Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session BP8: Poster Session I: Laser and Beam Driven Acceleration/Radiation; ICF I and Laser Plasma Interactions; Shape Control, Diagnostics, Reactor Design; Astrophysical Plasmas: Experiment and Theory |
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Room: Rosen Centre Hotel Grand Ballroom, 9:30am - 12:30pm |
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BP8.00001: LASER AND BEAM DRIVEN ACCELERATION/RADIATION |
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BP8.00002: Stability of monoenergetic electron beam generation in laser-driven plasma acceleration Eisuke Miura, Shin-ichi Masuda, Kazuyoshi Koyama, Susumu Kato We have so far reported the generation of monoenergetic electron beams with the energy up to 25 MeV by using a 2-5 TW, 50 fs laser pulse in laser-driven plasma acceleration.[1,2] In this paper, we will present the generation of more intense monoenergetic electron beams with higher energy. From a plasma with the electron density of 1.6 x10$^{19}$ cm$^{-3}$ produced by an 8 TW, 50 fs laser pulse, monoenergetic electron beams with the energy of 40-70 MeV and considerable amount of the charge up to 70 pC have been obtained. The shot-to-shot fluctuations of the energy, energy spread and charge of the monoenergetic beams were also evaluated. To improve the quality and stability of monoenergetic electron beams obtained by self-injection scheme, the optimum conditions including the plasma density, laser pulse duration, focusing geometry, and so on are investigated. A part of this work is supported by the Budget for Nuclear Research of the MEXT. \newline [1] E. Miura et al., Appl. Phys. Lett. 86 251501(2005). \newline [2] S. Masuda et al., Phys. Plasmas 14 023103 (2007). [Preview Abstract] |
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BP8.00003: Guiding of an intense, ultrashort laser pulse in a discharge-produced capillary plasma for electronacceleration application Takeshi Higashiguchi, Masafumi Hikida, Hiromitsu Terauchi, Kun Li, Noboru Yugami, Ryosuke Kodama Guiding of an intense laser pulse is supported today's advanced technology such as laser wakefield acceleration, x-ray lasers, high-order-harmonic generation, and inverse Compton scattering. The laser-matter interaction length of a focused laser pulse is fundamentally limited by diffraction to the order of the Rayleigh length, and is further restricted by ionization-induced refraction. We developed a plasma waveguide for propagating intense laser pulse by use of a capillary discharge plasma. The alumina capillary had a diameter of 300 $\mu$m and a length of 10 mm. For present work, the discharge peak voltage and current were 30 kV and 500 A with a pulse width of 100 ns (FWHM), respectively. The guiding experiments used the laser pulse of the central wavelength of 800 nm from a CPA Ti:sapphire laser with a pulse width of 130 fs (FWHM). A peak intensity of the laser pulse was $1 \times 10^{16}$ W/cm$^2$ with a spot diameter of 30 $\mu$m (FWHM) in vacuum. We demonstrated guiding of a laser pulse over length of up to 10 mm, which corresponded to 10 times the measured Rayleigh length. In addition, we also observed an electron acceleration of around 1 MeV by use of a 1- cm long gas-filled capillary waveguide at the discharge time of 150 ns. [Preview Abstract] |
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BP8.00004: Absolute Calibration of Imaging Plate for Electron Spectrometer measuring GeV-class electrons. Nobuhiko Nakanii, Kiminori Kondo, Toshinori Yabuuchi, Kazuki Tsuji, Kazuo Tanaka, Shinsuke Suzuki, Takao Asaka, Kenichi Yanagida, Hirohumi Hanaki, Takashi Kobayashi, Kazuhiro Makino, Takahisa Yamane An electron spectrometer (ESM) is designed and tested to measure high-energy electrons generated from ultra-intense laser plasma interactions. In this ESM, Fuji film imaging plate (IP) is often used as a detector for avoiding the influence by a strong electromagnetic noise. In previous study, IP was calibrated for electron up to 100-MeV in order to obtained absolute number of high-energetic electron. However, in more recent laser acceleration study 1-GeV monoenergetic electron beam was produced. Therefore, we performed the absolute calibration of IP for 1-GeV electrons experimentally at SPring-8. The 1-GeV electron beam was generated from Linac for injection to the storage ring at SPring-8. In the result, it has been proved that IP has sufficient sensitivity for 1-GeV electrons and the absolute sensitivity curve for electrons up to 1 GeV was obtained. [Preview Abstract] |
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BP8.00005: PIC Simulations and Analysis of CTR from Electron Bunches Exiting a Plasma Paul Mullowney, David Bruhwiler, Christine Roark, Bill Peter, Cameron Geddes, Eric Esarey, Wim Leemans, Guillerme Plateau Laser wakefield accelerator (LWFA) concepts are characterized by ultra-high gradients and ultra-short bunch lengths. These short, nano-Coulomb charge bunches can radiate strongly at THz frequencies via coherent transition radiation (CTR) as they exit the plasma. Accurate modeling of the CTR in simulations is challenging even for moderately energetic bunches of 5 MeV due to constraints imposed by the formation length, L, which scales as the inverse fourth power of the angle from the bunch propagation direction to the observer's position. If the EM fields on a virtual surface near to the plasma are used to calculate the far CTR radiation field, complications arise from the self-fields of the high-charge bunch. We present results and analysis of PIC simulations of characteristic electron bunches exiting a plasma. We show that by treating the surface currents as dipole-radiators, we can compute the CTR in the far-field. These techniques are being used to study the effect of plasma density ramps and other complicating factors. [Preview Abstract] |
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BP8.00006: Optimization of the LBNL Laser Wakefield Accelerator as a Compact, Powerful Terahertz Source Guillaume Plateau, Nicholas Matlis, Jeroen van Tilborg, Kei Nakamura, Cameron Geddes, Csaba Toth, Carl Schroeder, Eric Esarey, Wim Leemans At LBNL, laser wakefield accelerators (LWFA) routinely produce ultrashort electron bunches with energies up to 1 GeV [1]. As femtosecond electron bunches exit the plasma they radiate a strong burst in the terahertz range [2,3] via coherent transition radiation (CTR). Measuring the CTR properties allows non-invasive bunch-length diagnostics [4], a key to continuing rapid advance in LWFA technology. We present measurements demonstrating both the shot-to-shot stability of bunch parameters, and femtosecond synchronization between the bunch, the THz pulse, and the laser beam. In addition we present a technique for enhancing CTR generation from LWFA-produced electron beams, increasing its suitability for applications. \newline [1] W.P. Leemans et al., Nature Physics 2, 696 (2006); \newline [2] W.P. Leemans et al., PRL 91, 074802 (2003); \newline [3] C.B. Schroeder et al., PRE 69, 016501 (2004); \newline [4] J. van Tilborg et al., PRL 96, 014801 (2006) [Preview Abstract] |
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BP8.00007: Controlled laser plasma wakefield acceleration of electrons via colliding pulse injection in non-collinear geometry Csaba Toth, Kei Nakamura, Cameron Geddes, Dmitriy Panasenko, Guillaume Plateau, Nicholas Matlis, Carl Schroeder, Eric Esarey, Wim Leemans Colliding laser pulses [1] have been proposed as a method for controlling injection of electrons into a laser wakefield accelerator (LWFA) and hence producing high quality electron beams with energy spread below 1{\%} and normalized emittances $<$ 1 micron. The. One pulse excites a plasma wake, and a collinear pulse following behind it collides with a counterpropagating pulse forming a beat pattern that boosts background electrons into accelerating phase. A variation of the original method uses only two laser pulses [2] which may be non-collinear. The first pulse drives the wake, and beating of the trailing edge of this pulse with the colliding pulse injects electrons. Non-collinear injection avoids optical elements on the electron beam path (avoiding emittance growth). We report on progress of non-collinear experiments at LBNL, using the Ti:Sapphire laser at the LOASIS facility of LBNL. New results indicate that the electron beam properties are affected by the presence of the second beam. \newline [1] E. Esarey, et al, Phys. Rev. Lett 79, 2682 (1997) \newline [2] G. Fubiani, Phys. Rev. E 70, 016402 (2004) [Preview Abstract] |
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BP8.00008: Trapping and heating with colliding laser pulses Eric Esarey, C.B. Schroeder, E. Cormier-Michel, C.G.R. Geddes, W.P. Leemans, D. Bruhwiler, J.R. Cary Colliding pulse injection (CPI), in which two counterpropagating laser pulses intersect in a plasma, has been proposed as a method of injecting short electron bunches into a laser wakefield accelerator.\footnote{E. Esarey et al., Phys. Rev. Lett. 79, 2682 (1997); G. Fubiani et al., Phys. Rev. E 70, 016402 (2004).} When the two laser pulses overlap, the laser beat wave alters the momenta and phases of the electrons, allowing trapping in the wakefield. Recent experiments have demonstrated that CPI is capable of producing energetic electron bunches.\footnote{J. Faure et al., Nature 444, 737 (2006).} Here, theory and simulations of the beat wave injection process are presented, allowing the calculation of the bunch properties such as charge, energy, and duration. Methods for enhancing the amount of trapped charge are proposed and analyzed. [Preview Abstract] |
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BP8.00009: Optimization Studies of a Multi-GeV Single Stage Laser-Plasma Accelerator G.M. Tarkenton, B.A. Shadwick, C.B. Schroeder, E. Esarey Using a self-consistent Hamiltonian model of beam transport in a background plasma\footnote{B. A. Shadwick, G. M. Tarkenton and C. B. Schroeder, Bull. Am. Phys. Soc., \textbf{50}, 283 (2005).}, we consider the design of a single stage, multi-GeV plasma accelerator. In this model the beam is described by phase-space moments and the bulk plasma is taken to be a cold fluid. We present a detailed study of beam propagation in a resonant laser-wakefield accelerator with final energy gain between 5 and 10 GeV. We discuss optimization of the system with regard to energy gain and beam quality. [Preview Abstract] |
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BP8.00010: Reduced Phase-Space Models of Intense Laser-Plasma Interactions B.A. Shadwick, C.B. Schroeder, G.M. Tarkenton, E. Esarey We undertake a detailed comparison of a variety of reduced models --- moment based descriptions: warm\footnote{B. A. Shadwick, G. M. Tarkenton and E. H. Esarey, Phys.\ Rev.\ Lett.\textbf{93}, 175002 (2004).} and cold fluids as well as fixed-shape distributions: water bag, \textit{etc.} --- to direct solutions of 1-D Vlasov equation\footnote{B.~A. Shadwick, G.~M. Tarkenton, E. Esarey, and C.~B. Schroeder, ``Fluid and Vlasov Models of Low-Temperature, Collisionless, Relativistic Plasma Interactions,'' Physics of Plasmas {\bf 12}, 056710 (2005).}. We examine the quality of the agreement between the various models as a function of both initial plasma temperature and plasma wave amplitude. We determine parameter regimes of validity for the various reduced models and comment on applicability of these models to studying laser-driven plasma accelerators. [Preview Abstract] |
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BP8.00011: Simulation of 1 GeV laser wakefield accelerator experiments and scaling to 10 GeV Estelle Cormier-Michel, C.G.R. Geddes, W.A. Isaacs, N. Stinus, E. Esarey, C.B. Schroeder, W.P. Leemans, D.L. Bruhwiler, J.R. Cary Recent laser-plasma accelerator experiments at LBNL have demonstrated the production of high quality 0.5 and 1.0 GeV electron beams.\footnote{W.P. Leemans et al., Nature Physics 2, 696 (2006)} In these experiments, the 10-40 TW laser pulse was guided in a 3 cm long capillary discharge plasma channel. Particle-In-Cell (PIC) simulations provide information not accessible from experiments on the nonlinear laser-plasma interaction that governs the accelerator internal dynamics. Simulations show that high quality electron bunches are formed by self-trapping of electrons in the wake, followed by loading of the wake by the trapped bunch, creating a bunch of electrons isolated in phase space. A narrow energy spread beam is then obtained by extracting the bunch as it outran the accelerating phase of the wake. Simulations in 2D and 3D showing details on the electron bunch, wakefield, and laser evolution are presented and compared to experimental results. Simulations on scaling these experiments to the 10 GeV level are also presented. [Preview Abstract] |
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BP8.00012: Experimental Demonstration of 1 GeV Energy Gain in a Laser Wakefield Accelerator Anthony Gonsalves, Kei Nakamura, Csaba Toth, Carl Shroeder, Estelle Cormier-Michel, Wim Leemans, D. Bruhwiler, John Cary, Simon Hooker, Eric Esarey, Dmitriy Panasenko GeV-class electron accelerators have a broad range of uses, including synchrotron facilities, free electron lasers, and high-energy particle physics. The accelerating gradient achievable with conventional radio frequency (RF) accelerators is limited by electrical breakdown within the accelerating cavity to a few tens of MeV, so the production of energetic beams requires large, expensive accelerators. One promising technology to reduce the cost and size of these accelerators (and to push the energy frontier for high-energy physics) is the laser-wakefield accelerator (LWFA), since these devices can sustain electric fields of hundreds of GV/m. In this talk, results will be presented on GeV-class beams using an intense laser beam. Laser pulses with peak power ranging from 10-40TW were guided in gas-filled capillary discharge waveguides of length 15mm and 33mm, allowing the production of high-quality electron beams with energy up to 1 GeV. The electron beam characteristics and laser guiding, and their dependence on laser and plasma parameters will be discussed and compared to simulations. [Preview Abstract] |
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BP8.00013: A 10 GeV laser driven accelerator: the BELLA project W.P. Leemans, O. Albert, E. Esarey, C.G.R. Geddes, A. Gonsalves, N.H. Matlis, K. Nakamura, D. Panasenko, G.R. Plateau, C.B. Schroeder, Cs. Toth, D.L. Bruhwiler, J.R. Cary, M. Bakeman, E. Cormier-Michel, T. Cowan, S.M. Hooker GeV class beams have been generated from a laser driven accelerator \footnote{W.P. Leemans et al., Nature Physics \textbf{2}, 696-699 (2006); K. Nakamura et al., Phys. Plasmas \textbf{14}, 056708 (2007)}. The experiments used a cm-scale capillary discharge produced plasma channel to guide and control the process of acceleration, similar to the use of laser produced channels \footnote{C.G.R. Geddes et al., Nature \textbf{431}, 538-541 (2004)}, and 40 TW laser pulses. Lower plasma density and cm-scale channel length resulted in up to 1 GeV beams, in good agreement with simulations. This forms the basis for the next milestone experiment: a 10 GeV laser driven accelerator. As part of the BELLA project at LBNL, scaling of these experiments to the 10 GeV level is now underway. We will discuss experimental plans for the implementation of a 1 m scale channel guided laser wakefield accelerator and a petawatt class laser system. [Preview Abstract] |
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BP8.00014: A Novel Source of Injection Electrons for a Capillary Waveguide Accelerator Chris McGuffey, Takeshi Matsuoka, Michael Levin, Stepan Bulanov, Vladimir Chvykov, Galina Kalinchenko, Stephen Reed, Pascal Rousseau, Victor Yanovsky, Arie Zigler, Karl Krushelnick, Anatoly Maksimchuk Electron beams with quasimonoenergetic and/or broadband energy spectra have been produced by focusing a high-intensity (HI)laser (30TW 35fs Ti:Sapphire) through an ablated plasma plume. The plasma is ablated from a flat material by focusing a pulsed Nd:YAG laser. The characteristics of the electron beam produced are determined by the plasma density seen by the HI pulse, which can be controlled by varying the delay between ablation and the HI pulse, the ablation material, and the distance between the laser axis and ablation surface. These electron beams are candidates for injection into a capillary plasma waveguide. Capillary waveguides offer the possibility of staging or tapering to overcome the limitation of dephasing length. This injection scheme could lead to high charge, quasimonoenergetic beams with energy on the order of 1GeV. Such beams could assume some of the roles of conventional accelerators and may offer applications in medicine, biology, and solid state physics. [Preview Abstract] |
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BP8.00015: Analysis of Trajectories and Multi-Dimensional Phase Space Diagrams for Electrons Injected Orthogonal to Plasma Waves, and Laser and Scattered Radiation Arnesto Bowman, Ronald Williams Plasma waves have been shown to be capable of accelerating electrons over very short distances beyond what is capable by conventional accelerators. The orthogonal injection of electrons is being explored as a diagnostic to infer the amplitude of the large accelerating electric fields associated with plasma wave accelerators. The orthogonal geometry suggests that the particles have trajectories in three dimensions, and we will describe simulation studies of these trajectories. We will also discuss the results of our studies of the multi-dimensional phase space diagrams of electron dynamics in the lab and wave frames. We also describe the changes to the trajectories and diagrams that occur when transverse laser fields and scattered laser radiation fields are included with the longitudinal plasma wave fields. It is necessary for electron beams used for plasma wave diagnostics to have very small transverse emittance. We describe our studies of scanning and photographic methods for measuring beam emittance in our experiments. We are attempting to characterize our thermionic cathode and to compare the predicted emittance, based on power input, with the emittance measured using our apparatus. [Preview Abstract] |
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BP8.00016: Beam loading in the blowout regime of laser/plasma wakefield acceleration. Michail Tzoufras, Wei Lu, Chengkun Huang, Frank Tsung, Warren Mori, Jorge Vieira, Ricardo Fonseca, Luis Silva The amount of charge, the final energy, and the quality of the charged particle beam that is generated from a plasma-based accelerator depends on how the charge is loaded into the wake. In recent experiments the wakes are created by either the radiation pressure of the laser or the space-charge force of the electron beam expelling the plasma electrons outward. We present a theory for beam loading valid for such nonlinear multi-dimensional wakes. We start from the equation for the blowout radius derived by Lu et al. [1]. Analytical solutions are found for this equation when the wake is loaded by flat-top or trapezoidal electron beams. As a result expressions for the accelerating field, the shape of the bubble and the amount of charge are obtained. These are compared to those predicted by the linear theory of Katsouleas et al. [2]. We also discuss the optimum current profile to minimize the final energy spread while maximizing the mean energy and the number of particles. [1] W. Lu et al$,$ Phys. Rev. Lett. 96, 165002 (2006). [2] T. Katsouleas et al, Particle Accelerators, 1987, Vol. 22, pp. 81-99. [Preview Abstract] |
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BP8.00017: Simulation and calculation of particle trapping using a quasistatic simulation code Sepehr Morshed, John Palastro, Thomas Antonsen, Chengkun Huang, Warren Mori In LWFA schemes the laser pulse must propagate several centimeters and maintain its coherence over this distance, which corresponds to many Rayleigh lengths. These Wakefields and their effect on the laser can be simulated in quasistatic approximation [1, 2]. In this approximation the assumption is that the driver (laser) does not change shape during the time it takes for it to pass by a plasma particle. As a result the particles that are trapped and moving with near-luminal velocity can not be treated with this approximation. Here we have modified the 2D code WAKE with an alternate algorithm so that when a plasma particle gains sufficient energy from wakefields it becomes trapped to satisfy the trapping conditions. Similar implementations have been made in the 3D cod QUICKPIC [2]. We also have done simulation and comparison of results for centimeter scale GeV electron accelerator experiments from LBL [3] with WAKE. These changes in WAKE will give users a tool that can be used on a desk top machine to simulate GeV acceleration. [1] P. Mora and T. M. Antonsen Jr., Phys Plasma 4, 217 (1997) [2] C. Huang et al. Comp Phys. 217 (2006) [3] W. P. Leemans et al. Nature Phys 2, 696 (2006) Letters [Preview Abstract] |
