non-linear regime. Such transformer ratio can be maintained over four betatron wavelengths (or $\sim$2cm). The parameters for the electron bunches are chosen based on the current experiment running in the Brookhaven National Laboratory Accelerator Test Facility where the effects could be demonstrated. Reaching the weakly nonlinear is crucial to insure that the accelerating structure and the transformer ratio are maintained even in the presence of the transverse evolution of the bunch along the plasma caused by the transverse fields. In this presentation, we will investigate the wakefield evolution over very long plasma length (meter scale) and the parameters of a witness bunch following the drive train. [Preview Abstract] |
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BP9.00114: Excitation of plasma wakefields with tailored electron bunches Patric Muggli, Brian Allen, Yun Fang, Vitaly Yakimenko, Mikhail Fedurin, Karl Kusche, Marcus Babzien, Christina Swinson, Robert Malone Exciting plasma wakefields with a train of electron bunches or with a specially tailored bunch rather than with a single, Gaussian, short bunch allows for larger wakefield amplitude, larger transformer ratio and possibly better energy transfer efficiency. Driving wakefields in high-density plasmas (e.g., $>10\times^{16}cm^{-3}$) requires short, closely spaced ($<300\mu m$) bunches. By varying the plasma density over approximately four orders of magnitude, i.e., the frequency of the accelerator by approximately two orders of magnitude, we demonstrate that the resonant excitation can be achieved when the density is tuned such that the relativistic plasma wavelength is equal to the period between the drive bunches with approximately equal charge. Acceleration of a separate drive bunch with finite energy spread is also observed. As expected, maximum energy loss and gain are observed at that resonance. We also devise a method to tailor the charge along the bunch train to demonstrate transformer ratio enhancements. Initial experimental results of the interaction of this new bunch train, as well as of triangular current profile drive bunches will be presented. [Preview Abstract] |
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BP9.00115: Improvements and recent PWFA simulation results with QuickPIC Weiming An, Viktor Decyk, Wei Lu, Chan Joshi, Warren Mori, Chengkun Huang QuickPIC is a 3D parallel quasi-static Particle-In-Cell (PIC) code, which is developed with a PIC framework UPIC. Recently, a new 2D field solver for calculating the plasma response to the drive beam in QuickPIC has been developed. It is based on a new set of Maxwell equations (under the quasi-static approximation) which is using transverse Coulomb gauge. With this new solver, QuickPIC can obtain an accurate solution with only 1 iteration (3 or 4 iterations were needed with the old version). The new 2D field solver is also purely spectral (as compared to the older field solver which uses both finite difference and spectral method), which is not only more accurate. Furthermore, the new solver also reduce the total number of FFT calls, which led to a significant time saving. Comparisons between the results for the old and new solver will be given. In addition, we will show QuickPIC results on modeling two bunch FACET experiments, proton driven PWFA and parameters for a future collider based on PWFA stages. [Preview Abstract] |
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BP9.00116: Electron self-injection in a plasma wakefield accelerator in the strongly nonlinear regime due to inhomogeneous plasma density S.A. Yi, V. Khudik, T.H. Ratliff, G. Shvets We study self-injection into a plasma wakefield accelerator (PWFA) in the blowout (or bubble) regime with an inhomogeneous background plasma density. Using an analytic model and particle-in-cell simulations, we explore an injection mechanism into a PWFA, where a growing bubble causes reduction of the electron Hamiltonian in the co-moving frame, which leads to electron trapping [1]. In contrast to earlier work with steep density gradients, growth of the blowout region is caused by a slow decrease in plasma density along the propagation direction. To demonstrate this trapping mechanism, we generalize an analytic model for the wakefields inside the bubble [2], to derive expressions for the fields outside. With this extended model, we study the trapping of initially quiescent plasma electrons into the growing ultra-relativistic bubble, and show that a return current in the bubble sheath layer plays an important role in determining the trapped electron trajectories. We estimate the plasma density gradients and driver beam parameters required for self-injection, and compare our results with particle-in-cell simulations. This work is supported by the US DOE grants DE-FG02-04ER41321 and DE-FG02-07ER54945. \\[0pt] [1] S. Kalmykov {\it et al}, {\it Phys. Rev. Lett.} {\bf 103}, 135004 (2009).\\[0pt] [2] W. Lu {\it et al}, {\it Phys. Plasmas} {\bf 13}, 056709 (2006). [Preview Abstract] |
