Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session GP6: Poster Session III: Magnetic Reconnection; Waves, Oscillations, and Instabilities; Nonlinear Phenomena, Turbulence and Transport; Magnetic Fusion Technology |
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Room: Marsalis A/B, 9:45am - 12:45pm |
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GP6.00001: MAGNETIC RECONNECTION |
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GP6.00002: A four-field model for collisionless reconnection: Hamiltonian structure and numerical simulations Emanuele Tassi, Daniela Grasso, Francesco Pegoraro A 4-field model for magnetic reconnection in collisionless plasmas is investigated both analytically and numerically. The model equations are shown to admit a non-canonical Hamiltonian formulation with four infinite families of Casimir invariants [1]. Numerical simulations show that, consistently with previously investigated models [2,3], in the absence of significant fluctuations along the toroidal direction, reconnection can lead to a macroscopic saturated state exhibiting filamentation on microsocopic scales, or to a secondary Kelvin-Helmholtz-like instability, depending on the value of a parameter measuring the compressibility of the electron fluid. The novel feature exhibited by the four-field model is the coexistence of significant filamentation with a secondary instability when magnetic and velocity perturbations along the toroidal direction are no longer negligible. An interpretation of this phenomenon in terms of Casimir invariants is given.\\[0pt] [1] E. Tassi et al., Plasma Phys. Contr. Fus., 50, 085014 (2008)\\[0pt] [2] D. Grasso et al., Phys. Rev. Lett. 86, 5051 (2001)\\[0pt] [3] D. Del Sarto, F. Califano and F. Pegoraro, Phys. Plasmas 12, 012317 (2005) [Preview Abstract] |
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GP6.00003: Electron self-reinforcing process and nonsteady state magnetic reconnection Weigang Wan, Giovanni Lapenta We study the evolutions of collisionless nonsteady state magnetic reconnection with full kinetic particle-in-cell simulations. Simulations are setup for scenarios of forced reconnection with open boundary conditions. We find the change of reconnection rate is not empowered or dependent on the length of the EDR. During the early growing stage, the EDR is elongated while the reconnection rate is growing. During the later stage, the reconnection rate may significantly decrease but the length of the inner EDR is largely stable. Reconnection is controlled by the availability of plasma inflows from upstream, consistent with our previous discovery of the electron self-reinforcing process that drives fast reconnection [W. Wan and G. Lapenta, Phys. Rev. Lett. {\bf 101}, 015001 (2008)]. The Hall current induced by the quadrupole magnetic field is discovered to play an important role in this process through the electron pressure tensor. The electron super-Alfv\'{e}nic outflow jet structure could be elongated during the bipolar stage, and remains stable during steady state. The sufficiency of the electron inflow is crucial for the elongation. [Preview Abstract] |
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GP6.00004: Particle simulation of current sheet instabilities under finite guide field Xueyi Wang, Yu Lin, Liu Chen, Zhihong Lin The instability of a Harris current sheet under a broad range of finite guide field $B_{ G}$ is investigated using a gyrokinetic electron and fully kinetic ion particle simulation code. In this particle model, the rapid electron cyclotron motion is removed, while the realistic mass ratio \textit{mi }/\textit{me}, finite electron Larmor radii, and wave-particle interactions are kept. Firstly, a linearized $\delta f$ GKe/FKi simulation is carried out in the 2-D plane containing the guide field along $y $and the current sheet normal along $z$. It is found that for a finite $B_{ G}$/$B_{ x0}\le $1, where $B_{x0}$ is the asymptotic antiparallel component of magnetic field, three unstable modes, i.e., modes A, B, and C, can be excited in the current sheet. Modes A and C, appearing to be quasielectrostatic modified two-stream instability/whistler mode, are located mainly on the edge of the current sheet. Mode B, on the other hand, is confined in the current sheet center and carries a compressional magnetic field $B_{ y}$ perturbation along the direction of electron drift velocity. In the cases with extremely large $B_{ G}$/$B_{ x0}>>$1, the wave modes evolve to a globally propagating instability. Secondly, the effects of $k_{x}$ is calculated. Finally, nonlinear $\delta f$ GKe/FKi simulation is conducted to study the nonlinear physics of the unstable modes in the current sheet.. [Preview Abstract] |
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GP6.00005: Kinetic Simulations of Asymmetric Reconnection Michael Shay, Paul Cassak, Chris Bard Asymmetric magnetic reconnection is reconnection with different inflow conditions (density and magnetic field magnitude). A recent paper\footnote{Cassak, P. A. and M. A. Shay, Physics of Plasmas, 14, 102114, 2007.} predicted scaling relations for the diffusion region during asymmetric reconnection which were verified with MHD simulations. The structure of the diffusion region during kinetic reconnection, however, is less well understood. We will present kinetic PIC simulations of asymmetric reconnection with a focus on understanding the structure of the kinetic diffusion region. [Preview Abstract] |