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BP8.00018: Design and simulation of a single 100GeV stage Laser Wakefield Accelerator. Wei Lu, Michail Tzoufras, Chengkun Huang, Frank Tsung, Warren Mori, Jorge Vieira, Ricardo Fonseca, Luis Silva, James Cooley, Thomas Antonsen The design of a laser wakefield accelerator involves understanding and control of various plasma physics phenomena related to the laser evolution, the response of the plasma medium and its effect on the accelerating particles. Within the framework developed by W. Lu et al. [1] we study these phenomena in the weakly nonlinear blowout regime, where the laser power is similar to the critical power for self-focusing. High quality electron beams can be accelerated in this regime in a single stage with average gradient 3.6GeV/m to reach 100 GeV. Full and reduced particle-in-cell simulations are presented to illuminate the physics and verify the applicability of the design. \newline \newline [1] W. Lu et al, ``Generating multi-GeV electron bunches using single stage laser wakefield acceleration in a 3D nonlinear regime,'' Phys. Rev. ST Accel. Beams 10, 061301 (2007) [Preview Abstract] |
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BP8.00019: Detailed numerical modeling of electron injection in the Laser Wakefield Accelerator: Particle Tracking Diagnostics in PIC codes R.A. Fonseca, L. Gargat\'e, S.F. Martins, F. Peano, J. Vieira, L.O. Silva, W.B. Mori The field of laser plasma acceleration has witnessed significant development over recent years, with experimental demonstrations of the production of quasi mono-energetic electron bunches, with charges of $\sim $ 50 pC and energies of up to 1 GeV [1]. Fully relativistic PIC codes, such as OSIRIS [2] are the best tools for modeling these problems, but sophisticated visualization and data analysis routines [3] are required to extract physical meaning from the large volumes of data produced. We report on the new particle tracking diagnostics being added into the OSIRIS framework and its application to this problem, specifically targeting self-injection. Details on the tracking algorithm implementation and post processing routines are given. Simulation results from laser wakefield accelerator scenarios will be presented, with detailed analysis of the self injection of the electron bunches. \newline [1] W.P. Leemans et al, Nature Phys. 2 696 (2006) \newline [2] R. A. Fonseca et al., LNCS 2331, 342, (2002) \newline [3] R. A. Fonseca, Proceedings of ISSS-7, (2005) [Preview Abstract] |
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BP8.00020: Self-Steepening of intense laser pulses in plasmas Jorge Vieira, Frederico Fi\'uza, Luis Silva In state-of-the-art Laser Wake Field Acceleration (LWFA) experiments [1], the self-modulations of the laser pulse (both transverse and longitudinal) play an important role in the enhancement of the plasma wave, which can trap, accelerate and lead to quasi-mono-energetic electron beams. In this work, the self-steepening of intense laser pulses is studied analytically resorting to the photon-kinetic theory [2]. Rates for the growth of self-steepening in the early laser propagation are provided in the long and short pulse limits, and in the weakly and ultra relativistic regimes. Thresholds for the on-set, maximum and minimum growth of self-steepening are determined. We find very good agreement between the analytical model and one-dimensional PIC simulations with OSIRIS [3]. Implications of our results to state-of-the-art LWFA experiments are discussed. \newline [1] W.P. Leemans et al Nat. Phys., 2 (10), 696-699 (2006) \newline [2] L.O. Silva et al, IEEE TPS 28 (4) 1128-1134 (2000) \newline [3] R. A. Fonseca et al, LNCS 2331, 342-351, (Springer, Heidelberg, 2002). [Preview Abstract] |
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BP8.00021: Creation of a multi-centimeter low density plasma channel using high magnetic fields B. Pollock, D.H. Froula, P. Davis, J.S. Ross, A. Collette, L. Divol, P. Michel, N. Meezan, G. Tynan, S.H. Glenzer We will present experimental results that show the formation of a laser produced plasma channel when applying a large external magnetic field. This channel is suitable for guiding laser beams and is directly applicable to wakefield acceleration and short pulse laser amplification. This is accomplished by applying a technique that has been established at the Jupiter Laser Facility; an external magnetic field is used to prevent radial heat transport [D. H. Froula \textit{et.al.}, Phys. Rev. Lett. 98, 135001 (2007)] resulting in an increased temperature gradient. Temporally resolved Thomson-scattering measurements of the electron temperature profile in large magnetic fields show that the heat front, transverse to a high-power laser beam, is slowed resulting in extremely strong local heating. This strong local heating produces a density channel that is measured with interferometry for densities between 10$^{17}$ cm$^{-3}$ to 10$^{19}$ cm$^{-3}$. [Preview Abstract] |
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BP8.00022: Electron Acceleration by a Tightly Focused Laser Pulse K.I. Popov, V. Yu. Bychenkov, W. Rozmus, R. Sydora By using the test particle approach we have studied electron vacuum acceleration including nonadiabatic effects and synchronized trajectories which correspond to particles experiencing constant phase of electromagnetic fields and subluminous phase velocity. After the averaging over the laser field phase, the energy and emission angle distributions versus the electron positions in the focal region have been obtained. The most effective acceleration was found for electrons placed at laser beam axes at the distance comparable to the Rayleigh length before the best focus position. The correlations between electron energies and the emission angles were studied. We also obtained the dependence of the maximum electron energy on the focal spot size. Results of test particle studies guided 3D particle-in-cell simulations with thin foil targets for the best conditions for electron acceleration in the Coulomb explosion regime. [Preview Abstract] |
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BP8.00023: Petawatt laser-driven wakefield accelerator: All-optical electron injection via collision of laser pulses and radiation cooling of accelerated electron bunches. Serguei Kalmykov, Yoav Avitzour, S. Austin Yi, Gennady Shvets We explore an electron injection into the laser wakefield accelerator (LWFA) using nearly head-on collision of the petawatt ultrashort ($\sim$30 fs) laser pulse (driver) with a low- amplitude laser (seed) beam of the same duration and polarization. To eliminate the threat to the main laser amplifier we consider two options: (i) a frequency-shifted seed and (ii) a seed pulse propagating at a small angle to the axis. We show that the emission of synchrotron radiation due to betatron oscillations of trapped and accelerated electrons results in significant transverse cooling of quasi- monoenergetic accelerated electrons (with energies above 1 GeV). At the same time, the energy losses due to the synchrotron emission preserve the final energy spread of the electron beam. The ``dark current'' due to the electron trapping in multiple wake buckets and the effect of beam loading (wake destruction at the instant of beams collision) are discussed. [Preview Abstract] |
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BP8.00024: Relativistic self-focusing of multi-color laser pulses in plasmas. G. Shvets, S. Austin Yi, S. Kalmykov An intense laser beam (with a power less than critical for the relativistic self-focusing) can be guided in plasmas with the help of an additional small-amplitude co-propagating beam (few- percent of the main beam energy). The guiding is effective if the beams' frequency detuning is slightly below the electron plasma frequency. The enhanced guiding is caused mostly by the near-resonantly driven 3D electron density perturbation (plasma beatwave). Another intriguing effect of the nonlinear guiding of ultra-short ($<1/\omega_p$) radiation spikes is observed during the later stage of laser propagation. Periodic train of such spikes is self-consistently generated via electromagnetic cascading [S. Kalmykov and G. Shvets, Phys. Rev. Lett. {\bf 94} 235001 (2005); Phys. Rev. E {\bf 73} 046403 (2006)]. The guiding effect of the plasma wave partly suppresses the diffraction and results in a multi-centimeter guided propagation of the intense pulse train. Acceleration of externally injected electrons in the cascade-driven wake is quasi-monoenergetic and is characterized by low normalized transverse emittance and near-GeV energy gain. [Preview Abstract] |
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BP8.00025: Direct ion acceleration with variable-frequency lasers Fabio Peano, Jorge Vieira, Ricardo Fonseca, Luis Silva, Gianni Coppa, Roberta Mulas Laser-based ion acceleration commonly relies on indirect schemes, in which the ions are accelerated by the space-charge field in laser-irradiated solid targets, either via plasma-expansion processes [1], or resorting to electrostatic shock structures [2]. Here, we propose the production of monoergetic ion beams via direct acceleration by the laser field (in vacuum or in tenuous plasmas) [3]. The method exploits two counterpropagating lasers with variable frequency to drive a beat-wave structure with variable phase velocity: the ions are trapped in the beat wave and accelerated to high energies. The physical mechanism is described with a 1D theory, providing the general conditions for trapping and scaling laws for the relevant ion-beam features. The validity and the robustness of the method are confirmed by 2D PIC simulations with OSIRIS [4]. \newline [1] J. Fuchs\textit{ et al.}, Nature Phys. \textbf{2}, 48 (2006); L. Robson \textit{et al.}, Nature Phys. \textbf{3}, 58 (2007); B.M. Hegelich \textit{et al.}, Nature \textbf{439}, 441 (2006). \newline [2] L.O. Silva \textit{et al.}, Phys. Rev. Lett. \textbf{92}, 015002 (2004). \newline [3] F. Peano \textit{et al.}, submitted for publication (2007). \newline [4] R. A. Fonseca \textit{et al.}, Lect. Notes Comp. Sci. \textbf{2331}, 342 (Springer-Verlag, Heidelberg, 2002). [Preview Abstract] |
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BP8.00026: On The Possibility of Accelerating Positron on an Electron Wake Xiaodong Wang, Tom Katouleas, Patric Muggli, Rasmus Ischebeck A new approach for positron acceleration in non-linear plasma wakefields driven by electron beams is presented. Positrons can be produced by colliding an electron beam with a thin foil target embedded in the plasma. Integration of positron production and acceleration in one stage is realized by a single relativistic, intense electron beam. Simulations with the parameters of the proposed SABER facility at SLAC suggest that this concept could be tested there. [Preview Abstract] |
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BP8.00027: Study of relative ion acceleration efficiencies from laser-solid interactions Christopher Murphy, Enam Chowdhury, John Morrison, Linn Van Woerkom, Karl Krushelnick, Richard Freeman The production of high-energy proton and ion beams has important applications in many areas of science including inertial fusion energy, laboratory astrophysics and compact particle sources for use in radiography and medical oncology. Utilizing laser plasma interactions (LPI) for such a source is garnering support from the various communities due to its potential to be compact and mobile. Recent studies have suggested that while increasing the laser intensity in LPIs is important for high energy ion production, moving to ultrashort (sub-picosecond) laser pulses may not be as effective as increasing the energy. An experimental study of this hypothesis will be presented, comparing the ion beam spectrum, charge and spatial quality using both magnetic spectrometers and film stacks. [Preview Abstract] |
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BP8.00028: A high gradient plasma wakefield accelerator using two subpicosecond electron bunches Efthymios Kallos, Patric Muggli, Tom Katsouleas, Karl Kusche, Igor Pavlishin, Igor Pogorelsky, Daniil Stolyarov, Vitaly Yakimenko, Wayne Kimura A high gradient plasma wakefield accelerator was tested at the Accelerator Test Facility of Brookhaven National Lab. Two $\sim $100fs electron bunches with total charge of 0.5nC separated by $\sim $500fs were fed into a 6mm long high density (1e14/cc to 1e17/cc) plasma generated by an ablative capillary discharge. The drive bunch created a $\sim $300MV/m wakefield that was sampled by the short witness bunch. The relative position of the witness bunch with respect to the drive bunch wakefield could be adjusted by varying the plasma density, thus allowing controllable energy loss or energy gain with small energy spread. The experimentally observed energy shifts are in good agreement with 2D model predictions. [Preview Abstract] |
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BP8.00029: Beam head erosion in self-ionized plasma wakefield accelerators Miaomiao Zhou, Chris Clayton, Chengkun Huang, Chan Joshi, Wei Lu, Ken Marsh, Warren Mori, Tom Katsouleas, Patric Muggli, Erdem Oz, Melissa Berry, Ian Blumenfeld, Franz-Josef Decker, Mark Hogan, Rasmus Ischebeck, Richard Iverson, Neil Kirby, Robert Siemman, Dieter Walz In the recent plasma wakefield accelerator experiments at SLAC, the energy of the particles in the tail of the 42 GeV electron beam were doubled in less than one meter [1]. Simulations suggest that the acceleration length was limited by a new phenomenon -- beam head erosion in self-ionized plasmas. In vacuum, a particle beam expands transversely in a distance given by beta*. In the blowout regime of a plasma wakefield [2], the majority of the beam is focused by the ion channel, while the beam head slowly spreads since it takes a finite time for the ion channel to form. Beam/plasma parameter scan in a large range using simulations shows that in self-ionized plasmas, the head spreading is exacerbated compared to that in pre-ionized plasmas, causing the ionization front to move backward (erode). A theoretical analysis on the erosion rate dependence on beam/plasma parameters and its implications on future afterburner relevant experiments will be provided.~ [1] I. Blumenfeld et al., Nature 445, 741(2007) [2] J. B. Rosenzweig et al., Phys. Rev. A 44, R6189 (1991) [Preview Abstract] |
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BP8.00030: Spectral Modulation of Self-Guided Laser Pulses Arthur Pak, Joe Ralph, Ken Marsh, Chengkun Huang, Fang Fang, Chris Clayton, Chan Joshi In this paper the experimental results of spectral modulation of a self-guided laser pulse in an underdense plasma will be presented. Experiments were conducted using an ultrashort laser pulse ($\sim $50 fs) generated from the UCLA Ti:Sapphire laser system capable of delivering up to 10 TW of power. A gas jet was used to create a dense column of helium gas which the laser pulse ionized and self-guided through. By varying the laser pulse width, laser energy, gas jet density and gas jet length, different physical mechanisms of self-guiding were explored. In these experiments the guided laser pulse was spectrally and spatially resolved using a .25 m imaging spectrograph with 1.2 nm spectral resolution and 13 $\mu $m spatial resolution. Evidence of photon acceleration / deceleration due to the laser pulse interacting with density oscillations of a plasma wakefield will be presented and compared to simulation results. Additionally using the imaging spectrograph the percentage of the laser energy that was self-guided was determined. [Preview Abstract] |
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BP8.00031: 3D PIC Simulations of Laser Produced Plasma Expansion with Large Ion Larmor Radius Masanori Nunami, Akira Takata, Katsunobu Nishihara We have investigated expansion of laser produced cluster plasma in a strong magnetic field using 3D PIC simulation. Since initial electron pressure of laser heated cluster is much higher than magnetic pressure, electrons first expand and ions are accelerated outward due to electric field generated by expanding electrons. In the expansion of cluster plasma in magnetic field, ion Larmor radius is much larger than the initial cluster size, while electron Larmor radius is much smaller than the cluster size, namely, Re $<<$ Ro $<<$ Ri, where Re(i) is Larmor radius of electron (ion) and Ro is the initial cluster size. Accelerated ions expand up to about their Larmor diameter. Therefore magnetized electron surface separates from ion surface. The surface of magnetized electrons is unstable for the flute type instability mainly due to the inward-directed electric field created by streaming ions with large Larmor radius [1]. However we found that ion surface is relatively stable, which is different from previous works. \newline [1] B. H. Ripin et al, Phys. Fluids B5, 3491 (1993). [Preview Abstract] |
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BP8.00032: ICF I AND LASER PLASMA INTERACTIONS |
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BP8.00033: Update on Specifications for LMJ Ignition Targets Giorla Jean, Cherfils Catherine, Galmiche Didier, Gauthier Pascal, Laffite Stephane, Masse Laurent, Poggi Francoise, Quach Robert, Seytor Patricia The Laser M\'{e}gajoule (LMJ) facility will deliver up to 1.40~MJ of 0.35~$\mu $m light in 160~beams in a first step. The targets for the first ignition experiments rely on indirect drive and use plastic capsules doped with germanium. The target fabrication specifications are the result of an extensive robustness study where all fabrication, laser and experimental errors are taken into account. This study is complete for the `baseline' target A1040 designed for 240 beams and is in progress for lower laser energy targets. The target dispersions are regrouped into 1D errors, which keep the implosion spherical, and 3D errors, which induce a deformation of the DT shell. The 3D robustness is expressed in terms of non linear deformation at peak velocity and compared to the deformation threshold obtained with 2D simulations. We have performed an experimental design method based on 2000 1D-simulations, which gives the fusion energy as a function of the 22 1D-parameters and allows us to estimate the 1D-margin, as a function of DT aging and DT gas density for a given temperature law. [Preview Abstract] |
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BP8.00034: NIF ignition target requirements, margins, and uncertainties Steven Haan, Jay Salmonson, Daniel Clark, Debra Callahan, Bruce Hammel, Laurance Suter, John Edwards, John Lindl We describe simulations of NIF ignition targets, concentrating on a point design target that uses 1.3 MJ to drive a hohlraum to 285eV. The point design capsule has 5 layers of varying Cu dopant to minimize RT instability growth. A set of requirements has been developed that describes all aspects of the target, its fabrication and fielding, the laser pulse, and the features of the pre-ignition experiments that are needed to finalize the design. We describe a model that characterizes the margin of the target as a function of the input parameters and uncertainties. The model has been normalized to 1D, 2D, and 3D simulations. It has been used to define and update the point design, to quantify the impact of each requirement, and to ensure that the requirements are optimally defined. The model can be used to project the probability of ignition, as shot-to-shot variations and more globally given systematic errors. There are several backup targets that are being kept active, including other drive temperatures from 270 to 300eV, CH ablators, and high density C ablators. The relative performance, and specific pertinent issues, regarding these targets are described. [Preview Abstract] |