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BP9.00117: High energy low divergence electron beams generated with gas mixtures in sub-milimeter length gas cells Jessica Shaw, Navid Vafaei-Najafabadi, Ken Marsh, Chan Joshi Laser wakefield acceleration in underdense plasma has been an area of intense study as a source of high energy monoenergetic electron beams. In this work, we report on the acceleration of electrons with energies on the order of 100 MeV using sub millimeter gas cells with comparable lengths to the source's dephasing length. The gas cell design, used mainly to overcome the density inhomogeneity associated with gas jets, yielded low density homogeneous sub-millimeter length plasmas. A 50 fs, $\sim10$ TW Ti:Sapphire laser was focused with an OAP onto gas cells 300 $\mu$m long. Helium was used as the target gas with N$_2$ impurities added in order to induce ionization trapping of plasma electrons as previously reported [1]. The observed electron beams had divergences as low as 1.9 mrad and an unnormalized emittance as low as $3.7 \times 10^{-3}$ mm mrad. These results are to be presented and techniques to reduce energy spread to be explored.\\[4pt] [1] Pak, A., et. al. PRL, 104, 025003. [Preview Abstract] |
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BP9.00118: Beam quality from self and ionization induced trapping in the nonlinear LWFA regime Asher Davidson, Wei Lu, Chan Joshi, Luis Silva, Joana Martins, Ricardo Fonseca, Warren Mori In plasma based accelerators (LWFA and PWFA), the methods of injecting high quality electron bunches into the accelerating wakefield is of utmost importance for various applications. Understanding how injection occurs in both self and controlled scenarios is therefore important. We present results from high fidelity OSIRIS simulations on the beam quality that can be obtained from self and ionized induced trapping in the nonlinear LWFA regime. We compare trapping thresholds from the simulations to analytical expressions. We also quantify how the beam quality of 1.5-5 GeV beams can be improved through angle and energy selection as well as quantify the slice energy spread and emittance. We also study the effect of ion motion and the axial density profile. Preliminary results on inputting beams from OSIRS into the FEL code GENESIS will be presented. [Preview Abstract] |
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BP9.00119: Laser wakefield acceleration using lasers with longitudinal and transverse frequency chirp V.B. Pathak, J. Vieira, R.A. Fonseca, L.O. Silva We develop an analytical model, supported by multi-dimensional particle-in-cell simulations with OSIRIS, to study the effect of longitudinal and transverse frequency chirp on the laser evolution in the laser wakefield accelerator. On one hand, the longitudinal chirp leads to asymmetric temporal laser profile, and on the other hand, the transverse chirp leads to pulse-front tilt with respect to the laser-propagation direction. In the weakly relativistic regime, positive (negative) longitudinal chirp compresses (stretches) the laser pulse, increasing (decreasing) the peak vector potential and wakefield amplitude. In the blowout regime, longitudinal chirp can relax the self-guiding conditions at the laser front. Consequently, a laser with longitudinal positive chirp leads to higher self-injection rate, and thus to higher self-trapped electrons in comparison to a negatively chirped laser. Tilted-front laser, due to the transverse chirp, excites asymmetric plasma waves, thus causing off-axis injection. Simulations show that the self-injected bunch propagate along the axis of the asymmetric wakefield. Thus, the transverse electron bunch dynamics can be controlled by the transverse frequency chirp. [Preview Abstract] |