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GP6.00006: Numerical Simulations of Turbulent MHD Reconnection Nuno Loureiro, Dmitri Uzdensky, Alexander Schekochihin, Tarek Yousef Magnetic reconnection is a very important process in a large number of laboratory, space, and astrophysical plasmas. It is usually believed that in the resistive MHD regime, the reconnection rate obeys the classical Sweet-Parker scaling, proportional to the square root of the resistivity, and is thus very slow. Whether reconnection can be significantly accelerated in the presence of MHD turbulence\footnote {A.~Lazarian \& E.~Vishniac, ApJ, 517, 700 (1999).} is still an unresolved question. In this study, we use high-resolution incompressible resistive magnetohydrodynamic numerical simulations to investigate the effect of externally imposed small-scale turbulence on the reconnection of a large-scale magnetic field. We characterize the turbulent enhancement of the reconnection rate over the laminar Sweet-Parker rate as a function of the resistivity, turbulent driving scale and amplitude. [Preview Abstract] |
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GP6.00007: Particle Simulations of Collisionless Magnetic Reconnection in Turbulent Medium H. Karimabadi, W. Daughton, V. Roytershteyn, J. Scudder Many instances of reconnection occur in systems where turbulence is ubiquitous. Examples include solar wind, dayside magnetopause, coronal heating of turbulent accretion disks, the dynamo in the sun's convection zone, and turbulent tokamak plasmas during disruptions. The studies of reconnection in turbulent medium has been traditionally based on resistive MHD and Hall MHD limits but to the best of our knowledge we are not aware of any fully kinetic treatment of this problem. Here we present the first kinetic simulations of reconnection in turbulent medium where the effects of turbulence is modeled through wave-particle scattering. A comparison of the results with simulations with binary collisions is presented. We show results as a function of turbulence level and demonstrate the changes in the reconnection layer and associated reconnection rates. [Preview Abstract] |
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GP6.00008: Influence of Coulomb Collisions on Undriven Magnetic Reconnection in Large-Scale Systems W. Daughton, B.J. Albright, V. Roytershteyn, K.J. Bowers, L. Yin, H. Karimabadi The influence of Coulomb collisions on magnetic reconnection is studied in neutral sheet geometry using fully kinetic particle-in-cell (PIC) simulations in which binary collisions are modeled by a Monte-Carlo technique.\footnote{T. Takizuka and H. Abe, J. Comput. Phys., v. {\bf 25}, p. 205, 1977} This approach describes a full Fokker-Planck collision operator and thus permits a first-principles study of the transition between collisionless and collisional reconnection. For sufficiently collisional regimes, this approach recovers the well-known Sweet-Parker scaling from resistive MHD. As the collisionality is reduced, a transition to faster reconnection rates is observed and the structure of the layer is dramatically altered. Although certain aspects of this transition are consistent with expectations from two-fluid theory, there are significant differences in the structure and time dependence within the weakly collisional regime. In particular, there is a basic tendency for the diffusion region to form an elongated current sheet for all collisionality regimes. For large-scale systems, these elongated current layers are unstable to secondary-island formation leading to a time-dependent reconnection process for both collisional and collisionless regimes. Results are presented to illustrate how the structure of the diffusion region is modified by Coulomb collisions and how the reconnection dynamics scales with system size in the various regimes. [Preview Abstract] |
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GP6.00009: Local Dynamics and Global Size Coupling during Magnetic Reconnection C.M. Jacobson, J.A. Breslau, S.C. Jardin, H. Ji Magnetic reconnection is an important physical process not only in small systems such as laboratory plasmas, but also in large systems such as solar flares. The reconnection rate increases with resistivity $\eta$ and decreases with the current sheet length $L$. Recent experimental results suggest that these parameters are not independent, but anti-correlate such that $\eta L$ is kept roughly constant; thus the reconnection rate is a function of both local dynamics and global size [1]. In order to verify these results and further extend the system size, a numerical MHD model [2] is used. This code allows simulation of either two-fluid or single-fluid resistive MHD reconnection of colliding flux tubes on a 2D grid. The resistivity and system size are systematically varied, and scalings of the ion skin depth, collisionality, and reconnection rate due to these quantities are presented. Results are compared to experimental data, and findings are projected to solar flare scales. [1] A. Kuritsyn \textit{et al.} Geophys. Res. Lett. 34, L16106 (2007) [2] J. A. Breslau and S. C. Jardin, Comput. Phys. Commun. 151, 8 (2003) [Preview Abstract] |