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BP8.00035: Plans for and Progress Towards and Inertial Confinement Fusion Code from the Crestone Project John Wohlbier, Charles Wingate, Thomas Masser, Gregory Bowers, Michael Sorice The Crestone Project at Los Alamos National Laboratory has recently released {\em cassio}, a code for use in simulating Inertial Confinement Fusion. The initial release of {\em cassio} included a radiation hydrodynamics capability, a 3T plasma physics model~[1], and a laser ray tracing capability~[2], all implemented on an Eulerian AMR mesh. Future enhancements to the code will include higher order radiation transport, improved plasma models (e.g., electron, ion, and radiation temperatures per material in mixed cells), charged particle transport, thermonuclear burn, and conformal AMR meshes to ensure symmetric capsule implosions. In this paper we detail the existing models in {\em cassio} and lay out our plans for the future code enhancements. \newline [1] J.G.~Wohlbier, Los Alamos National Laboratory Report, LAUR pending, (2007). \newline [2] M.~Sorice, Los Alamos National Laboratory Report, LAUR pending, (2007). [Preview Abstract] |
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BP8.00036: Colliding plasmas in laser irradiated cavities studied with soft x-ray interferometry Jorge Filevich, Mike Purvis, Jonathan Grava, Mario C. Marconi, Jorge Rocca, James Dunn, Stephen J. Moon, Vyacheslav Shlyaptsev, Ela Jankowska Electron density maps of dense converging plasmas created by laser irradiation of semi-cylindrical and V--shaped targets at I = 1 x 10$^{12}$ W/cm$^{2}$ were obtained with soft x-ray laser interferometry ($\lambda $ = 46.9 nm). In the case of the cylinders, the plasma expands off the target surface converging in a focal region, creating a concentrated plasma where the electron density build-up exceeds 1 x 10$^{20}$ cm$^{-3}$. The plasma in the V-shaped targets concentrates along the symmetry plane of the target where collisions redirect the plasma forming, early on in the evolution, a narrow jet-like plasma. The measurements were compared with simulations obtained using the code HYDRA. [Preview Abstract] |
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BP8.00037: Progress in High-Energy-Density Plasma Jet Theoretical Research Chiping Chen, Jing Zhou A self-consistent phase-space moment description is developed for high-energy-density plasma jets. The phase-space moment theory is the truncated moment average of the kinetic equation. Using the phase-space moment theory, the root-mean-square (rms) envelope equations, which describe the orientation and size of the plasma jet, are derived for high-energy-density plasma jets. The envelope equations are demonstrated to agree with the virial theorem. To study the role of magnetic field helicity in plasma jet compression, a simplified model of a plasma jet is employed, and a complete set of equations governing the plasma jet is derived. The characteristic distance over which the compression occurs is calculated. [Preview Abstract] |
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BP8.00038: Acceleration of thin flyer foils with a 1 MA pulsed power device for shock-wave experiments in clumpy foam targets Stephan Neff, Jessica Ford, David Martinez, Christopher Plechaty, Sandra Wright, Radu Presura The dynamics of shock waves in clumpy media are important for understanding many astrophysical processes, including the triggering of star formation in interstellar gas clouds by passing shock waves. This phenomena can be studied in the laboratory by launching a flyer plate into a low density foam with clumps. Low density foams offer the advantage of relative low sound speeds (a few hundred meters per second) compared to normal solids, thus reducing the flyer speed required to create shock waves. In first experiments aluminum foils with thicknesses between 20 micrometer and 130 micrometer were accelerated to speeds up to 2.3 km/s. In addition, the impact of the flyers on plexiglas targets was studied. Additional measurements will focus on optimizing the flyer properties (thicker flyers, higher velocities) and on characterizing the flyer in more detail (temperature of the flyer and plasma ablation from the flyer). The results of these measurements will be used to design an experiment studying the dynamics of shock waves in clumpy foams, using the 100 TW laser system Leopard for back-lighting the foam target. [Preview Abstract] |
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BP8.00039: Impact of High-Z Coatings on the Ablation Pressure of Laser Driven Targets. Andrew Mostovych, Jaechul Oh, Andrew Schmitt, James Weaver Recent hydrodynamic experiments [1] with planar high-Z coated targets at the Naval Research Laboratory and spherical implosion experiments with high-Z coated shell targets [2] at the Omega facility all show significant improvement in target stability as a result of the high-Z coatings. For better understanding of the hydrodynamic processes it is important to know the changes in ablation pressure as a result of the high-Z layers. Using the Nike Laser, we have conducted new experiments to measure the change in shock speed of planar CH targets that are irradiated with and without the presence of a 200 Ang. gold high-Z coating. The evolution of shock propagation inside the targets is diagnosed with VISAR probing while average shock velocities are also measured by shock breakout detection from the stepped rear surface of the targets. We find that the high-Z layers produce a time dependent ablation pressure which is detected via the observation of non-steady shocks in the targets. Experimental results and comparisons to hydrodynamic simulations will be presented. Work supported by U. S. Department of Energy. \newline [1] S.P. Obenschain et al., Phys. Plasmas 9, 2234 (2002). \newline [2] A.N. Mostovych et al., APS Abstracts DPPFO3002M, (2005). [Preview Abstract] |
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BP8.00040: Transverse modulation instability of white light in plasmas L.O. Silva, R. Bingham, B. Brandao, J. Santos The transverse modulation, or filamentation, instability of intense radiation with arbitrary spatial coherence in plasmas is analyzed with generalized photon kinetic theory [1,2], which includes both forward and backward scattered radiation. The instability threshold as well as the growth rate dependence with the laser bandwidth are determined. The analytical results are compared with numerical solutions of the generalized dispersion relation and with full PIC simulations. Consequences of white light driven parametric instabilities in ICF and fast ignition scenarios are also discussed. \newline [1] J. P. Santos, and L. O. Silva, Journal of Mathematical Physics 46, 102901 (2005) \newline [2] J. E. Santos, L. O. Silva, R. Bingham, Physical Review Letters 98, 235001 (2007), also arXiv:0704.2831 (http://arxiv.org/abs/0704.2831) [Preview Abstract] |
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BP8.00041: Measurements of the energy flow and plasma dynamics in a laboratory analogue of N-waves in the solar atmosphere M. Taylor, J. Foster, P. Graham, A. Moore, S. MacLaren, P. Young, G. Glendinning, A. Reighard, C. Sorce, C. Back, J. Hund, B. Blue Density perturbations in the solar atmosphere are coupled to the X-ray radiation field and so their evolution is difficult to simulate. To assess the fidelity of our current modeling capabilities, a series of experiments have been performed on the LLE OMEGA laser. These used a hohlraum to drive X-rays through a tantalum aerogel disk containing a machined slot. The dynamic evolution of this system is diagnosed with several complementary methods. The energy flow was measured using both direct flux and hohlraum calorimetry, which are compared to assess the best technique. 2D X-ray self-emission images of the data reveal structure in the radiation front seeded from localized spatial perturbations in the foam areal density. Point projection radiography down the axis of the hohlraum was utilized to determine the associated density structures, including complex irregular flows. [Preview Abstract] |
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BP8.00042: Nonlocal heat transport using optically-smoothed lasers in direct-drive ICF Michael Keskinen, Denis Colombant, Andrew Schmitt, Wally Manheimer Electron thermal conduction is important in direct-drive inertial confinement fusion. Since it is responsible for transporting laser energy absorbed near the critical surface into the overdense region, it can directly affect ablation and implosion dynamics. For high laser intensities, nonlocal transport models need to be used to accurately calculate populations of fast electrons which may lead to target preheat. Optically smoothed laser radiation changes on a coherence time scale. For these reasons we are developing a Fokker-Planck (FP) code which is coupled to an electromagnetic full-wave Maxwell solver. We present results from this coupled model for a range of laser intensities, using different collisional operators, e.g., Krook, and radiation transport effects for high-Z targets. [Preview Abstract] |
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BP8.00043: Plasma Heating and Fusion Neutron Production in Collisions of Planar CD Foils at Velocities Above 400 km/s. A.L. Velikovich, S.T. Zalesak, A.J. Schmitt, N. Metzler, M. Murakami, T. Sakaiya, K. Shigemori, H. Shiraga, S. Fujioka, T. Watari, H. Saito, H. Azechi Interest in experiments on colliding planar CD foils has recently been stimulated by (a) the Impact Fast Ignition approach to laser fusion [1], which involves the collision of a shell accelerated to $\sim $1000 km/s with high-density DT fuel, and (b) the approach to a high-repetition rate ignition facility based on direct drive with the KrF laser and a very high implosion velocity, $\sim $450 km/s, to reduce the ignition threshold and increase gain [2]. Studies of planar foil collisions at hyper-velocities help test feasibility of both concepts. We present the results of modeling the recent experiments at ILE, where collisions of CD planar foils produced fusion neutron yields of the order of 1E6. Analytical formulas for the neutron yield and the results of numerical simulations are compared to the experimental data. \newline [1] M. Murakami \textit{et al}., Nucl. Fusion \textbf{46}, 99 (2006).\newline [2] S. P. Obenschain \textit{et al}., Phys. Plasmas \textbf{13}, 056320 (2006). [Preview Abstract] |
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BP8.00044: Non-Maxwellian distributions in an IEC device for fusion break even Evstati Evstatiev We explore the effect of non-Maxwellian distributions on the maximum sustainable ion density in the core of an Inertial Electrostatic Confinement (IEC) device. Ref. 1 considered a negative potential well created by injecting radially monoenergetic electrons. It showed that potential well of 100 KV can be sustained with about 100 A of total injection current. Then Ref. 1 showed that purely monoenergetic, radially moving ions cannot be trapped in a well so created. On the other hand, Maxwellian radial ions can be trapped but there is an upper limit on the core ion density, $n_0$. If break even balance of fusion yield/ injected (electron) power is considered, this limitation translates into a required electron injection current of the order of $10^{14}$~A. Clearly, this number is impractical. Natural questions arise from the calculations presented in Ref. 1: Is it possible to find a non-Maxwellian ion distribution (sustainable by some means) such that the necessary electron injection current for creating a trapping potential can be lowered to reasonable numbers? If yes, is such ion distribution energetically viable for fusion break even? Ongoing work indicates positive answer to the first question. We will discuss the energetics of such scheme. \newline [1] W. C. Elmore, J. L. Tuck, K. M. Watson, Phys. Fluids, vol. 2, 239 (1959). [Preview Abstract] |
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BP8.00045: Investigation of Stimulated Raman Scattering Using a Short-Pulse Single-Hot-Spot at the Trident Laser Facility J.L. Kline, D.S. Montgomery, L. Yin, K.A. Flippo, B.J. Albright, T. Shimada, R.P. Johnson, H.A. Rose, E.A. Williams, R.A. Hardin A new short-pulse version of the single-hot-spot configuration has been implemented to enhance the performance of experiments to understand Stimulated Raman Scattering. The laser pulse length was reduced from $\sim $200 to $\sim $4 ps. The reduced pulse length improves the experiment by minimizing effects such as plasma hydrodynamics and ponderomotive filamentation of the interaction beam. In addition, the shortened laser pulses allow full length 2D particle-in-cell simulations of the experiments. Using the improved single-hot-spot configuration, a series of experiments to investigate k$\lambda _{D}$ scaling of SRS has been performed. Quantitative comparisons of the experiments have been made with the VPIC$^{\dag }$ particle-in-cell code with favorable agreement. In addition, the measurements of the backscatter SRS spectra possibly show evidence of a direct observation of a nonlinear frequency shift due to electron trapping. Details of the experimental setup and initial results will be presented. [Preview Abstract] |
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BP8.00046: Mitigation of Stimulated Raman Scattering in Hohlraum Plasmas D.S. Montgomery, J.L. Kline, H.A. Rose, S.R. Goldman, D.H. Froula, J.S. Ross, R.M. Stevenson One aspect of the research at LANL to control Stimulated Raman Scattering (SRS) in hohlraum plasmas is the investigation of risk mitigation strategies for indirect drive inertial confinement fusion. Beam spray of the laser, due to thermally-enhanced forward Brillouin scattering, results in a decrease in the longitudinal coherence lengths of the laser, which in turn reduces SRS. Since thermal effects depend on $Z^{2}$, a small amount of a high $Z$ dopant, 1-2\%, can have a large effect. Experiments have been conducted at the Omega laser to test this theory by varying the amount of Xe dopant in $C_{5}H_{12}$ gas filled hohlraums, and do show a decrease in SRS backscatter as Xe dopant is added. However, there are still uncertainties regarding the responsible mechanism. The second strategy investigated is using high $k\lambda_{D}$ plasmas to reduce SRS backscatter. Experiments conducted at the Omega laser in hohlraum plasmas determined the critical onset intensity for a range of $k\lambda_{D}$. A scaling of the critical onset intensity as a function of $k\lambda_{D}$ has been determined. The scaling is compared with theoretical predictions. Results for both mitigation strategies will be presented, as well as suggested implementation strategies for ignition-relevant hohlraums. [Preview Abstract] |
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BP8.00047: Multidimensional non local effects in hot spot relaxation in laser-produced plasmas P.H. Nicolai, J.L. Feugeas, X. Ribeyre, M. Grech, G. Schurtz The control of parametric instabilities, such as filamentation and stimulated scattering is a necessity for the Inertial Confinement Fusion (ICF). The plasma temperature and density distribution directly affect the laser beam propagation and the energy deposition. Under sharp gradients created by non-uniform laser heating, the size of hot spots is often comparable to the electron mean free path and the electron heat transport becomes nonlocal. Furthermore, the hot spot form is not necessarily spherical and a one dimensional analysis is insufficient. This work presents the multi-dimensional effects of the non local electron transport on the plasma response induced by a single hot spot or multi hot spots. In addition, in non spherical speckles, we show that crossed gradients of density and temperature generate vortical flows and magnetic fields. These self generated magnetic fields combined with nonlocal heat transport effects [Ph. Nicolai et al Phys. PLasmas {\bf 13}, 032701 (2006)] could strongly change the life time of hot spots. Thanks to the use of a 2D multi-physics hydrodynamic code, we investigate the LIL facility quadruplet conditions for long time periods and large plasma conditions [the LIL facility is a full scale bundle of 4 Laser Mega Joule (LMJ) beams]. It appears that in a realistic case, our model indicates a dramatic change of the temperature and density distributions. [Preview Abstract] |
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BP8.00048: Parametric instabilities and their control in multidimensional nonuniform gain media Mathieu Charbonneau-Lefort, Bedros Afeyan, Martin Fejer In order to control parametric instabilities in large scale long pulse laser produced plasmas, optical mixing techniques seem most promising [1]. We examine ways of controlling the growth of some modes while creating other unstable ones in nonuniform gain media, including the effects of transverse localization of the pump wave. We show that multidimensional effects are essential to understand laser-gain medium interactions [2] and that one dimensional models such as the celebrated Rosenbluth result [3] can be misleading [4]. These findings are verified in experiments carried out in a chirped quasi-phase-matched gratings in optical parametric amplifiers where thousands of shots can be taken and statistically significant and stable results obtained. \newline \newline [1] B. Afeyan, et al., IFSA Proceedings, 2003. \newline [2] M. M. Sushchik and G. I. Freidman, Radiofizika 13, 1354 (1970). \newline [3] M. N. Rosenbluth, Phys. Rev. Lett. 29, 565 (1972). \newline [4] M. Charbonneau-Lefort, PhD thesis, Stanford University, 2007. [Preview Abstract] |
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BP8.00049: Particle-in-Cell Simulations of the 2$\omega_p$ Instability F.S. Tsung, W.B. Mori, B.B. Afeyan A particle-in-cell code (OSIRIS) is used to investigate the two-plasmon decay instability in nonuniform plasmas of various density profiles. We find good agreement between the simulation and linear theory by Afeyan and Williams (Phys. Plas. {\bf{4}}, 3827, 1997.) under a variety of laser and plasma conditions relevant to ICF. So far the theory has been tested for linear density profiles and parabolic density profiles where the perfect phase matching (PPMP) point is at the parabolic peak density. We will also test the theory's predictions concerning growth rates and eigeneconditions when the PPMP is in the transition region between the peak density of the parabolic profile and down on the flanks where strictly linear profile behavior is recovered. These simulations allow a check on linear theory, and also demonstrate the ability of PIC codes to study this instability in small regions of ICF relevant targets. Building on these experiences, we have now begun to investigate nonlinear effects on a longer time-scale, such as the saturation mechanism, the spectrum of the fast electrons at saturation, the relaxation and recurrence of the instability, and ion effects. [Preview Abstract] |
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BP8.00050: Simulation of stimulated Raman scattering in 2D Wojciech Rozmus, P.-E. Masson-Laborde, Zhongling Peng, V.Yu. Bychenkov, C.E. Capjack Results of particle-in-cell (PIC) simulations of the stimulated Raman scattering (SRS) in one and two spatial dimensions are discussed. With the focus on plasma conditions corresponding to large k$\lambda _{D}$ values of SRS driven Langmuir waves (k$\lambda _{D} \quad >$ 0.2) we examine secondary instabilities of plasma waves in the presence of trapped particles. For k$\lambda _{D} \quad >$ 0.3 transverse trapped particle modulational instability (Rose, Phys. Plasmas \textbf{12}, 2005) dominates nonlinear evolution of SRS. We have studied interplay between Langmuir decay and modulation instability in the intermediate regime of k$\lambda _{D}\sim $0.2. New effects are examined in two spatial dimensions where large fraction of trapped particles gives rise to electric current of fast electrons and the generation of magnetic field. Magnetic field and the transverse ponderomotive force of localized Langmuir waves modify trapped particle dynamics and alter frequency shift and side loss damping of Langmuir waves. Experimental signatures of the 2D effects such as angular broadening of the backscattered light are discussed. [Preview Abstract] |