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BP9.00120: Controlling nonlinear optical evolution of the laser pulse for dark-current-free electron acceleration in the blowout regime S.Y. Kalmykov, B.A. Shadwick, A. Beck, E. Lefebvre Electron density bubble maintained by radiation pressure guides a relativistically intense laser pulse in a rarefied plasmas and accelerates (self-)injected electrons to GeV-scale energy. Optical evolution of the pulse causes slow variations in the bubble shape and potentials, resulting in self-injection of initially quiescent plasma electrons. Spot size oscillations and pulse self-steepening during self-guiding result in massive continuous injection (dark current), jeopardizing quasi-monoenergetic acceleration [1,2]. Using nonlinear plasma lenses [2], as well as a large negative chirp of the laser pulse frequency [3], mitigate these adverse nonlinear optical effects and stabilize the shape of the bubble, suppressing the polychromatic, low-energy background, enabling production of high quality, GeV-scale energy, nC-charge electron beams.\\[0pt] [1] S. Y. Kalmykov et al., Phys. Plasmas 18, 056704 (2011). [2] S. Y. Kalmykov et al., Plasma Phys. Control. Fusion 53, 014006 (2011); [3] S. Y. Kalmykov et al., Physics of Quasi-Monoenergetic Laser-Plasma Acceleration of Electrons in the Blowout Regime, in Laser Pulses/Book 3, (InTech, Rijeka, Croatia; www. intechweb.org); ISBN 978-953-308-56-9. [Preview Abstract] |
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BP9.00121: Modeling GeV-class single-stage laser-plasma electron acceleration in the blowout regime K. Bunkers, S.Y. Kalmykov, B.A. Shadwick, D.P. Umstadter, A. Beck, E. Lefebvre, B.M. Cowan, D.L. Bruhwiler Electron density bubble maintained by radiation pressure guides the laser pulse in a rarefied plasma and accelerates (self-)injected electrons to GeV-scale energy. The quasistatic nature of the bubble and quasi-paraxial behavior of the pulse allow us to identify the optimal regime, ruling out continuous self-injection, at minimal computational cost using the quasi-static code WAKE with test particles. The most promising regimes are modeled directly, using fully explicit, 3D PIC codes such as VORPAL (with a perfect-dispersion algorithm) or the quasi-cylindrical code CALDER-Circ with a poloidal-mode decomposition of EM fields [1]. Using this computationally effective strategy, we elucidate the physics of electron self-injection and assess the risk factors associated with the nonlinear evolution of the self-guided driver. It is shown that phase self-modulation and self-steepening transform an initially smooth driver into a relativistic piston, which causes rapid expansion of the bubble followed by continuous injection and generation of polychromatic tails in electron spectra [2]. The work is partly supported by the U.S. DoE.\\[0pt] [1] S. Y. Kalmykov et al., NJP 12, 045019 (2010).\\[0pt] [2] S. Y. Kalmykov et al., Phys. Plasmas 18, 056704 (2011). [Preview Abstract] |
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BP9.00122: Numerical methods for laser-plasma interactions J. Paxon Reyes, B.A. Shadwick We have studied numerical methods for solving the fluid equations in a moving window coordinate system. Previously we have solved the full fluid model using explicit methods and we compare these results to new results using the quasistatic approximation (QSA) with full- and reduced-wave equations. The new results were obtained using the Crank-Nicolson method which permits larger time steps due to its unconditional stability and these new codes are orders of magnitude faster, at similar levels of accuracy, than the previous explicit fluid code. Although the QSA models show nearly identical laser evolution as the full fluid model, the plasma response in the QSA models develops a phase error in time. Also, there is only a forward-propagating mode in the reduced-wave equation and the dispersion relation agrees with that of the full-wave equation only to second order; we have found a superluminal window velocity $\beta_t$ that can reconcile the dispersion relation for one particular wave number, producing a nominal improvement in the phase error. The plasma behavior with the full-wave and QSA model is closest to that of the full fluid model, and it is clear that the discrepancy originates with the quasistatic assumption. [Preview Abstract] |