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BP8.00051: pF3d simulations of nonlinear laser propagation in a multi-speckle environment E.S. Dodd, B. Bezzerides, D.F. DuBois, H.X. Vu Current design practice focuses on estimating LPI growth with linear analysis and using the average laser intensity [1]. However, LPI growth can be dominated by nonlinear effects, and by the distribution of intensities from the multi-speckle nature of the beam. Recent work on stimulated Raman scattering (SRS) has shown that above a threshold, due to trapped electrons, the reflectivity is greatly increased [2,3]. This threshold also has a dependence on local plasma conditions that differs from the SRS growth rate. In this poster we discuss current work that attempts to understand how the onset of nonlinear Langmuir wave behavior is affected by inter-speckle interactions with the pF3d code [4]. The current work shows that the distribution function for speckle intensities must be taken into account and that the average intensity currently used is insufficient. [1] R. Berger, E. A. Williams, and A. Simon, \textit{Phys. Fluids B} \underline {1} 414 (1989). [2] H. X. Vu, D. F. DuBois, and B. Bezzerides, \textit{Phys. Plasmas} \underline {9} 1745 (2002). [3] H. X. Vu, D. F. DuBois, and B. Bezzerides, \textit{Phys. Plasmas} \underline {14} 012702 (2007). [4] R. L. Berger, C. H. Still, E. A. Williams, and A. B. Langdon, \textit{Phys Plasmas} \underline {5} 4337 (1998). [Preview Abstract] |
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BP8.00052: SRS modeling of NIF ignition designs using pF3D Edward Williams, Denise Hinkel, Laurent Divol, A. Bruce Langdon, Pierre Michel, C.H. Still The laser plasma interaction code pF3d is used to model the propagation of high intensity laser beams through plasma, including filamentation and stimulated Raman and Brillouin scattering. Making these calculations feasible for ignition-related applications require that the equations for the light and Langmuir waves be enveloped in both space and time in a ``paraxial'' approximation. For the SRS light, the time dependence is enveloped around the peak of the (anticipated) spectrum. The Langmuir wave is enveloped around the corresponding parametric matching frequency and wave-number. For homogeneous plasmas, one can arrange for the properties (frequency, damping rate, group velocity, ponderomotive response) of the Langmuir wave, modeled by an enveloped fluid equation, to match those of a kinetic model. This is no longer the case when the plasma conditions span a large range of electron density and temperature. Some compromise is required. In this paper we describe modifications to our pF3d SRS model and compare them with benchmarks. We show simulations of the inner beam of NIF ignition designs, focusing on the behavior of SRS. [Preview Abstract] |
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BP8.00053: Driven Plasma Waves Relevant to Stimulated Raman Scattering Jay Fahlen, Benjamin Winjum, John Tonge, F.S. Tsung, Viktor Decyk, Warren Mori In fully self-consistent particle-in-cell (PIC) simulations the saturation of Stimulated Raman Scattering (SRS) is quite complicated. To better understand possible saturation mechanisms of SRS, we study the excitation of plasma waves by imposing an external ponderomotive force in 1D electrostatic PIC simulations. By varying the phase velocity and the drive frequency (detuning) with respect to the linear frequency, several saturation mechanisms are explored, including fluid and kinetic nonlinear frequency shifts, sideband generation, and particle trapping. The simulations indicate that simple frequency shift models are inadequate in describing the wave saturation. Wave harmonics are also observed and these can contribute to the non-linear frequency shift. A theory for harmonic-generated frequency shifts in the absence of particle trapping is presented along with corroborating simulation data. Further, the simulations are used to understand the effects necessary for developing a consistent harmonic, kinetic theory. Work supported by DOE under DE-FG52-06NA26195 and NSF under NSF-Phy-0321345. Simulations performed on the DAWSON Cluster. [Preview Abstract] |
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BP8.00054: The Role of Pump Depletion in Stimulated Raman Scattering for NIF Parameters B.J. Winjum, J. Fahlen, F.S. Tsung, W.B. Mori Using the full-PIC code OSIRIS in 1D, we have studied stimulated Raman scattering (SRS) in a range of parameters relevant to NIF. In recent years, a wide range of trapped particle effects have been implicated in the behavior of SRS in this regime: detuning due to a kinetic frequency shift, beam modes, electron-acoustic Thompson scattering from these beam modes, and sidebands (the trapped-particle instability). Relatively little mention has been made of pump depletion. We will present results demonstrating that for some parameter ranges, pump depletion due to a convecting scattered packet is the primary mechanism for wave saturation. Furthermore, once pump depletion saturates the instability, the laser can still Raman scatter off the nonlinear, convective plasma wave groups. Once a localized pulse of plasma waves has convected out of the system, or traveled a distance sufficient for convective growth to recur, the instability may restart again. We also show that the behavior changes dramatically when the plasma length becomes much longer than the convective gain length. Work supported by DOE under DE-FG52-06NA26195 and NSF under NSF-Phy-0321345. Simulations performed on the DAWSON Cluster. [Preview Abstract] |
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BP8.00055: LPI Experiments at the Nike Laser* J. Weaver, J. Oh, B. Afeyan, L. Phillips, J. Seely, C. Brown, M. Karasik, V. Serlin, S. Obenschain, L.-Y. Chan, D. Kehne, D. Brown, A. Schmitt, A. Velikovich, U. Feldman, G. Holland, Y. Aglitskiy Advanced implosion designs under development at NRL for direct drive inertial confinement fusion incorporate high intensity pulses from a krypton-fluoride (KrF) laser to achieve significant gain with lower total laser energy (E$_{tot}\sim $500 kJ). These designs will be affected by the thresholds and magnitudes of laser plasma instabilities (LPI). The Nike laser can create short, high intensity pulses (t $<$0.4 ns; I$>$10$^{15}$ W/cm$^{2})$ to explore how LPI will be influenced by the deep UV (248 nm), broad bandwidth (2-3 THz), and induced spatial incoherence beam smoothing of the NRL KrF laser systems. Previous results demonstrated no visible/VUV signatures of two-plasmon decay (2$\omega _{p})$ for overlapped intensities $\sim $2x10$^{15}$ W/cm$^{2}$. We have increased the laser intensity and expanded the range of targets and diagnostics. Single and double pulse experiments are being planned with solid, foam, and cryogenic targets. In addition to spectrometers to study SRS, 2$\omega _{p}$, SBS, and the parametric decay instability, hard x-ray spectrometers (h$\nu >$2 keV) and a scintillator/photomultiplier array (h$\nu >$10 keV) have been deployed to examine hot electron generation. *Work supported by U. S. DoE. [Preview Abstract] |
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BP8.00056: Additional Considerations for Laser Plasma Instability Mitigation in Ignition-scale Hohlraums. William Kruer Control of laser plasma instabilities in ignition-scale hohlraums is an important physics challenge, Current hohlraums [1] are designed to minimize the linear instability gains of stimulated Raman and Brillouin backscatter. To complement this work, attention is here given to other possibilities for the excitation of laser plasma instabilities in large hohlraums. Topics addressed include excitation of the two plasmon decay instability, especially by the inner beams in the ablator plasma, as well as cooperative excitation [2] of stimulated scattering by overlapped beams near the laser entrance holes. Particular attention is given to estimating gains and identifying signatures for the cooperative scattering. It is also found that diffraction of the Raman-scattered light wave can reduce the stimulated Raman gain in a speckle but can improve the communication between different ranks of speckles. \newline [1] D. Callahan, N. Meezan, D. Hinkel, et. al., (private communication) \newline [2] D. DuBois, B. Bezzerides, and H. Rose, Phys. FluidsB4, 241(1992) [Preview Abstract] |
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BP8.00057: Diagnosing Large Simulations of Laser-Plasma Interaction for NIF ignition targets. Bruce Langdon, Denise Hinkel, Steve Langer, Bert Still, Ed Williams We have deployed a variety of diagnostics for the pF3d laser-plasma interaction (LPI) simulation code, which includes paraxial wave optics, multi-species hydrodynamics, and models for stimulated scattering. We present a survey of the diagnostics we use to process the data from the simulations and the directions of their development for very large massively-parallel simulations in support of upcoming 96 beam experiments at NIF next year and ignition. Two examples: Now that we can simulate over the entire beam path in the complex interior of an indirect-drive ignition target, we need to be able to form the spatial distribution of the power absorption of the laser and backscattered light. Such post-processing is itself a parallel processing endeavor due to the large number of spatial cells involved. To compare with experimental near-field streak spectra of backscattered and transmitted light, obtained at the ``full aperture backscatter stations'', we form synthetic near field streak spectra. For forensic purposes we can also calculate spectra inside the target, which are experimentally inaccessible. [Preview Abstract] |
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BP8.00058: SHAPE CONTROL, DIAGNOSTICS, REACTOR DESIGN TECHNOLOGY |
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BP8.00059: Microwave Cavities for Pellet Mass Detection on JET S.K. Combs, J.B.O. Caughman, L.R. Baylor, D.A. Rasmussen, A. Geraud, D. Homfray Resonant microwave cavities have been built for measuring the mass of pellets that will be created by the new high frequency pellet injector (HFPI) on JET. Two smaller cavities (TE010 mode) have been made for measuring 1 mm size pellets (for ELM mitigation), and two larger cavities (TM010 mode) have been made for measuring 4 mm size pellets (for fueling). Two of the cavities, one each for measuring both size pellets, will be placed near the injector, while the other two will be placed in curved guide tubes located closer to the JET vacuum vessel. Frozen deuterium pellets have been shot through all four cavities at ORNL, with signal levels ranging from 0.7 to 5.0 volts. Pellet mass is determined by measuring the frequency shift caused by the pellet traversing the cavity. The cavities have been calibrated against each other and as a function of their frequency response. Details of the design and testing results will be presented. [Preview Abstract] |
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BP8.00060: The Ignitor High Speed Pellet Injector$^*$ F. Bombarda, S. Migliori, A. Frattolillo, L.R. Baylor, J.B.O. Caughman, S.K. Combs, D. Fehling, C. Foust, J.M. McJill, G. Roveta A joint ENEA-Frascati and ORNL program for the development of a four barrel, two-stage pellet injector for the Ignitor experiment is in progress. At 4 km/s, pellets can penetrate close to the plasma center when injected from the low field side even for the plasma temperatures expected at ignition. Recent activities carried out at ORNL include improvements to the cryostat, the addition of miniature adjustable heaters in the the freezing zone, and of four close-coupled valves for rapid evacuation of gas after a shot. The LabView application software was successfully used to control the simultaneous formation of D$_2$ pellets, from 2.1 to 4.6 mm in diameter, that were launched at low speed. ORNL developed, specifically for this application, the light gate and microwave cavity mass detector diagnostics that provide in-flight measurements of the pellet mass and speed, together with its picture. The ENEA two-stage propelling system, now ready for shipping to ORNL, makes use of special pulse shaping valves, while fast valves prevent the propulsion gas from reaching the plasma chamber. Novel experiments, e.g. to create high pressure plasmas in existing devices using this innovative facility, have been envisioned and are being simulated. \newline $^*$Sponsored in part by ENEA of Italy and by the U.S. D.O.E. [Preview Abstract] |
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BP8.00061: Design of the IGNITOR Plasma Start-up and Scenarios$^*$ G. Ramogida, G. Cenacchi, A. Coletti, A. Cucchiaro, F. Villone, F. Rubinacci, B. Coppi IGNITOR is a high field, high plasma current compact experiment designed to be first to reach and study ignited plasma conditions. The design is characterized by a high degree of flexibility obtained by mean of a higher number of poloidal coils and a ``large'' volume available to the plasma relative to the machine overall dimensions. The most advanced operation scenario (11 MA, 13 T) is based on one that involves the optimal filling of the plasma chamber (``extended First Wall configuration''). The double X-point plasma configuration (X- points on the plasma chamber) enables it to reach ignition with a relatively modest amount auxiliary heating and a sufficient magnetic safety factor in the H-regime. This scenario involves a plasma current of 9 MA with the 13 T maximum toroidal field. Other plasma scenarios with reduced performances are based on a 9 T toroidal field and involve plasma currents of 7 or 6 MA, in the extended First Wall or the double X-point configuration, respectively. The plasma start-up phase has been carefully studied and an optimal choice of the poloidal field coils currents has led to obtaining a relatively large area with a nearly null and flat magnetic field, without reducing the available maximum flux swing (up to 36 Wb) from the Poloidal Field coils system. $^*$Sponsored in part by ENEA and the US D.O.E. [Preview Abstract] |
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BP8.00062: Advances in the IGNITOR Plasma Control$^*$ F. Villone, R. Albanese, G. Ambrosino, A. Pironti, F. Rubinacci, G. Ramogida, F. Bombarda, A. Coletti, A. Cucchiaro, B. Coppi The IGNITOR vertical position and shape controller has been designed on the basis of the CREATE${_-}$L linearized plasma response model, taking into account the engineering constraints of the machine and the features of the burning plasma regimes to be obtained. Special care has been devoted to the design of a robust control system, that can operate even when a degradation of the performance of the electro-magnetic diagnostics may occur. The coupling between the vertical position control and the plasma shape control has been analyzed, in order to allow the plasma vertical position to be stabilized also in the case where a shape disturbance is provoked by a change of the main plasma parameters. Simulations of the control system response have been carried out using realistic models of the electrical power supply system. The non-linear computation of equilibrium flux maps before and after the perturbation shows that the system is able to recover from all the assumed disturbances with this control scheme. In addition, the control of the plasma current and of the separatrix of the double-null plasma configuration is being studied.\\ $^*$Sponsored in part by ENEA and the US D.O.E. [Preview Abstract] |
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BP8.00063: Plasma Position Diagnostics for the Ignitor Experiment G. Pizzicaroli, F. Alladio, F. Bombarda, A. Licciulli, M. Fersini, D. Diso, E. Paulicelli Prototype coils of the electromagnetic diagnostics for the Ignitor experiment have been manufactured adopting innovative methods to improve the ceramic insulator resilience to neutron and gamma radiation. Thus, real time plasma position measurements should be possible over a broader range of high performance plasma regimes with D-D and D-T fuel. An alternative method is under study to provide the necessary spatial information also at the highest parameters that the Ignitor experiment can achieve ($B_T\simeq13$ T, $I_p\simeq11$ MA, neutron yield$\simeq 3\times10^{19}$ n/s), where the electromagnetic diagnostics may fail. The new instrument is based on the diffraction and detection of the soft X-ray radiation emitted at the plasma edge. Gas Electron Multiplier (GEM) detectors are considered as the best candidates to provide signals with high counting rates ($>$1 MHz) and high S/N ratios, to be used by the control system\footnote{D. Pacella, et al, \textit{Nucl. Instr. Meth. A} \textbf{508}, 414 (2003)}. A curved Multilayer Mirror placed inside one of the equatorial ports will diffract the radiation onto a properly shielded GEM detector that is located outside the machine vacuum and not in direct view of the plasma. [Preview Abstract] |
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BP8.00064: IGNITOR Plasma Chamber Assembly Procedure and Welding Processes$^*$ G. Toselli, G. Barbieri, B. Carmignani, G. Celentano, F. Cognini, A. Cucchiaro, U. De Maio, A. Ierino, T. Minghetti, G. Panzani, S. Sangiorgi, M. Timpanaro, D. Trestini, D. Visparelli, B. Coppi The appropriate welding techniques to be adopted for the assembly sequences of the 12 sectors of the Plasma Chamber, are described. The last welds, joining two assembled 180$\rm ^o$ sectors of the plasma chamber, need to be carried out automatically, at the inside of the Chamber, guided and controlled by the remote handeling system. The deformations and the displacements due to these welds have to be very limited in order to comply with the design geometry of the closed torus and its functions (e.g. support of the First Wall structure). Numerical simulation of the relevant welding processes have been carried out. Two different welding techniques have been chosen.\\ $-$Laser welding for the junction of 4 mm of the thickness of adjacent sectors of the plasma chamber \\ $-$TIG-NG welding with filler material for the remaining thickness \\ Experimental tests and corresponding simulations have been made, for both of these welding processes, on suitable samples which reproduce some aspects and geometrical characteristics of the chamber sectors. The most significant results obtained are described and discussed. \\ $^*$Sponsored in part by ENEA of Italy and by the U.S. D.O.E. [Preview Abstract] |
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BP8.00065: CATIA-V 3D Modeling for Design Integration of the Ignitor Machine Load Assembly$^*$ A. Bianchi, B. Parodi, F. Gardella, B. Coppi In the framework of the ANSALDO industrial contribution to the Ignitor engineering design, the detailed design of all components of the machine core (Load Assembly) has been completed. The machine Central Post, Central Solenoid, and Poloidal Field Coil systems, the Plasma Chamber and First Wall system, the surrounding mechanical structures, the Vacuum Cryostat and the polyethylene boron sheets attached to it for neutron shielding, have all been analyzed to confirm that they can withstand both normal and off-normal operating loads, as well as the Plasma Chamber and First Wall baking operations, with proper safety margins, for the maximum plasma parameters scenario at 13 T/11 MA, for the reduced scenarios at 9 T/7 MA (limiter) and at 9 T/6 MA (double nul). Both 3D and 2D drawings of each individual component have been produced using the Dassault Systems CATIA-V software. After they have been all integrated into a single 3D CATIA model of the Load Assembly, the electro-fluidic and fluidic lines which supply electrical currents and helium cooling gas to the coils have been added and mechanically incorporated with the components listed above. A global seismic analysis of the Load Assembly with SSE/OBE response spectra has also been performed to verify that it is able to withstand such external events. \\ $^*$Work supported in part by ENEA of italy and by the US D.O.E. [Preview Abstract] |
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BP8.00066: Ignition and Burning Plasma Regimes in the Double Null Configuration of Ignitor$^*$ G. Cenacchi, A. Airoldi, B. Coppi A new operating scenario for Ignitor with {$B_{T} \approx 13$ T, $I_{p} \approx 9$ MA} and a double X-point configuration (X- points just outside the first wall) has been investigated. The analyses carried out are directed to optimizing the plasma volume, the magnetic configuration and the relevant ``safety factor'' near the first wall. A transport analysis has been performed to simulate the current density evolution (important for the considered sequences of equilibrium configurations) and to verify the possibility of accessing H-regimes. The H-regime power threshold has been estimated from recent scalings based on a variety of experiments. This threshold power is consistent with that available from the provided ICRH system, combined with the Ohmic and $\alpha$-particle heating. In the numerical simulations a volume average density {$\langle n_{e} \rangle\approx 3\times 10^{20} \rm m^{-3}$}, an average {$Z_ {eff} \approx 1.5$}, and 3 MW of ICRH a power absorbed by the plasma have been considered. Ignition and advanced parameters as those expected for the ``standard'' 11 MA scenario with the ``extended'' first wall configuration of Ignitor can be reached. Even without accessing the H-regime and with pessimistic assumptions about the energy confinement time, plasma conditions of relevance to the physics of burning plasmas can be attained.\\$^*$Sponsored in part by ENEA and CNR of Italy and by the US D.O.E.\\ [Preview Abstract] |
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BP8.00067: ICRH Physics in the Ignitor Experiment$^*$ A. Cardinalli, G. Cenacchi, A. Airoldi, B. Coppi The Ignitor ICRH sytem can operate in a broad frequency range (80-120 MHz) and with significant levels of delivered power (4 to 12 MW). The frequency band is consistent with the use of magnetic fields in the range 9-13 T. In this work a review of the ICRH physics is presented for i) full performance scenarios, ii) reduced parameters scenarios, and iii) double X- point configurations at 13 T and 9 MA. In all cases the ICRH is used to control the plasma temperature, to accelerate the achievement of ignition in the extended first wall configuration ($I_p \cong$ 11 MA), and to help the transition to the H-regime in the X-point configuration. The power deposition profiles on ions and electrons are obtained by using a full wave code in a toroidal geometry configuration and are used as input data for a transport analysis. In particular, calculations show that a small fraction of $^3$He (1-2$\%$) improves the wave absorption on ions near the center of the plasma column, while a substantial fraction of the coupled power, owing to the n$_\|$-spectrum radiated by the antenna, is damped on the electrons in a broad radial interval of the plasma column. The conclusion is that in Ignitor, given the flexibility of its ICRH system, it is possible to control the plasma temperature and the transition to the H-mode in the X- point scenarios with relatively modest amounts of ICRH power ($<$ 8 MW).\\$^*$Sponsored in part by ENEA and by the U.S. D.O.E. [Preview Abstract] |
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BP8.00068: The IGNITOR ICRH Antenna Design with TOPICA Riccardo Maggiora, Volodymyr Kyrytsya, Daniele Milanesio, Orso Meneghini, Giuseppe Vecchi A flexible auxiliary Ion Cyclotron Resonance Heating (ICRH) system (f = 80 -- 120 MHz) has been included in the IGNITOR machine design. ICRH systems have been successfully tested on a number of existing devices especially at high density. Ignition can be accelerated significantly by relatively low levels of ICRH (about 5 MW, a fraction of the final fusion heating) when applied during the current ramp-up. In addition, ICRH provides a useful tool to control the evolution of the current density profile. Four antennas, each composed by 4 straps independently fed by 4 matching systems, can deliver a minimum RF power of about 12 MW in the entire adopted frequency range. The possibility of adding two more antennas has been considered. The antenna design and optimization have been based on the simulation results obtained with TOPICA (Torino Polytechnic Ion Cyclotron Antenna code)[1]. \newline \newline [1] V. Lancellotti et al., Nuclear Fusion, \textbf{46} (2006) S476-S499 [Preview Abstract] |
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BP8.00069: High temperature transient heating experiments on C in a Be seeded plasma in PISCES B. J. Hanna, D. Nishijima, R.P. Doerner, M. Baldwin, K.R. Umstadter, R. Seradarian, R. Hernandez, R. Pugno An experimental investigation of the effects of transient heating on Be films on C substrates in deuterium plasmas has been conducted in PISCES-B. It has been shown previously that Be film growth on C can form carbide layers that reduce the chemical erosion of C during deuterium ion bombardment. Results from transient heating up to 1200$^{\circ}$C have also been reported. In this presentation, results on the chemical erosion and on deuterium retention in C targets with heat pulses up to 2000$^{\circ}$C will be presented. A scaling expression for chemical erosion suppression due to Be2C formation developed previously will be extended to include transients with varying peak surface temperature and duty cycle using an integrated time-temperature, with a temperature range of 300 to 2000$^{\circ}$C. [Preview Abstract] |
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BP8.00070: The Effect of MHD Noise on the Vertical Observer in Tokamaks Gianpaolo Turri, Stefano Coda, Yves Martin, Jean-Marc Moret, Olivier Sauter The spurious perturbations induced by MHD instabilities on the Tokamak \`{a} Configuration Variable (TCV) vertical position observer are investigated. The study is performed for ITER-relevant ELMs (involving high heat fluxes, and with geometry compatible with the ITER constraints), sawteeth, and magnetic islands on the q=2-3 rational surfaces. In addition, ``infernal'' mode instabilities appearing in reverse shear are analyzed. A modified observer is calculated such as to minimize the MHD noise propagation for each case, based on three constraints: 1) equivalent response to a real vertical displacement event (VDE), based on VDE analysis; 2) minimized distance in the least squares sense from the optimized default observer; 3) orthogonality with the magnetic response to an MHD event. The result of the minimization is applied to the single instability for which it is designed, to verify the validity of the approach. In addition to that, mixed-mode observers are used to evaluate the effect of noise propagation in discharges with multiple modes. The results confirm that the design of a discharge-specific vertical observer should be sufficient for safely controlling the vertical position of the plasma in ITER. [Preview Abstract] |
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BP8.00071: X-ray diagnostic for monitoring the charge state distribution in an ECR ion source Brian Cluggish, Ioan-Niculae Bogatu, Liangji Zhao, Jin Soo Kim FAR-TECH, Inc. is developing a non-invasive X-ray spectral diagnostic to monitor the charge state distribution (CSD) in an electron cyclotron resonance ion source (ECRIS). The ECRIS is magnetic mirror confined plasma device in which electrons undergo ECR heating. Consequently the electron distribution function (EDF) develops a non-Maxwellian ``tail'' with energies over 10 keV. This in turn results in an ion CSD dominated by multiply charged ions. FAR-TECH, Inc. will determine the EDF by measuring the bremsstrahlung X-ray spectrum between 3 and 100 keV. The CSD can then be determined using our Generalized ECRIS Model (GEM) which calculates the rate of ionization to each charge state using the EDF. Measurements of the X-ray spectrum and the resulting EDF's will be presented. The simulations will be compared to Faraday cup measurements of the CSD of ions extracted from an ECRIS. [Preview Abstract] |
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BP8.00072: Ion Velocity Distribution Functions in a Compact, Expanding, Helicon Plasma Daniel Lewis, Alexander Hansen, Earl Scime Previous laser induced fluorescence (LIF) measurements of ion velocity distribution functions in a compact, expanding helicon plasma were limited by the available laser power and optical access [\textit{Keesee et al}., Phys. Plasmas \textbf{12}, 093502 (2005)]. Here we present LIF measurements of the ivdf in argon plasmas in the CHEWIE compact helicon source as a function of fill pressure, source magnetic field, and partial pressure of argon. The LIF measurements were accomplished with a ring dye laser tuned to 611.662 nm (vacuum wavelength) to pump the Ar II 3$d^{2}$G$_{9/2}$ metastable state to the 4$p^{2}$F$_{7/2}$ state and observing the fluorescent emission at 460.96 nm photons. RF power up to 600 W is used to create a steady state plasma in the 12 cm long, 6 cm diameter Pyrex source chamber. One end of the source chamber is connected to a 30 cm long, 15 cm diameter expansion chamber. The magnetic field strength ranges from 0 to 850 Gauss. We will present measurements of argon ivdfs for gas flow rates of 10, 20, 30 sccm at constant rf power and magnetic field. [Preview Abstract] |
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BP8.00073: Absorption Spectroscopy Measurements of Ion Velocity Distribution Functions in Argon Plasmas. Earl Scime, William S. Przybysz The scarcity of strong absorption lines in accessible tuning ranges along with plasma saturation due to low ion population densities makes absorption spectroscopy of helium ions notoriously difficult. Helicon plasmas, with their characteristically high ion densities, are a good candidate for initial helium ion spectroscopy experiments. However, preliminary measurements of Doppler broadened ion velocity distribution functions (ivdf) involving injecting a tunable infrared diode laser, tuned to 1012.36 nm and chopped roughly at 1kHz, along the axis of a 1.5m long helicon plasma have yielded erratic and irreproducible measurements. Here we present absorption spectroscopy measurements of ivdfs in argon helicon plasma using a tunable diode laser at 668.43 nm to pump the Ar II metastable 3d$^{4}$F$_{7/2 }$level to the 4p$^{4}$D$_{5/2}$ level. The optimized multi-pass optical configuration and the ratioing detector will be described and initial measurements presented. Once the absorption measurement technique is optimized for the well-known and more easily diagnosed Ar II transition, the same experimental configuration will then be used for the infrared helium ion absorption measurement [Preview Abstract] |
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BP8.00074: Optical diagnostic suite for measuring plasma velocity, laser-ablated metal topography and electron density Anthony Valenzuela, George Rodriguez, Steven Clarke We report on the capabilities of our diagnostic system to measure plasma properties including longitudinal velocity, topography of an ablation region and transverse electron density. We generate a plasma by ablating a thin metal film on a transparent substrate with a nanosecond millijoule laser pulse. We use a heterodyne-based Photonic Doppler Velocimeter (PDV) system to measure the velocity of the plasma plume. We also use a Shack-Hartmann interferometer (DOTS) to record the topography of the ablated metal. Also, we extended the functionality of DOTS to examine the transverse electron density of plasma channel in air generated by an intense, self-focused laser pulse. We compare the experimental data in the first two cases to hydrodynamic simulations to provide a feedback loop to improve our theoretical models. [Preview Abstract] |
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BP8.00075: Method to Estimate the Electron Temperature and the Neutral Density in a Plasma from Spectroscopic Measurements Using Argon Atoms and Argon Ions Collisional-Radiative Models Ella M. Sciamma, Roger D. Bengtson, Kevin E. Casey, W.L. Rowan, Amy M. Keesee, Charles A. Lee, Dan Berisford, Kevin Lee, Kenneth Gentle We present a method to infer the electron temperature in argon plasmas using a collisional-radiative model for argon ions [1] and measured electron density to interpret absolutely calibrated spectroscopic measurements of argon ion (Ar II) line intensities. The neutral density, and hence the degree of ionization of this plasma, can then be estimated using spectroscopic argon atoms (Ar I) line intensities and a collisional radiative model for argon atoms [2]. This method has been tested for plasmas generated on two different devices at the University of Texas at Austin: the helimak experiment and the helicon experiment. We present results that show good correlation with Langmuir probe measurements. \newline [1] http://adas.phys.strath.ac.uk \newline [2] Amy. M. Keesee and Earl E. Scime. Rev. Sci. Instrum. 77, 10F304 (2006). [Preview Abstract] |
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BP8.00076: A Novel X Ray Source for Cancer Radiotherapy Ilija Draganic, Jacek Capala, John Gillaspy There is a growing interest in exploring the possibility of replacing conventional broadband x-rays used in biomedicine with narrowband x-rays. In a form of binary therapy, drugs containing heavy elements are made to preferentially concentrate in cancer cells, and then the x-ray wavelengths are tuned to match the photo absorption peaks of the heavy elements. Synchrotrons can provide the necessary x-ray beams, but are impractical for routine therapy. An EBIS/T device may be a suitable alternative. EBIS/T devices can produce slow highly charged ions (HCI) which can be easily transported into the body through a cannula, where they produce relatively monoenergetic x rays within the tumor. In order to achieve the highest charge states with sufficient fluence for use in biomedicine, a better understanding of the EBIS/T ion trap dynamics may be required. This work will address the optimal ion temperatures, spatial distributions, densities, and related parameters of HCI in the trap using optical spectroscopy. [Preview Abstract] |
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BP8.00077: ASTROPHYSICAL PLASMAS: EXPERIMENT AND THEORY |
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BP8.00078: The Magnetothermal Instability And Its Role In Angular Momentum Transport in Hot, Dilute Magnetized Accretion Tanim Islam Recent observations have demonstrated the prevalence of underluminous accretion flows in massive and supermassive central galactic black holes, for which the best studied example is that of Sagittarius A* at the center of our Milky Way. In addition, circular polarization measurements of millimeter-wavelength radiation from Sagittarius A* has shown the existence of measurable magnetic fields in the source. These flows are characterized by the radiatively inefficient accretion of a hot, mildly collisional to highly collisionless, and optically thin plasma onto a black hole. The energy generated through the accretion of matter down a gravitational well cannot be efficiently radiated and therefore must be advected outwards. We show that the collisionless and mildly collisional MTI, an MHD mode of a dilute rotationally supported plasma, can destabilize these dilute, magnetized, radiatively inefficient flows and can carry out angular momentum and energy in order to allow accretion to occur. [Preview Abstract] |
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BP8.00079: Finite Larmor Radius Effects on the Magnetorotational Instability N.M. Ferraro The linear dispersion relation for the magnetorotational instability (MRI) has been derived including gyroviscosity, which represents finite Larmor radius (FLR) effects in the Braginskii equations [1]. It is shown that FLR effects are the most important effects in the limit of weak magnetic fields for ionized disks, and are much more important than the Hall effect when $\beta_i \gg 1$, where $\beta_i$ is the ratio of the ion thermal pressure to the magnetic pressure. FLR effects may completely stabilize even MRI modes having wavelengths much greater than the ion Larmor radius. Some implications for astrophysical accretion disks are discussed. The results of fluid simulations of accretion disks using M3D-$C^1$, a toroidal axisymmetric extended-MHD code which includes two-fluid and gyroviscous effects, are presented.\newline [1] N.~M. Ferraro, \textit{ApJ} \textbf{662}(1):512 (2007) [Preview Abstract] |
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BP8.00080: Poloidal rotation and its effect on toroidal rotation O.D. Gurcan, P.H. Diamond, T.S. Hahm A transport model, which describes the self-consistent evolution of poloidal and toroidal rotation in addition to density and pressure is suggested. The model is self consistent in the sense that Er shear is used for symmetry breaking, but its effect on turbulence is also considered. We also solve the poloidal ion momentum equation together with the radial force balance relation. The full study involves a rigorous gyrokinetic derivation of the model, and numerical solutions of the simple 1-D transport model. Notice that the simplicity of the model allows parameter scans that require many runs. The results of this study will be presented. [Preview Abstract] |
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BP8.00081: Initial results from the Princeton Magnetorotational Instability Experiment using liquid metal M.D. Nornberg, E. Schartman, H. Ji, M.J. Burin, W. Liu, J. Goodman The Magnetorotational Instability (MRI) is regarded as the dominant mechanism for accretion disk turbulence and its associated angular momentum transport. A series of experiments using both water and liquid metal in a novel wide-gap Taylor-Couette apparatus are conducted to elucidate the relative importance of sub-critical hydrodynamic turbulence to the MRI. Reynolds stress measurements using two-component laser Doppler velocimetry in water demonstrate that keplerian-like flows have extremely weak angular momentum transport even at Reynolds numbers up to $10^6$ when end effects are suppressed by differentially rotating end caps. By switching the working fluid to a liquid Gallium alloy (Ga-In-Sn), this quiescent flow can be destabilized by applying an axial magnetic field of up to 5~kG. The growth rate of the MRI is determined from external magnetic field measurements using radially-aligned induction coils and Hall probes. Secondary circulation induced by the MRI, which is a local instability, is distinguished experimentally from magnetically-induced Ekman circulation generated by the boundary layers through comparisons between flows which are stable and unstable to the MRI. This work is supported by DOE, NASA, and NSF. [Preview Abstract] |
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BP8.00082: The Princeton MagnetoRotational Instability (MRI) Experiment - Apparatus and Diagnostics Ethan Schartman, Mark Nornberg, Hantao Ji, Michael J. Burin, Jeremy Goodman The Princeton MRI experiment investigates instabilities believed to be responsible for angular momentum transport in accretion disks. The apparatus consists of fluid confined between a pair of concentric spinning cylinders. The shear flow developed shares with accretion disks the properties of linear hydrodynamic stability and outwardly-decreasing angular velocity. Onset of instability in this apparatus is relevant to studies of turbulent transport in astrophysical disks. Using water or a liquid Gallium alloy we investigate Subcritical Hydrodynamic Instability (SHI) or the MRI. The cylinder end caps are divided into two pairs of differentially-rotatable rings to reduce the impact of the vertical boundaries on the bulk flow. When using water the Reynolds stress is directly measured using 2-component Laser Doppler Velocimetry. During Gallium operation a 5kG axial magnetic field is applied. Radial motions of the fluid generate a fluctuating radial component of the magnetic field which is detected by an array of magnetic B-dot coils external to the flow. For the largest flow shear in our apparatus the radial fields will be generated by the MRI. To test the operation of the B-dot coils the magnetic field is applied to a Rayleigh-unstable flow. Supported by DOE, NASA and NSF. [Preview Abstract] |