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BP9.00123: Incremental chemical etching of CR-39 detectors for nondispersive proton spectroscopy with high resolution Chao Gong, Sergei Tochitsky, Dan Haberberger, Chan Joshi Experiments on shock wave proton acceleration in a hydrogen gas plasma using multi-terawatt CO$_{2}$ laser have produced $\sim$20MeV proton beams with a narrow energy spread [D.Haberberger et al, Proceedings of PAC2011, New York, Paper TuOBN6]. The laser-accelerated proton beam is detected by a stack of 1 mm thick CR-39 with a 100$\times$100 mm$^{2}$ area. This nondispersive imaging spectrometer, located at 150 mm from the plasma,provided a superb spatial resolution but its spectral resolution was limited due to the 1 mm CR-39 thickness. In order to increase the spectral resolution, the incremental layer etching technique has been developed and tested using a computer control system for proton pits counting and analysis. Using this etching technique we reached spectral resolution $\leq60$KeV per etching step and confirmed the generation of mono-energetic proton beam centered around 20MeV with an energy spread dE/E around 1\%. Results on bulk etching rate and proton related track size evolution as well as limitations of this method will be presented. [Preview Abstract] |
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BP9.00124: Laser-driven shock acceleration of monoenergetic ion beams Elisabetta Boella, Frederico Fiuza, Ricardo A. Fonseca, Luis O. Silva, Dan Haberberger, Sergei Tochitsky, Chao Gong, Warren B. Mori, Chan Joshi Ion acceleration from laser-plasma interactions is a promising approach for compact and bright ion sources. However, the conditions for optimization of the beam quality and energy are not yet fully understood. We show that the use of tailored critical-density targets, with a steep density ramp at the front and an exponential ramp at the back, which can be obtained in realistic experimental conditions, enables the generation of high quality and high energy ion beams accelerated by a laser-driven electrostatic shock. The laser deposits most of its energy in a localized region at critical density, heating the electrons and generating an electrostatic shock. The shock can then reflect most of the ions from the back of the target to high energies before competing accelerating fields (like TNSA) develop significantly, leading to high quality beams. Our PIC simulation results illustrate the possibility of generating high quality proton beams with energies in the required range for medical applications (100-300 MeV) with moderate laser intensities (a$_{0} \quad \sim $ 10). [Preview Abstract] |
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BP9.00125: Medical Implication of Quasi-monoenergetic Proton Generated from Laser Acceleration of Ultra-thin Multi-Ion Foil Tung-Chang Liu, Xi Shao, Jao-Jang Su, Chuan-Sheng Liu, Minqing He, Bengt Eliasson, Roald Sagdeev Recent work by Liu et al. [2011] (presented in this conference) shows that high quality quasi-monoenergetic proton beams can be generated in laser acceleration of an ultra-thin multi-ion, i.e. carbon-proton, foil. The proton acceleration is due to the combination of radiation pressure and heavy-ion Coulomb repulsion. Using a normalized peak laser amplitude of $a_0 = 5$ and a carbon-proton target with 10\% protons, our PIC simulation shows that the resulting quasi-monoenergetic (energy spread $<$ 10\%) proton energy is $\sim$ 70 MeV. To assess the feasibility of laser-proton cancer therapy with such a proton accelerator, simulations are carried out to model the interaction of protons with water and determine the radiation dosage deposition for particle beams produced from the PIC simulation of laser acceleration of multi-ion target. We used the SRIM code to calculate the depth and lateral dose distribution of protons energized by laser radiation pressure. The overall dosage deposition map from the proton beam is derived by superposing the radiation dosage contributed from each particle fed from the PIC simulation. Comparison between the dosage map produced from quasi-monoenergetic protons generated from laser acceleration of single ion and multi-ion targets is also presented. [Preview Abstract] |