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BP8.00083: Axisymmetric Simulation of the Magnetorotational Instability in a Magnetized Taylor-Couette Flow Wei Liu, Jeremy Goodman, Hantao Ji The magnetorotational instability (MRI) is probably the main cause of turbulence and accretion in sufficiently ionized astrophysical disks. Despite much theoretical and computational work, however, the nonlinear saturation of the MRI is imperfectly understood. We present non-ideal magnetohydrodynamic simulations of the Princeton MRI experiment. In vertically infinite or periodic cylinders, MRI saturates in a resistive current-sheet with significant reduction of the mean shear, and with poloidal circulation scaling as the square root of resistivity. Angular momentum transport scales as the reciprocal square root of viscosity but only weakly depends on resistivity. For finite cylinders with insulating end caps, a method to implement full insulating boundary condition is introduced. MRI grows with a clear linear phase from small amplitudes at rates in good agreement with linear analysis. In the final state one inflowing ``jet" opposite to the usual Ekman ``jet" is found near the inner cylinder. Angular momentum transport has a weaker scaling with Reynolds number and is dependent hardly on Lundquist number. Under proper condition our experimental facility is a good testbed to show that MRI could be suppressed by a strong magnetic field. [Preview Abstract] |
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BP8.00084: Numerical Studies of Boundary Layers in the Princeton MRI Experiment Austin Roach, Hantao Ji, Wei Liu, Jeremy Goodman The Princeton MRI experiment uses a Taylor-Couette apparatus to generate rotating shear flows for the investigation of the magnetorotational instability. Discrepancies have been observed between the experimentally measured fluid flow profiles and those expected from hydrodynamic simulation of the experimental apparatus with the 2-dimensional code ZEUS-2D. While experimental adjustments have been made to produce the desired flow profiles in the apparatus for the investigation of the MRI, an explanation for the difference between the experimental measurements and computational prediction of the fluid flows has not yet been found. An attempt is now being made to account for these differences by adding to the simulations additional effects, such as noisy boundary layers and a more detailed description of the geometry of the experiment. Results of these computational investigations and a comparison to experimental results will be presented. [Preview Abstract] |
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BP8.00085: An Experimental Study of Turbulent Boundary Layers in Free-Surface MHD Flows J. Luc Peterson, Mark Nornberg, Hantao Ji, Alex Gill, Dimitris Giannakis Descriptions of the turbulent boundary layers in astrophysical and laboratory plasmas require an understanding of free-surface magnetohydrodynamic (MHD) stability. The dynamics of the plasma ocean on the surface of nuetron stars in binary systems could relate to X-Ray burst phenomena, while turbulent flows in liquid metal diverters are useful for effective heat transport from the core of a fusion reactor. The Liquid Metal eXperiment (LMX) at PPPL is a small-scale laboratory experiment using liquid gallium alloy designed to study free-surface MHD stability and wave propagation. LMX is a short wide-aspect ratio channel (1 by 15 by 70 cm) designed for Reynolds numbers of 10000 under an imposed magnetic field of 7 kG. Extensive hydrodynamic experiments have demonstrated the ability to create stable turbulent boundary layers in short open channels. The transition to liquid metal operations will be discussed, with particular attention paid to new flow velocity diagnoistic tools and techniques for creating stable turbulent boundary layers. This work supported by DoE under contract \#DE-AC02-76- CH03073. [Preview Abstract] |
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BP8.00086: Laboratory Study of Magnetorotational Instability (MRI) in a Helicon Plasma H. Ji, J. Foley, F. Levinton, B. Fetroe, Y. Raitses, J. Kefeli, M. Nornberg, S. Zweben, M. Yamada Fast angular momentum transport in accretion disks has been an outstanding problem in astrophysics for more than three decades. Classically estimated transport due to molecular viscosity of a neutral fluid is too small to account for the fast observed accretion rates. The magnetorotational instability (MRI) has been identified as a powerful mechanism to transport angular momentum. Experiments using liquid metal are underway to study the MRI in the incompressible MHD limit. A new frontier in accretion disk research is to explore physics beyond the incompressible MHD. Possible new effects include compressibility, multiple-fluid effects, kinetic effects, ion-neutral collisions, radiation pressure, and dust grains. In order to study some of these effects, a new, small-scale experiment using a helicon plasma has been constructed. A preliminary analysis, addressing the two-fluid or Hall effect based on a local Hall MHD formulation, shows large differences in the growth rate between the cases when magnetic field is parallel and anti-parallel to the rotation axis for the experimentally achieved parameters. This is a clear sign of Hall effects on MRI. The detailed analyses and experimental results will be presented when available. [Preview Abstract] |
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BP8.00087: Magnetorotational instability in electrically driven flow: theoretical predictions and experimental observations Ivan Khalzov, Andrei Smolyakov, Victor Ilgisonis The electrically driven flow of liquid metal in circular channel is efficient way to test magnetorotational instability (MRI) in laboratory. The main body of this flow has the equilibrium rotation law $\Omega(r)\propto1/r^2$, which is stable hydrodynamically but can be unstable with respect to MRI. We study numerically the linear stability of such flow in the circular channel of finite hight in the presence of vertical magnetic field in the frame of dissipative incompressible magnetohydrodynamics (MHD). Marginal stability curves in the plane Hartmann number -- Reynolds number are calculated for the range of azimuthal wave-numbers $m=0\div200$. It is shown that for larger Hartmann numbers the threshold of instability is determined by modes with higher $m$. Our numerical results are found to be in a good agreement with available experimental data. [Preview Abstract] |
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BP8.00088: Physics of Plasma Accreting Structures$^*$ F. Rousseau, B. Coppi Plasma accretion is considered to take place within thin differentially rotating structures, (sequence of density rings\footnote {B. Coppi and F. Rousseau, \textit{Ap. J.}, \textbf{641}, 458 (2006)}) in the prevalent gravity of a central object where the vertical confinement is provided by the Lorentz force associated with internal toroidal currents. The factors that are needed to complete the solution of the equations$^1$ that describe ``ring configurations'' are identified and included in the relevant analysis. The relationship between poloidal flows and ``seed'' magnetic fields is uncovered and analyzed\footnote {B. Coppi and F. Rousseau, Paper O4.034, {Proceedings of the 2007 E.P.S. Conference on Plasma Physics} and MIT-LNS Report 07/06}. The significance of the symmetries of the poloidal currents that are found to be associated with the presence of an effective viscosity is pointed out. The problem of having a radial inflow velocity in a two-dimensional configuration with internal currents has been dealt with in the limit of very small ``seed'' magnetic fields by finding a narrow family ``open'' magnetic surfaces on which the plasma can spiral toward the central object$^2$. $^*$Sponsored in part by the US D.O.E. [Preview Abstract] |
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BP8.00089: Non-axisymmetric Ballooning Modes in Thin Accretion Structures Chris Crabtree, Bruno Coppi The consideration of non-axisymmetric modes in thin accretion structures has been shown to be relevant to the issue of angular momentum transport [1]. To this end we consider the stability of differentially rotating, $u_{\phi}=R\Omega(R)$, axisymmetric equilibria with cylindrical magnetic flux surfaces to perturbations with large toroidal mode numbers $n_0\gg 1$. Assuming the equilibrium plasma rotation is approximately Keplerian about a central compact object and the rotation speed is smaller than the thermal speed, the equilibrium pressure and density depend on both $R$ and $z$. We seek linear solutions to the ideal MHD equations by expanding in the small parameter $1/n_0$ and satisfying requirements similar to those used for ballooning modes in magnetically confined toroidal plasmas: 1) the wavelength parallel to the magnetic field is relatively small, 2) the mode approximately corotates with the plasma, and 3) the compressibility is small but non-zero. Normal mode solutions are constructed using a radially dependent toroidal mode number which is inversely proportional to the rotation frequency such that $k_{\phi} \sim n_0 \Omega_0/u_{\phi}(R)$ where $\Omega_0$ is a constant. Toroidally periodic perturbations are then constructed with a formalism that is typically used in the theory of ballooning modes in toroidal configurations to enforce periodicity in the poloidal direction in the presence of magnetic shear. [1] {B. Coppi, P. Coppi {\it Ann. Phys.} \textbf{291}, 134 (2001)} *Supported by U.S. D.O.E. [Preview Abstract] |
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BP8.00090: Modeling the spectral evolution of prompt GRBs and X-ray flares S. Pothapragada, S. Reynolds, S. Graham, M.V. Medvedev We use the detailed theory of jitter radiation from relativistic shocks containing small-scale magnetic fields and relativistic shock kinematics to build a numerical model of spectral variability of GRB emission. It is, then, applied to the conditions of the internal shocks in order to model the prompt phase and X-ray flares. We derive the lighcurves, spectral evolution in time within each sub-pulse of a prompt GRB and during an X-ray flare. Correlations of spectral parameters are also deduced. We demonstrate that the model lightcurves and spectra agree well with observation data. We discuss how one can deduce certain parameters of the plasma of the shock and the ejected material. [Preview Abstract] |
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BP8.00091: Phase-space distribution of accelerated electrons in Weibel-mediated relativistic GRB shocks S. Graham, S. Pothapragada, S. Reynolds, M.V. Medvedev The shock model of gamma-ray bursts (GRBs) contains two equipartition parameters: the magnetic energy density and the kinetic energy density of the electrons relative to the total energy density of the shock, $\epsilon_B$ and $\epsilon_e$, respectively. These are free parameters within the model. Whereas the Weibel shock theory and PIC simulations fix $\epsilon_B$ at the level of $\sim$few$\times(10^{-3}...10^{-4})$, no understanding of $\epsilon_e$ existed until recently. Medvedev (2006) has demonstrated that it inevitably follows from the Weibel shock theory that $\epsilon_e\simeq\sqrt{\epsilon_B}$. Extrapolating the theory to GRB afterglow shocks, we find that observational data agree with our theoretical prediction. It has been suggested that the $\epsilon_e-\epsilon_B$ relation can be used to reduce the number of free parameters in afterglow models. Here we further develop the model of non-Fermi acceleration of electrons in prompt GRBs. We developed a numerical code, which computes full phase space distribution of electrons in Weibel electromagnetic fields. This distribution is further used to compute the electron energy distribution, the distribution over pitch-angle, the angular pattern of jitter emissivity, etc. [Preview Abstract] |
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BP8.00092: Jitter radiation produced by electrons with anisotropic distribution in Weibel turbulence of GRBs and lab experiments S. Reynolds, S. Graham, S. Pothapragada, M.V. Medvedev Radiation emitted by relativistic electrons propagating through the random, skin-depth-scale magnetic fields produced by the Weibel instability is referred to as the Jitter radiation. These fields are associated with current filaments and, hence, their spatial distribution is anisotropic. This anisotropy of fields has been shown to result in anisotropic radiation spectrum (even if the electron distribution is isotropic) that depends on the angle between the line of sight and the direction of current filaments. The study of the electron distribution in the Weibel turbulence shows that the electron PDF is also anisotropic, with the direction of anisotropy being along the filament orientation. Therefore, accurate calculation of the emitted spectra shall account for the PDF angular structure as well. Here we formulate the jitter radiation theory in a general set-up, thus generalizing previous results to an arbitrary PDF. We calculate model spectra and discuss implications of the results to emission from ultra-relativistic shocks of gamma-ray bursts and to Laboratory Astrophysics experiments with petawatt-scale lasers. [Preview Abstract] |
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BP8.00093: Transport Properties of Laser Plasma and H-Rich White Dwarf Stars Vithal L. Patel, Jaechul Oh An extended survey of spectroscopic observations of hydrogen-rich white dwarf stars indicates existence of magnetic fields of 10kG-10MG [1]. High intensity laser interactions with matter generates magnetic fields of several 100s MG [2]. In both laboratory laser plasma as well as astrophysical plasmas, weakly Landau quantization may be present. We consider transport properties of such plasmas following work of Potekhin [3]. With some simplification, estimate of transport coefficients can be made without evaluation of elaborate integration. This research was performed in Laser Plasma Branch, Plasma Physics Division, Naval Research Laboratory and was supported by DOE/NNSA. \newline [1] A. Kawka et. al., ApJ, 654, 499, 2007 \newline [2] S. Eliezer et. al., Phys. Plasmas, 12, 052115, 2005 \newline [3] A. Y. Potekhin, Astron. Astrophys., 346, 345, 1999 [Preview Abstract] |
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BP8.00094: Numerical study of ultra-relativistic electromagnetic filamentation in boosted frames S.F. Martins, R.A. Fonseca, W.B. Mori, L.O. Silva We address the simulation of relativistic shocks in astrophysics, namely the numerical implications of ultra-relativistic particles and the large time/space scales associated with these systems. It was recently shown [1] that performing simulations in optimized Lorentz frames can decrease simulation run times by orders of magnitude, completely changing computational resources required. The Lorentz transformation for a boosted frame was implemented in osiris 2.0 [2] and encompassed several difficulties that will be discussed. For instance, transformation of quantities back to the laboratory frame may require massive data handling and complex diagnostic/visualization, and can cancel the potential gains from the time scale reduction due to the boost. The possibility of using a boosted frame to suppress numerical noise (e.g. due to numerical Cerenkov radiation) will also be explored. \newline [1] J.-L. Vay, PRL 98, 130405 (2007) \newline [2] R. A. Fonseca et al, Lecture Notes in Computer Science 2329, III-342 (Springer-Verlag, 2002) [Preview Abstract] |
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BP8.00095: Colliding Laser-Produced Plasmas on LaPD Andrew Collette, Walter Gekelman The expansion and interaction of dense plasmas in the presence of a magnetized background plasma is important in many astrophysical processes, among them coronal mass ejections and the many examples of plasma jets from astrophotography. Turbulence is expected to be present in many such configurations. We describe a series of experiments which involve the collision of two dense (initially, $n > 10^{15}{\mbox{cm}}^{-3}$) laser-produced plasmas within an ambient, highly magnetized plasma. The laser-produced plasmas form diamagnetic cavities in which a large percentage of the background magnetic field (600G) has been expelled. First-stage observations using a fast (3ns exposure) camera indicate complicated structure at late times, in addition to coherent corrugated structures on the bubble surfaces. The data hint at the presence of turbulence in the interaction. The second stage of observation consists of direct investigation of the magnetic field using probes. A novel diagnostic system composed of small (300-500 micron) 3-axis differential magnetic field probes in conjunction with a ceramic motor system capable of extremely fine (sub-micron) positioning accuracy is currently under development. An ensemble of magnetic field data from fixed and movable probes makes possible the calculation of the cross-spectral function. [Preview Abstract] |
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BP8.00096: Three-dimensional RAGE Simulations of Strong Shocks Interacting with Sapphire Balls B.H. Wilde, R.F. Coker, P.A. Rosen, J.M. Foster, P. Hartigan, R. Carver, B.E. Blue, J.F. Hansen The goal of our 2007-2008 NLUF experiments at the OMEGA laser facility is to investigate the physics associated with the interaction of strong shocks and jets with clumpy media. These experiments have close analogs with structures observed in a variety of astrophysical flows, including jets from young stars, outflows from planetary nebulae, and extragalactic jets. In these experiments, a multi-mega bar shock is created in a plastic layer by heating a hohlraum to 190 eV temperature with 5 kJ of laser energy. The shock enters a 0.3 g/cc RF foam into which are embedded 500 micron diameter sapphire balls. The shock shears off the ball such that it creates thin two-dimensional sheets of sapphire which subsequently break up and undergo the three-dimensional Widnall instability (Widnall, S. E., Bliss, D. B., {\&} Tsai, C. 1974, J. Fluid Mech., 66, 35). The time evolution of the ball/balls is diagnosed with dual-axes point-projection radiography. In this poster, we discuss the results of high-resolution three-dimensional radiation-hydrodynamic simulations with the adaptive-mesh-refinement RAGE code of single and multiple balls. Comparisons with data from our August shots will be made. [Preview Abstract] |
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BP8.00097: Using Hydrodynamic Codes in Modeling of Multi-Interface Diverging Experiments for NIF M.J. Grosskopf, R.P. Drake, C.C. Kuranz, T. Plewa, N. Hearn, D. Arnett, C. Meakin, A.R. Miles, H.F. Robey, J.F. Hansen, B.A. Remington, W. Hsing, M.J. Edwards Using the Omega Laser, researchers studying supernova dynamics have observed the growth of Rayleigh-Taylor instabilities in a high energy density system. The NIF laser hopes to generate the energy needed to expand on these experiments to a diverging system. We report scaling simulations to model the interface dynamics of a multilayered, diverging Rayleigh-Taylor experiment for NIF using a combination of 1D and 2D Hyades, a Lagrangian 3-temperature, 1-fluid hydrodynamic simulation code used within the high energy density physics community, and CALE, a hybrid adaptive Lagrangian-Eulerian code developed at LLNL and used extensively throughout the hydro community. The simulations will assist in the target design process and help choose diagnostics to maximize the information we receive in a particular shot. This will be critical given that early experiments on NIF will get few shots and take a considerable amount of time and money to prepare. [Preview Abstract] |
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BP8.00098: Effect of relaxation and radiation transport on the structure of electro-magnetically driven strong shock waves Kotaro Kondo, Mitsuo Nakajima, Tohru Kawamura, Kazuhiko Horioka Strong shocks appear in many astrophysical phenomena, such as supernova remnants. Ion-electron relaxation process and radiative transport affect the structure of strong shock waves. Study on these phenomena should contribute to get the well understanding of astrophysical phenomena. We investigate electro-magnetically driven shock in the laboratory experiments. The pulse power device with tapered electrodes can generates a quasi steady and 1-D shock [1], which enables to analyze ion-electron relaxation and radiation processes. The shock Mach number can be up to 250, which correspond to 45 km/s. We will discuss the structure of electro-magnetically driven shock, especially ion-electron relaxation process accompanied with radiation transport, based on experimental results. \newline \newline [1] K. Kondo, M. Nakajima, T. Kawamura and K. Horioka, Rev. Sci. Instr. \textbf{77}, 036104 (2006). [Preview Abstract] |
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BP8.00099: Laboratory experiments to study supersonic astrophysical flows interacting with clumpy environments P.A. Rosen, J.M. Foster, B.H. Wilde, R. Coker, B.E. Blue, R.J.R. Williams, F. Hansen, C. Sorce, P. Hartigan, R. Carver, J. Palmer A wide variety of objects in the universe drive supersonic outflows through the interstellar medium which is often highly clumpy. These inhomogeneities affect the morphology of the shocks that are generated. The hydrodynamics is difficult to model as the problem is inherently 3D and the clumps are subject to a variety of fluid instabilities as they are accelerated and destroyed by the shock. Over the last two years, we have been carrying out experiments on the University of Rochester's Omega laser to address the interaction of a dense-plasma jet with a localized density perturbation. More recently, we have turned our attention to the interaction of a shock wave with a spherical particle. We use a 1.6-mm diameter, 1.2-mm length Omega hohlraum to drive a composite plastic ablator (which includes bromine to prevent M-band radiation from preheating the experiment). The ablator acts as a ``piston'' driving a shock into 0.3 g/cc foam containing a 0.5-mm diameter sapphire sphere. We radiograph along two orthogonal lines of sight, using nickel or zinc pinhole-apertured x-ray backlighters, to study the subsequent hydrodynamics. We present initial experimental results and multi-dimensional simulations of the experiment. [Preview Abstract] |
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BP8.00100: Plasma Jet and Shock Experiments Using High-Power Lasers Youichi Sakawa, A. Oya, S. Dono, T. Kimura, N. Ozaki, Y. Kuramitsu, T. Kato, H. Nagatomo, K. Shigemori, R. Kodama, T. Norimatsu, H. Takabea, B. Loupais, M. Koenig, J. Waugh, N. Woolsey We investigate laboratory laser-plasma experiments to address questions connected to the formation and collimation of astrophysical jets and shock waves. In particular, we plan to scale our experiments to non-relativistic jets associated with Young Stellar Object. Experiments were performed with Gekko / HIPER laser system (3w, 500 ps, 300 - 750 J, $<$ 10$^{15}$ W/cm$^{2})$ at ILE, Osaka Univ. Several types of targets were used; 10 $\mu $m thick plane or 600 $\mu $m diameter hemisphere CH shell targets with an Au-cone, foam-filled cone targets targets, and hemisphere CH. These targets were used to create plasma jets. In order to produce a shock wave, a He gas jet system was placed on the bottom on the vacuum chamber. The plasma jet and shock were measured with a Mach-Zender interferometry diagnostic, using a probe laser pulse in the transverse direction; an ICCD camera with 200 ps gate time and a S1 streak camera were used. As rear-side diagnostics of self-emission, we used HISAC and SOP for time-evolution measurements of 2-D self-emission profile and temperature, respectively. In all types of the targets, clear jets were observed. [Preview Abstract] |
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BP8.00101: Hydrodynamic and atomic-kinetic modeling of photoionised neon plasmas Iain Hall, Tunay Durmaz, Roberto Mancini, Jim Bailey, Gregory Rochau, Michael Rosenburg, David Cohen, Igor Golovkin, Joseph MacFarlane, Manolo Sherrill, Joseph Abdallah, Robert Heeter, Mark Foord, Siegfried Glenzer {\small Photoionised plasmas are common in astrophysical environments and in recent years, high resolution spectra from such sources have been recorded by the Chandra and XMM-Newton satellites. These have motivated several recent efforts to understand the atomic-kinetic and radiative characteristics of such plasmas in detail. The Z-pinch facility at the Sandia lab is the most powerful terrestrial source of X-rays and provides an opportunity to produce photoionised plasmas in a well characterised radiation environment. We present modeling work and experimental design considerations for a forthcoming experiment at Sandia in which the X-ray emission from a collapsing Z-pinch will be used to photoionise low density neon contained in a gas cell. View factor calculations were used to evaluate the radiation environment at the gas cell. These were used to design shielding which maximises the contribution of Z-pinch emission to the total X-ray flux incident on the gas cell. The Helios-CR code was used to examine the hydrodynamic characteristics of the gas cell, in particular looking at the heating, temperature and ionisation of the neon and the absorption of radiation by the plastic walls enclosing the gas.} [Preview Abstract] |
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BP8.00102: Chaotic magnetic fields due to asymmetric current configurations - application to cross-field diffusion of particles in cosmic rays B. Dasgupta, A.K. Ram The observed cross-field diffusion of charged particles in cosmic ray transport is assumed to be due to chaotic nature of the interplanetary/intergalactic magnetic fields. The particles are accelerated and energized by the temporal fluctuations of the magnetic field. The generation of chaotic magnetic fields is {\it ad hoc} and the characteristics of the fields are chosen to satisfy the observations. We consider simple current configurations consisting of circular loops and straight wires that generate asymmetric, nonlinear, steady-state magnetic fields in three spatial dimensions. These magnetic fields are completely deterministic, and, for certain range of parameters, chaotic. We will present analytical and numerical studies on the generation of chaotic magnetic fields and the nature of these fields. The motion of charged particles in these magnetic fields can be described by the Lorentz equation. An analysis of the particle motion will also be presented. A particle moving in a chaotic magnetic field superposed on a uniform background magnetic field is found to undergo spatial transport. This shows that chaotic magnetic fields can produce cross-field diffusion. [Preview Abstract] |
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BP8.00103: Dynamics of Cosmic Ray acceleration in a modified shock Alexander Pushkin, Mikhail Malkov, Patrick Diamond Backreaction of accelerated cosmic rays (CRs) on the structure of the shock which supports their acceleration results in bifurcation of stationary solutions. Therefore the process of nonlinear shock acceleration of CRs in strong astrophysical shocks cannot be fully understood within a steady state approach. We analyze stability and evolution of the structure of a nonlinearly modified shock in the framework of a zero-dimensional time dependent system of ODEs. The system is derived from the convection-diffusion equation which describes the acceleration and transport of cosmic rays coupled to the plasma flow profile near the shock. The main variables of the resulting system are the shock strength (in form of subshock and precursor compression ratios) and the maximum CR energy. A self-consistent dependence of the injection rate on the subshock compression is included. This study enables: (i) selection of possible acceleration regimes (ii) analysis of stability of different steady state solutions (iii) time dependent description of the dynamics of coupled CR-shock system. [Preview Abstract] |
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BP8.00104: Relativistic MHD Jets and Their Interactions with the Intra-cluster Medium Hui Li, Masa Nakamura, Shengtai Li, Hao Xu We present the formulation of relativistic MHD flows, likely occurring in the environment of accretion onto supermassive black holes. Three-dimensional relativistic MHD simulations will be presented on how the energy outflow will partition among different physical components and on the collimation and stability of such systems. The interaction between such flows with their environment, e.g., the intra-cluster medium, will be discussed. [Preview Abstract] |
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BP8.00105: Deformation of a magnetized plasma jet upon collision with a neutral gas cloud A.L. Moser, P.M. Bellan The Caltech spheromak formation and astrophysical jet simulation experiment uses a coplanar spheromak gun with magnetically linked concentric electrodes. This geometry and the electric field applied between the electrodes provide an analog to the accretion disk environment and allow production of collimated plasma jets similar to those found in astrophysical settings. The experiment has been used to study the evolution of a plasma jet when it collides with a localized neutral gas cloud. The experiment produces a collimated, magnetized dense plasma jet that propagates through a very low pressure environment before colliding with a high density neutral gas cloud. A high speed camera and a magnetic probe array characterize the physical and magnetic field structure of the plasma jet with and without a target gas cloud present. [Preview Abstract] |
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BP8.00106: MHD kink instability driven by differential rotation Christopher Carey, Carl Sovinec, Sebastian Heinz Recent observations of extragalactic outflows from active galactic nuclei suggest that some of these jets maintain a large scale helical magnetic structure [1]. The kink instability is known to create similar magnetic structures in laboratory plasmas. Thus, extragalactic jets may resemble a screw pinch topology and be susceptible to the current driven kink instability. We are conducting numerical MHD simulations which will address the issues of collimation and stability of the extragalactic jet system. In these simulations an initial seed field is twisted by a differentially rotating flow boundary condition. Three dimensional nonlinear calculations show that the magnetic column produced is kink unstable and that the instability saturates to a helical magnetic structure. The kink instability in the numerical system leads to conversion of the toroidal magnetic flux, which is injected by the differentially rotating boundary, to poloidal magnetic flux. Examination of this flux conversion process could lead to a better understanding of how the jet distributes magnetic energy to the medium which surrounds it. A synthetic diagnostic has been implemented for calculating the synchrotron emission of the numerical jet. This synthetic synchrotron emission is compared to observations of actual extragalactic jet systems. \newline [1] D.M. Worrall, M. Birkinshaw, et. al., Mon. Not. R. Astron. Soc., May, 2007 [Preview Abstract] |
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BP8.00107: Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes A.G. Lynn, Y. Zhang, S.C. Hsu, H. Li, W. Li, M. Gilmore, Christopher Watts Recent work in plasma astrophysics has suggested that magnetic energy features prominently in the large-scale evolution of active galaxies. The Plasma Bubble Expansion Experiment (PBEX) will conduct laboratory experiments and coordinated numerical modeling to address outstanding nonlinear plasma physics issues related to how magnetic energy and helicity carried by extra-galactic jets interacts with the intergalactic medium to form extra-galactic radio lobe structures. Experiments will be conducted in the 4 meter long, 50 cm diameter HELCAT linear plasma device at UNM. A new pulsed coaxial gun will form and inject magnetized plasma bubbles into a lower pressure background plasma formed by the helicon and/or hot cathode source in HELCAT. Experimental parameters will be adjusted so that important dimensionless parameters are relevant to the astrophysical context. Preliminary magnetic probe measurements and MHD modeling will be presented. [Preview Abstract] |
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BP8.00108: Instabilities and Structure Evolution in Radiative Shocks F.W. Doss, R.P. Drake, A.J. Visco, C.C. Kuranz, M.J. Grosskopf, A.B. Reighard, J. Knauer Radiative shocks, systems in which radiation transport across the shock front contributes substantially to the properties and dynamics of the shock, occur frequently in astrophysical systems, motivating our high-energy-density experiments. Recent laser-driven experiments have produced collapsed shocks by launching 10-20 $\mu$m drive disks of Be into shock tubes of Xe gas at atmospheric pressure. This method produces strongly radiative shocks at well over 100 km/sec. Experiments using x-ray pinhole radiography of collapsed radiative shocks have revealed evidence of structure evolution, perhaps through instability mechanisms. Recent experiments provided simultaneous normal and oblique data. Theoretical work related to structure growth will also be reported. This research was sponsored by the NNSA through DOE Research Grants DE-FG52-07NA28058, DE-FG52-04NA0064, and the NNSA Stewardship Science Graduate Fellowship. [Preview Abstract] |
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BP8.00109: DYNAMOS AND HELIOPHYSICAL PLASMA PHYSICS |
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BP8.00110: Overview of Recent Upgrades to the Madison Dynamo Experiment Elliot Kaplan, Cary Forest, Roch Kendrick, Carlos Parada, Zane Taylor, Mark Nornberg, Erik Spence The Madison Dynamo Experiment is designed to function as a simply-connected, homogeneous dynamo. A turbulent flow of liquid sodium is driven by two counter-rotating impellers in a one-meter-diameter sphere. The experiment is presently undergoing upgrades to it's magnetic diagnostics and seed field coils to better refine the measurement of turbulence driven currents. A high current amplifier, to drive the experiment's seed magnetic field coils, is under development that will be able to generate a $>$200 gauss sinusoidal magnetic field in the .1-5 Hz frequency band. The current wave form is generated by applying pulse-width-modulated square waves to a set of four IGBT switches in an H Bridge configuration which allows the current to flow in either direction through the external field coils. The duty cycle is determined through one of two methods: An analog circuit generates a reference sine wave and a modulating triangle wave in an intersective PWM circuit; a Labview Realtime control that uses a PID feedback loop to calculate the duty cycle. This is replacing the present system of a single IGBT turning on a DC current through the coils. The primary physics goal for this hardware is to measure the electrical skin depth of large scale magnetic perturbation and unravel the nature of the turbulent resistivity of the experiment. [Preview Abstract] |
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BP8.00111: Direct Numerical Simulation of Mechanically Driven Turbulent Dynamos in Spherical Geometry Klaus Reuter, Frank Jenko, Cary Forest, Adam Bayliss A parallel version of a nonlinear pseudo-spectral MHD code for the simulation of liquid metal dynamos in spherical geometry was developed using a domain decomposition technique. The parallel code exhibits ideal scaling going up to 8 CPUs on shared-memory machines. At 16 CPUs, it still achieves efficient speedups between $14$ and nearly $16$. Given today's computational speed, it is now possible to resolve fluid Reynolds numbers of $\mathrm{Re} \approx 4000$ in the simulations, whereas previous serial computations were limited to $\mathrm{Re} \approx 1500$. Direct numerical simulations are performed to explore the dynamo threshold $\mathrm{Rm_{crit}}$ (the critical magnetic Reynolds number) in $\mathrm{Re}$-$\mathrm{Rm}$-space for the flow profile of the Madison Dynamo Experiment. The shape of $\mathrm{Rm_{crit}}$ has been determined up to $\mathrm{Re} \approx 3000$. Furthermore, the code was adapted to model the driving of a new generation of dynamo experiment using plasma instead of liquid metal. By employing pure toroidal driving in two thin counter-rotating hemispherical shells along the walls, numerical simulations show that the system reaches a quasi-stationary state with a self-excited magnetic field. [Preview Abstract] |
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BP8.00112: Optimization of flows in the Madison Dynamo Experiment N.Z. Taylor, C.B. Forest, N.S. Haehn, E.J. Kaplan, R.D. Kendrick, C.A. Parada, C.R. Weber, M.D. Nornberg, E.J. Spence In the Madison Dynamo Experiment two counter-rotating impellers in a one meter-diameter sphere drive turbulent flows of liquid sodium. One goal of the experiment is to observe the spontaneous onset of a large scale dynamo in the presence of background turbulence. The time-averaged flows created by the impellers are expected to be dynamos (assuming that the time-averaged flows are laminar). The role of the fluctuations is expected to increase the threshold for self-excitation. To observe a dynamo in the experiment, two sets of optimizations are being implemented. First, the mean-flow is being optimized using a combination of a Computational Fluid Dynamics (CFD) simulation of the experiment coupled with an eigenmode code. The CFD simulation predicts the velocity field due to the addition of vanes mounted on the outer wall to change the overall pitch of the mean flow. The vane pitch has been optimized by minimizing the critical magnetic Reynolds number. The second optimization is to reduce the overall turbulent fluctuation amplitude by introducing an equatorial baffle to partially separate the two hemispheres of the experiment. These optimizations are in the process of being tested in a water version of the experiment and are being implemented in the sodium experiment. [Preview Abstract] |
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BP8.00113: A Plasma, Magnetorotational Instability Experiment C. Collins, C.B. Forest, R. Kendrick, A. Seltzman A new experiment is underway at the University of Wisconsin to investigate the magnetorotational instability in a plasma. Magnetorotational instability (MRI) is a likely mechanism that could account for the observed accretion rates in astrophysical objects. The instability occurs when a weak magnetic field is present, so that tension in perturbed field lines transfers angular momentum outward while mass moves towards the center. In the Plasma Dynamo Experiment Prototype, a cylindrical, axisymmetric, ring cusp confinement geometry is used to produce a large unmagnetized plasma, confined by a highly localized magnetic field at the plasma boundary. The plasma is stirred by a novel axisymmetric electrode set that can control the rotation (angular momentum profile). The feasibility of observing the MRI will be discussed and initial results from a protoype experiment will be presented. [Preview Abstract] |
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BP8.00114: A Plasma Dynamo Experiment based upon Ring Cusp Confinement and Electrostatic Stirring Cary Forest, Gennady Fiksel, Noah Hershkowitz, Roch Kendrick, Steve Cowley, Erik Spence A new plasma experiment to investigate the self-generation of magnetic fields is proposed. The experiment consists of a spherical vacuum chamber with a series of permanent magnetics, with electrically insulated pole faces, in a ring cusp geometry (poles facing inward with alternating polarity along the vessel wall). The resulting field is axisymmetric and decays quickly away from the walls providing a large, magnetic field free region in the central region of the device. To stir the plasma, cathodes positioned between the magnet rings are biased such that the resulting electric field induces plasma rotation through the ExB drift. The flow drive principle is quite general and by controlling the poloidal profile of the toroidal rotation, high magnetic Reynolds number plasmas flows can in principle be generated that result in magnetic field self-generation or plasma flows unstable to the magnetorotational instability. Use of a plasma for such an experiment may allow the magnetic Reynolds number (the dimensionless parameter governing self-excitation of magnetic fields) to be approximately a factor of 10 larger than in liquid metal experiments and will be the first experiment to investigate self-excited dynamos in a plasma,the state of matter that makes up most naturally occuring astrophysical dynamos. [Preview Abstract] |
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BP8.00115: Measuring and imaging bulk flows in laboratory plasma loops E.V. Stenson, P.M. Bellan Arched plasmas similar to solar coronal loops are made in the lab by means of a magnetized plasma gun. These plasma structures are created in a process resembling that used to make spheromaks, exhibit behavior that is also seen in the sun, and demonstrate some very general flow phenomena. It has been proposed that in a current-carrying flux tube with nonuniform cross-section, plasma jets flow from more constricted to less constricted regions (P. M. Bellan, Phys. Plasmas 10, 1999 (2003)). By making arched plasmas from two different gas species - one at each of the two footpoints of the arch - we see that this is indeed the case. High-speed imaging with optical filters reveals a jet emanating from each footpoint. With velocities on the order of the Alfven speed, these jets move much faster than both the sound speed of the neutral gas and the thermal velocity of the ions. The technique of using two gases will next be used for experiments wherein two adjacent plasma arches merge. Each will be made of a different gas, so that the process by which the two combine can be resolved. [Preview Abstract] |