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BP9.00126: Laser Radiation Pressure and Shock Acceleration of Quasi-Monoenergetic Protons in Thin Gas Target Min-Qing He, Xi Shao, Chuan-Sheng Liu, Tung-Chang Liu, Jao-Jang Su, Galina Dudnikova, Roald Sagdeev, Zheng-Ming Sheng Recently, there have been increased interests in CO2 laser acceleration of hydrogen gas target for energetic proton generation. We present a scheme of laser thin gas target acceleration for quasi-monoenergetic proton generation. The scheme uses gas target of thickness about several laser wavelengths with spatial density distribution of square-sine shape. In the simulation, a compressed electron/ion layer is formed with enhanced density peak within a region of sub-wavelength scale. The acceleration of proton is a combination of radiation pressure and shock acceleration. During the radiation pressure acceleration of the compressed layer, the protons behind the shock front are also accelerated by the shock electric field. With normalized laser amplitude $\sim $ 5, target thickness = 2.5$\lambda $ and peak density 20n$_{c}$, the proton energy reaches $\sim $15 MeV. We also analyzed the dependence of laser gas target acceleration on the target thickness, density profile and the incident laser energy. [Preview Abstract] |
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BP9.00127: Monoenergetic ion beam production through the generation of ion solitary waves in relativistically transparent plasma with a high intensity circularly polarized laser B.J. Albright, L. Yin, D. Jung, K.J. Bowers, R. Shah, S. Palaniyappan, J.C. Fern\'andez, B.M. Hegelich Experiments at the LANL Trident user facility have yielded quasi-monoenergetic ion beams from the interaction of an ultraintense, circularly polarized laser with a solid density, nm-scale target under conditions of ultrahigh laser pulse contrast [1]. Kinetic modeling shows that after a brief radiation pressure acceleration phase, the plasma turns relativistically transparent and nonlinear ion density spikes propagate across the plasma in a manner that efficiently couples laser energy into ion kinetic energy [2]. Understanding the governing physics is possible with an application of analytic theory, shown to reproduce the features of these solitary waves. This theory will be discussed along with how to optimize energy and degree of monoenergeticity of this novel class of laser-generated ion beams. \\[4pt] [1] D. Jung et al. ``Monoenergetic ion beam generation by driving ion solitary waves with circularly polarized light,'' Phys. Rev. Lett. (submitted). \\[0pt] [2] L. Yin et al., Phys. Plasmas 18, 053103 (2011). [Preview Abstract] |
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BP9.00128: Study of the plasma expansion produced on ultra-thin foil targets with a high intensity and ultrashort laser pulse Semen Gnedyuk, Sylvain Fourmaux, Sebastien Buffechoux, Bruno Albertazzi, Francois Martin, Jean Claude Kieffer INRS-EMT, Universit\'{e} du Qu\'{e}bec, 1650 Lionel Boulet, Varennes J3X 1S2, Qu\'{e}bec, Canada LULI, UMR 7605, CNRS - CEA - Universit\'{e} Paris 6 - Ecole Polytechnique, Palaiseau, France Abstract: A high intensity ultrashort laser pulse, with an intensity of the order of 10$^{19}$ W/cm$^{2}$, focused onto a thin foil target generates a plasma and highly energetic ion (including proton) beams from its front and rear sides which propagate along the target normal. Another interest of laser plasma interaction with ultra-thin foil is the possibility to deposit energy in the entire laser absorption depth before any expansion thus enabling target isochoric heating. With a target thickness of 30 or 15 nm the laser pulse should interact in volume and enable to reach very high temperature while the target is still at solid density. The resulting cooling of the target will then be ultra-fast and potential X-ray emission should be ultrashort. The 100 TW class laser system at the Advanced Laser Light Source facility enables laser plasma interaction study with femtosecond laser pulses, ultra thin foil targets and high contrast laser pulse intensity ratio. We used a shadowgraph diagnostic with a femtosecond laser probe to characterize the plasma expansion. [Preview Abstract] |
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BP9.00129: Conversion Efficiency Enhancement for Laser Generated Protons in Reduced Mass Targets Alessio Morace, Teresa Bartal, Louise Willingale, Joohwan Kim, Anatoly Maksimchuk, Karl Krushelnick, Mingsheng Wei, Bhooshan Paradkar, Dimitri Batani, Nicola Piovella, Richard Stephens, Farhat Beg We demonstrate experimentally that minimizing the area and maximizing the isolation of proton beam sources, can increase the efficiency with which they convert energy into protons. The experiment was performed on the Tcubed laser facility at the University of Michigan. The hybrid Ti:Sapphire/ Nd-glass laser delivers up to 5 J on target in 400 fs, with a peak intensity of 2x10$^{19}$ W/cm$^{2}$. Micro machined Cu foils, 10 $\mu $m thick, were used as reduced mass targets. These 150 $\mu $m x 150 $\mu $m Cu targets, were connected to the supporting mount foil by identical legs at their corners, of the same thickness and 3 varying widths: 21$\mu $m, 42 $\mu $m and 84 $\mu $m. Detailed experimental data and simulations will be presented. The work was performed under the auspices of the U.S. Department of Energy under contract DE-SC0001265. [Preview Abstract] |