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BP8.00116: Observing Energetic Bursts in the Caltech Solar Coronal Loop Simulation R.J. Perkins, G.S. Yun, P.M. Bellan X-ray bursts have previously been observed in a solar coronal loop experiment (J.F. Hansen, S.K.P. Tripathi, and P.M. Bellan, Phys. Plasma 2, 3177(2004)) where two parallel plasma-filled flux tubes merged in either a co-helicity or counter-helicity arrangement. These x-ray bursts were observed with a set of x-ray photo-diodes. We are developing means to observe these bursts with additional diagnostics. A low cost photo-electric detector was thoroughly tested on a test chamber using a xenon flashlamp. The detector utilizes the low work function of magnesium to measure ultraviolet radiation. A photo-scintillator is being developed to detect hard x-ray emission down to 10 keV. A smaller photo-scintillator was previously constructed and used in the Caltech spheromak experiment; we have enlarged the scintillating volume in hopes of increasing sensitivity. Finally, in a single loop experiment, spectroscopic measurements detect the onset of oxygen impurity lines at the loop apex; the onset is simultaneous with the formation of a bright spot at the same location. Future spectroscopic measurements are planned to investigate the apex region during merging. [Preview Abstract] |
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BP8.00117: Measurement of Electrical Currents in the Solar Corona Steven Spangler Some theories for heating of the solar corona invoke Joule heating by electrical currents. Observations of spatially-extended radio sources through the corona show times when there is a difference in the Faraday rotation between two lines of sight separated by about 33,000 km in the corona. Ampere's Law is used to relate these observations to the presence of electrical current flowing between the two lines of sight. I infer a current of $2.5 \times 10^9$ Amperes in the case of the strongest signal, and a current of $2.3 \times 10^8$ Amperes in another, marginally significant detection. A model of coronal current sheets is used to interpret the current measurements, and estimate the volumetric heating rate due to Joule dissipation. The model uses the Spitzer resistivity. The model heating rate is approximately 6 orders of magnitude less than independent, observational estimates in the relevant part of the corona. Either the currents detected play a negligible role in coronal heating, or the effective resistivity in the corona is 6 orders of magnitude larger than the Spitzer value. [Preview Abstract] |
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BP8.00118: Lagrangian Simulations of Current Sheet Formation During Relaxation of an Unstable Line-Tied Equilibrium Liwei Lin, C.S. Ng, A. Bhattacharjee Our recent theory, based on reduced MHD equations, predicts the formation of current sheets (tangential discontinuities) in an ideal line-tied plasma when an unstable equilibrium relaxes to a state of minimum energy [C. S. Ng and A. Bhattacharjee, Phys. Plasmas {\bf 5}, 4028 (1998)]. This mechanism has important implications for the heating of the solar corona, first envisioned by E. N. Parker. Testing of this prediction using conventional Eulerian simulations is subjected to the intrinsic numerical difficulty that the magnetic field line mapping is not kept fixed explicitly, as required by the line-tied condition. In fact, field line mapping can change substantially by reconnection due to numerical resistivity. To overcome this obstacle, we have developed a Lagrangian relaxation algorithm to simulate the evolution of an unstable equilibrium by following the movement of magnetic field lines explicitly. Preliminary simulation results will be presented. [Preview Abstract] |
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BP8.00119: Solar Coronal Heating and Magnetic Energy Build-Up in a Tectonics Model M. Gilson, C.S. Ng, A. Bhattacharjee Observations from SOHO and TRACE have shown that the solar surface is covered with a so-called ``magnetic carpet,'' in which small-scale magnetic flux loops are continually emerging and interacting. The magnetic flux at the photosphere is thus being replaced in every 10-40 hours. This magnetic carpet has important implications for the problem of coronal heating. We have extended a tectonics model of coronal heating [E. Priest, J. Heyvaerts and A. Title, Astrophys. J. {\bf 576}, 533 (2002)] and shown, based on analysis and numerical simulations, that the heating rate is independent of the Lundquist number as well as the photospheric coherence time, if the magnetic footpoints are subject to random photospheric motion. We have also found that magnetic energy can be built up to a statistically high level before the energy is released by some mechanisms, such as instabilities and/or magnetic reconnection. We have also shown that even if such processes limit the build-up of magnetic energy, the overall heating rate is still independent of the Lundquist number. [Preview Abstract] |
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BP8.00120: The Effect of Magnetic Turbulence Energy Spectral Scaling on the Heating of the Solar Wind D. Munsi, C.S. Ng, A. Bhattacharjee, P.A. Isenberg Recently, a phenomenological solar wind heating model based on a turbulent energy cascade prescribed by the Kolmogorov theory has produced reasonably good agreement with observations on proton temperatures out to distances of the order of 70 AU, provided the effect of turbulence generation due to pickup ions is included in the model. Without the inclusion of pickup ions, the Kolmogorov scaling law appears to predict a proton temperature profile that drops off too rapidly with radial distance from the Sun. In this study, we have incorporated in the heating model the energy cascade rate based on Iroshnikov-Kraichnan (IK) scaling, derivable from incompressible magnetohydrodynamics. We show that the model can produce significantly higher proton temperatures, within the range of observations, with or without the inclusion of pickup ions. Moreover, the turbulence correlation lengths prescribed by IK scaling seem to follow better the trend of observations, as compared with previous results based on Kolmogorov scaling, which showed a qualitatively different trend. [Preview Abstract] |
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BP8.00121: Alignment of Velocity and Magnetic Fluctuations in Simulations of Anisotropic MHD Turbulence C.S. Ng, A. Bhattacharjee There has been recent theoretical interest in the effect of the alignment of velocity and magnetic fluctuations in three-dimensional (3D) MHD turbulence with a large-scale magnetic field [Boldyrev 2005, 2006]. This theory predicts that the angle $\theta$ between the velocity and magnetic fluctuation vectors has a scaling of $\theta \propto \lambda^{1/4}$, where $\lambda$ is the spatial scale of the fluctuations. There have also been simulations on 3D forced MHD turbulence that supports this prediction [Mason {\em et al.} 2006, 2007]. The scaling has also been tested against observations of solar wind turbulence [Podesta {\em et al.} 2007]. We report here simulation results based on decaying 2D turbulence. The scaling of $\theta \propto \lambda^{1/4}$ and Iroshnikov-Kraichnan scaling has also been observed within a range of time interval and spatial scales, despite the fact that Boldyrev's theory was developed for fully 3D turbulence in the presence of a strong external field. As the external field is reduced in magnitude and becomes comparable to the magnitude of magnetic fluctuations or lower, the scale-dependent alignment is weakened. Implications for observations of solar wind turbulence will be discussed. [Preview Abstract] |
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BP8.00122: Scattering of Suprathermal Electrons in the Solar Wind: Particle-in-Cell Simulations S. Peter Gary, Shinji Saito Properties of the narrow, magnetic-field-aligned strahl electron velocity distributions are sensitive indicators of collisionless processes in the solar wind. Three distinct signatures have been observed in the characteristics of this suprathermal (70 eV $<$ Energy $<$ 1 keV) component: 1) Pitch-angle widths that decrease with increasing energy, 2) Pitch-angle widths that increase with increasing energy, and 3) Pitch-angle widths that have a distinct maximum as a function of energy. This presentation describes results from particle-in-cell simulations which have used three different sources of enhanced fluctuations to demonstrate how each of these signatures can arise. Signature 1) is well-known as being due to scattering by Coulomb collisions, but the simulations have shown that it may also arise as a consequence of scattering by the whistler anisotropy instability driven by a $T_{\perp}/T_{\parallel} >$ 1 condition on the electron core component. Signature 2) has been shown by quasilinear theory to arise due to scattering by a broadband spectrum of whistler fluctuations; our simulations confirm that conclusion. Signature 3) arises from scattering due to the electrostatic electron/electron instability. The simulations demonstrate how the latter two signatures change with various plasma parameters. [Preview Abstract] |
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BP8.00123: Remote Measurements of Ion Temperatures in the Terrestrial Magnetotail Amy Keesee, Earl Scime The plasma in the terrestrial magnetotail plays a central role in magnetospheric storms and substorms. Reconnection in the magnetotail yields flows in the tailward and earthward directions, redistribution of energetic particles throughout the inner magnetosphere, and possibly direct ion heating by waves. The magnetotail, out to 30-40 Earth radii, lies in the field of view of the instruments on the Imager for Magnetopause-to-Aurora Global Explorer (IMAGE) satellite when the spacecraft is in a favorable position in its orbit. \textit{McComas et al.} (2002) showed that the Medium Energetic Neutral Atom (MENA) imager onboard IMAGE measures significant neutral flux from this region during periods of intense magnetospheric activity, i.e., when the plasmasheet density is enhanced by plasma injections from the solar wind and ionospheric outflows. We present remote ion temperature measurements calculated from MENA neutral flux measurements from 1 -- 60 Earth radii during a substorm on 4-5 October 2000 (DOY 278-279). During the evolution of the substorm, a wave of increasing ion temperature appears to propagate earthward through the magnetotail. [Preview Abstract] |
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BP8.00124: Hybrid Simulations of Mini Magnetospheres in the Laboratory Luis Gargate, Ruth Bamford, Robert Bingham, Ricardo Fonseca, Luis Silva We use a massively parallel 3D hybrid particle code, dHybrid, to simulate the deflection of plasma beams by a dipole like magnetic field in a laboratory environment. Dipole magnetic fields, along with a plasma injection source to inflate the magnetic field, are now being studied as means of deflecting solar wind and Energetic Particles away from spacecrafts [1,2,3]. We have considered three setups, consistent with the experiments, with a plasma beam fired at i) a dipole field with no plasma injection, ii) a plasma injection source with no dipole field and iii) a dipole field with a plasma injection source. The hybrid simulations help understand the relevant physical phenomena, and enable extrapolation to space plasma scenarios, where setups are similar but plasma parameters differ significantly. The simulation results consistently show the plasma beam being deflected by the dipole field, in the first scenario, with the deflecting distance determined by the magnetic field intensities. The other two scenarios are also studied via hybrid simulations and the main physical differences between setups are highlighted. Comparisons with experimental results are discussed. [1] D. Winske et al, Phys. Plasmas 12 (2005) [2] Hai-Bin Tang et al, Phys. Plasmas 14 (2007) [3] http://www.ukssdc.ac.uk/twiki/bin/viewauth/Minimag/WebHome [Preview Abstract] |
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BP8.00125: 3D hybrid simulation of the Titan's plasma environment Alexander Lipatov, Edward Sittler Jr., Richard Hartle Titan plays an important role as a simulation laboratory for multiscale kinetic plasma processes which are key processes in space and laboratory plasmas. A development of multiscale combined numerical methods allows us to use more realistic plasma models at Titan. In this report, we describe a Particle-Ion--Fluid-Ion--Fluid--Electron method of kinetic ion-neutral simulation code. This method takes into account charge-exchange and photoionization processes. The model of atmosphere of Titan was based on a paper by Sittler, Hartle, Vinas et al., [2005]. The background ions H$^+$, O$^+$ and pickup ions $H_2^+$, $CH_4^+$ and $N_2^+$ are described in a kinetic approximation, where the electrons are approximated as a fluid. In this report we study the coupling between background ions and pickup ions on the multiple space scales determined by the ion gyroradiis. The first results of such a simulation of the dynamics of ions near Titan are discussed in this report and compared with recent measurements made by the Cassini Plasma Spectrometer (CAPS, [Hartle, Sittler et al., 2006]). \newline E C Sittler Jr., R E Hartle, A F Vinas, R E Johnson, H T Smith and I Mueller-Wodarg, J. Geophys. Res., 110, A09302, 2005.\\ R E Hartle, E C Sittler, F M Neubauer, R E Johnson, et al., Planet. Space Sci., 54, 1211, 2006. [Preview Abstract] |
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BP8.00126: Simulations of plasmas pentrating magnetic barriers Herbert Gunell, Tomas Hurtig, Mark Koepke, Nils Brenning, Hans Nilsson Perturbed currents perpendicular to the magnetic are generated by plasma motions in which the equilibrium magnetic field (and the corresponding equilibrium currents) are compressed, stretched, and deformed. One example of this is the Earth's magnetopause with its ever-present equilibrium transverse currents and its strong perturbations. Experiments have recently been performed using a plasma gun to shoot a plasma at a magnetic barrier (Brenning, et al., PoP, 2005). It was found that, at a critical drift that is about 2-3 times the ion thermal speed, non-linear oscillations in the lower hybrid range give rise to a resistivity which is at least 200-300 times the Spitzer resistivity. We present simulations of the above scenario for different values of the plasma kinetic energy density. We find waves with frequencies on the order of the plasma frequency. These waves contribute to the electron heating that has been observed both in the experiments and in previous simulations (Hurtig, et al., PoP, 2003). [Preview Abstract] |
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BP8.00127: Spectral Characteristics of Weakly-Collisional Stationary Alfv\'en Waves S.M. Finnegan, M.E. Koepke, D.J. Knudsen The spectral properties of weakly-collisional stationary Alfv\'en (StA) waves are presented. StA waves, stationary electromagnetic structures generated by plasma flow across magnetic field-aligned current sheets, have a stationary parallel electric field structure that can energize electrons along magnetic field lines [Knudsen, \textbf{JGR} (1996)]. Knudsen's model has been generalized to include collisional and thermal effects. Ion-neutral collisions are shown to introduce long-wavelength \textbf{k}-space spectral features which produce ``quasi-dc'' field-aligned electron acceleration for both StIA and stationary kinetic Alfv\'en (StKA) waves. Ion- neutral collisions are also shown to broaden spectral features, without shifting the dominant perpendicular wavenumber for StIA waves. For StKA waves both ion-neutral and electron collisions broaden spectral features and increase the number of Fourier components. StIA wave spectra and spatial scales are consistent with measurements of dispersive Alfv\'en wave turbulence observed by various rocket and satellite missions. [Preview Abstract] |
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BP8.00128: Generation of Lower Hybrid Waves in an Oxygen Dominated Plasma Manish Mithaiwala, Leonid Rudakov, Gurudas Ganguli A previous work considered the generation of ULF waves in the inner Magnetosphere due to a heavy ion ring distribution such as Lithium, Cesium or Barium [Ganguli et al., 2007]. We extend this analysis by considering the generation of Lower-hybrid waves in an Oxygen dominated plasma at R$\sim $500km due to a Barium ion ring distribution. In this situation it is possible to generate ion-Bernstein modes or Lower-hybrid waves. We demonstrate the criteria in which Lower-hybrid waves are produced. The growth rate is found to be strongly dependent on the ion thermal velocity. For a broad range of parameters the growth rate $\gamma >\Omega _{Ba} $ so that the Barium ions are considered to be unmagnetized which simplifies the analysis. The analysis is compared with space experiments done several decades ago. Ganguli, G., L. Rudakov, M. Mithaiwala, and K. Papadopoulos (2007), Generation and evolution of intense ion cyclotron turbulence by artificial plasma cloud in the magnetosphere, \textit{J. Geophys. Res.}, $112$, A06231, doi:10.1029/2006JA012162. [Preview Abstract] |
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BP8.00129: Laboratory experiments to investigate auroral cyclotron emission processes Sandra McConville, Kevin Ronald, Alan Phelps, David Speirs, Karen Gillespie, Adrian Cross, Colin Whyte, Craig Robertson, Robert Bingham, Barry Kellett, Irena Vorgul, Alan Cairns In the auroral regions of the Earth's magnetosphere, particles are accelerated downwards into an increasing magnetic field. Magnetic compression leads to the formation of a velocity distribution in the shape of a horseshoe due to conservation of the magnetic moment. Kilometric radiation is observed in association with this process, polarised in the X-mode. The RF output power has been estimated at 10$^{7}$-10$^{9}$W, corresponding to a beam-wave conversion efficiency of 1-2{\%}. A cyclotron maser instability driven by the horseshoe distribution is thought to be the source of this \textbf{A}uroral \textbf{K}ilometric \textbf{R}adiation (AKR). A scaled laboratory experiment was created, to simulate this naturally occurring phenomenon. Measurements of the radiation conversion efficiency, mode and spectral content were obtained and seen to be in close agreement with numerical predictions and also with satellite observations in the magnetosphere. The experiment is currently being modified by introducing a background plasma to give a better representation of the natural environment. The latest results of this modification shall be presented. [Preview Abstract] |
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BP8.00130: Ionospheric Interaction Experiments With Gigawatts J.P. Sheerin, J.M. Gerres, M.E. Bacon, B.J. Watkins, W.A. Bristow, S.I. Oyama, C.J. Heinselman, K.M. Groves High power HF transmitters have induced a number of plasma instabilities in the interaction region of overdense ionospheric plasma. We report results from the first such experiments to use over one gigawatt of HF power (ERP) in comprehensive studies of strong Langmuir turbulence (SLT) and particle acceleration. Among the effects observed and studied are: SLT spectra including the outshifted plasma line or free-mode, appearance of a short timescale ponderomotive overshoot effect, artificial field-aligned irregularities (AFAI), the aspect angle dependence of the intensity of the plasma line, and suprathermal electrons. We explore the observed magnetic-zenith effect of enhanced turbulence backscatter with the HF pump wave directed up the field line. Experimental results are compared to previous high latitude experiments and predictions from recent modeling efforts. [Preview Abstract] |
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