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BP9.00130: Radiation Pressure Dominant Acceleration: Polarization and Radiation Reaction Effects in 3D PIC Simulations M. Tamburini, T.V. Liseykina, F. Pegoraro, A. Macchi Polarization and Radiation Reaction (RR) effects in the interaction of a superintense laser pulse ($I > 10^{23} W/cm^2$) with a thin plasma foil are investigated with three dimensional Particle-In-Cell (PIC) simulations. For a linearly polarized laser pulse, strong anisotropies such as the formation of two high-energy clumps in the plane perpendicular to the propagation direction and significant radiation reactions effects are observed. On the contrary, neither anisotropies nor significant radiation reaction effects are observed for circularly polarized laser pulses. In both cases, the deformation of the initially flat plasma foil leads to the self-making of a quasi-parabolic shell that focuses the impinging laser pulse to an intensity up to over nine times the initial peak intensity. [Preview Abstract] |
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BP9.00131: Self-proton/ion radiography from high-intensity laser interactions with thin foil targets Y. Paudel, A.Ya. Faenov, Ph. Nicolai, E. d'Humieres, V.L. Kantsyrev, A.S. Safronova, I. Shrestha, M.E. Weller, G.C. Osborne, V.V. Shlyaptseva, N. Renard-Le Galloudec Protons and multicharged ions generated from high-intensity laser interactions with thin foil targets have been studied at Nevada Terawatt Facility (NTF). Protons/ions with energies up to 10 MeV/u are accelerated either from front or rear surface of the target material. We have observed the proton/ion accelerated from the front surface of the target, opposite to the laser propagation direction, are pulled back to the rear surface, towards laser propagation direction, by the self-generated magnetic field. This proton/ion beam is able to create a radiograph of target and glass stalk holding the target itself on the RCF. Details as a function of material and target thicknesses will be presented and discussed. [Preview Abstract] |
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BP9.00132: ABSTRACT WITHDRAWN |
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BP9.00133: Directional Neutron Beams Using High-Intensity Ultrashort Laser Pulses A. Maksimchuk, F. Dollar, L. Willingale, G.M. Petrov, V. Chvykov, G. Kalinchenko, V. Yanovsky, C. Zulick, J. Davis, A. Thomas, K. Krushelnick Neutron production using high energy protons or deuterons from p-Li or d-Li reactions are superior in terms of the number and directionality to that from d-d reactions [1]. These schemes require a pitcher-catcher target method instead of using laser-driven fusion neutron production d(d,n)$^{3}$He from bulk deuterated plastic targets. The experiments performed with 400 fs T-cubed laser focused to maximum intensities of up to 3.10$^{19}$ Wcm$^{2}$ onto the bulk deuterated plastic targets produced neutrons beamed preferentially in the laser propagation direction with a flux of 2.10$^{4}$ neutrons/J/steradian [2]. In recent experiments at the Hercules facility with 30 fs laser pulses focused to intensity of 2.10$^{21}$ Wcm$^{2 }$on thin CH plastic targets very high neutron yield of $\sim $10$^{8}$ neutrons/J/steradian was produced from (p,n) reactions in the LiF catcher target. Neutron energies peaked at 4 MeV were determined through TOF using fast PMT with a large area plastic scintillator. This work was supported by DTRA and the NRL. \\[4pt] [1] J. Davis et al., PPCF 52, 045015 (2010). \\[0pt] [2] L Willingale et al., POP (2011) (submitted). [Preview Abstract] |
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BP9.00134: LSP simulations of fast deuteron generation from CD$_{2}$ foils by high-intensity laser pulses Bin Qiao, D.P. Higginson, R.B. Stephens, G. Petrov, M.S. Wei, F.N. Beg High-energy, high-flux neutron sources have been extensively used in many areas, such as crystallography, radiography, detection of nuclear material and probing of material properties. Previous studies of nuclear reactions through either the D(d,n)$^{3}$He or $^{7}$Li(p,n)Be$^{7}$ reactions have encountered difficulties in producing neutrons with energies $>$ 10 MeV, required for some of the above applications. A recent novel approach [1,2] that uses the $^{7}$Li(d,n)Be$^{8}$ reaction has the advantage of producing energetic neutron beams ($>$10 MeV) with a short pulse laser produced deuteron beam. In this meeting, we report LSP simulation results on the production of fast deuterons from CD2 foils by high-intensity laser pulses. The properties of the deuteron beam, laser to deuteron conversion efficiency, and impact of hydrocarbon contaminants on the fast deuteron acceleration will be discussed.\\[4pt] [1] J. Davis et al., Plasma Phys. Control. Fus. 52, 045015 (2010).\\[0pt] [2] D. P. Higgingson et al., submitted to Phys. Plasmas (2011). [Preview Abstract] |
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BP9.00135: Hot Electron generation from Laser and High-Z targets interaction Xin Wang, Edison Liang We did two dimensional particle-in-cell (PIC) simulation about laser and High-Z target interaction, as a comparison of experiments on Texas Petawatt Laser (TPL). We used High-Z target as gold, and Aluminum for comparison, then diagnosed the plasma characters in various conditions. We found that pre-pulse, initial electron density, target thickness and main pulse duration affected the hot electron spectrum. Combine with the experiments, we look forward to find out the optimization to generate above 1MeV high density electron bunch, even to find out a new mechanism. [Preview Abstract] |
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BP9.00136: Observation of highly intense THz radiation from relativistic solid plasmas Sven Herzer, Amrutha Gopal, Albrecht Schmidt, Andreas Reinhard, Wolfgang Ziegler, Gerhard Paulus, Torsten May, Hans Georg Meyer We present the first study of THz generation during relativistic laser solid interaction. THz pulses of few micro joules were detected at intensities of 10$^{19}$ W/cm$^{2}$. The observed radiation has highly non-collinear emission direction. The spectral distribution was studied using a set of bandpass filters. A correlation between the target properties, the ion spectra and THz emission is presented. [Preview Abstract] |
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BP9.00137: Focusing Betatron Radiation Produced by Laser Wakefield Accelerated Electrons with a Spherically Curved Crystal M. Vargas, W. Schumaker, F. Dollar, V. Chvykov, G. Kalintchenko, V. Yanovsky, A. Maksimchuk, K. Krushelnick, A.G.R. Thomas Laser Wakefield Acceleration in the bubble regime can be used to accelerate electrons to GeV energies while simultaneously wiggling them to produce a synchotron like x-ray radiation. Using HERCULES, a 100TW TiSapphire laser, 30fs pulses are focused onto a 5mm He gas jet to accelerate electrons in the bubble regime. The betatron x-rays produced by the transverse motion of the accelerated electrons are focused onto a detector by a spherically curved quartz, and other crystals. This result shows the feasibility of dynamic studies of crystal diffraction, with femtosecond level accuracy, using pump probe techniques. [Preview Abstract] |
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BP9.00138: Controlling the Betatron Oscillations of Accelerated Electron Beams by Temporally-Asymmetric Laser Pulses in LWFA Inhyuk Nam, Min Sup Hur, Han Sup Uhm, Nasr A.M. Hafz, Hyyong Suk We investigated the betatron oscillations of accelerated electron beams in laser wakefield acceleration by temporally-asymmetric laser pulses via two-dimensional particle-in-cell simulations. By using an asymmetric laser pulse having sharp rising and slow falling time scales, the accelerated electron beam can interact directly with the falling part of the laser field and the electrons will have transverse oscillations due to the phase-slip with the laser field. This oscillation can be matched with the betatron oscillation by the focusing force of the ions, which results in large transverse oscillation amplitude due to the resonance between two frequencies. Furthermore, the electron beam can be micro-bunched at the laser wavelength, which may provide the possibility for generation of a coherent synchrotron radiation. In this presentation, details of the phenomena are shown. [Preview Abstract] |
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