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GP6.00010: Laboratory demonstration of the onset threshold for 3D Sweet-Parker reconnection Xuan Sun, Tom Intrator, Giovanni Lapenta, Ivo Furno Magnetic reconnection is important in the magnetic self-organization of plasmas in many diverse fields of physics that include plasmas, magnetic fields, and current systems. It is one of these processes that can transport, generate, and restructure magnetic field, plasma flow, and thermal energy. Nearly all theoretical and experimental work on magnetic reconnection starts with boundary or initial conditions where the state of driven reconnection is the presumed starting point. But in nature and the laboratory the onset of reconnection must be forced naturally by some evolution of plasma mediated forces. How and why the onset of reconnection occurs is commonly recognized as a scientifically urgent question. Using two flux ropes, we show an experimental example of self consistent, kink instability driven, onset of 3D Sweet Parker reconnection. The local magnetic and kinetic pressure pile up and and can react back on the drive pressure, and may even stall out the reconnection process [Preview Abstract] |
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GP6.00011: Current sheet dynamics in transient magnetic reconnection experiment Yoshinori Hayashi, Takuma Yamada, Michiaki Inomoto, Yasushi Ono The current sheet dynamics was studied in the TS-4 torus plasma merging device using controlled external force and guiding field. Under strong guiding field - five times as much as reconnecting field, the half-width $\delta$ of current sheet was always longer than ion Larmor radius $\rho_i$, so that the low resistivity sheet made the reconnection quasi-steady like the Sweet-Parker model. Without external driving force, $\delta$ decreased to $\rho_i$ where dissipation was subject to anomalous resistivity and aspect ratio of current sheet $\Delta/\delta$ was determined by resistivity. When the reconnection inflow is strongly driven by the coil current, the plasma and magnetic flux inflow exceeded the outflow, causing density piled-up in the current sheet. This pile-up effect increased the reconnection speed without anomalous resistivity effect. With strong guiding field magnetic island (plasmoid) grew in the current sheet and ejected from reconnection region reconnection occasionally. The plasmoid ejection made the reconnection rate maximum when its acceleration was maximized, indicating another transient effect as a new fast reconnection mechanism. [Preview Abstract] |
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GP6.00012: The Wheaton Impulsive Reconnection Experiment D. Craig, E. Braun, J. Schroeder, D. Stapleton, R. Stegink, J. Whitmore A new experiment is under construction at Wheaton College for the study of impulsive magnetic reconnection in three dimensions. The experiment is composed of two parallel electrodes, linked by a magnetic arcade that is generated by a coil surrounding the electrodes. Current is driven along the arcade from one electrode to another, causing the arcade to inflate and become sheared. During the subsequent nonlinear evolution, the arcade may become unstable with strong driving and/or break off into a separated plasmoid. Fast imaging diagnostics and magnetic probe arrays will be used to follow the evolution of the arcade and identify sites where reconnection is taking place. The experimental components have been constructed and assembled. Power supply and fueling tests are underway with first plasmas anticipated within the next year. Work supported by U.S.D.O.E. grant DE-FG02-08ER55002. [Preview Abstract] |
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GP6.00013: On fast reconnection in pair plasmas A. Zocco, L. Chacon, A. Simakov, V. Lukin The relevance of two-fluid effects to fast magnetic reconnection in standard electron-proton plasmas is well-known.\footnote{J. Birn et al., {\em J. Geophys. Res.} {\bf 106} (A3), pp. 3715--3719 (2001) } The currently accepted view is that such fast reconnection is enabled by fast dispersive waves,\footnote{M. A. Shay et al., {\em Geophys. Res. Lett.} {\bf 26}, 2163 (1999); B. N. Rogers et al., {\em Phys. Rev. Lett.} {\bf 87}, 195004 (2001)} which originate in the ion-electron mass difference. However, electron-positron (pair) plasmas do not feature such mass difference, and thus do not support fast dispersive waves. Nevertheless, recent kinetic\footnote{See e.g. S. Zenitani and M. Hoshino, {\em Astrophys. J.} {\bf 562}, L63 (2001); N. Bessho and A. Bhattacharjee, {\em Phys. Rev. Lett.} {\bf 95}, 245001 (2005); W. Daughton and H. Karimabadi, {\em Phys. Plasmas} {\bf 14}, 72303 (2007).} and fluid\footnote{L. Chac\'on, A. N. Simakov, V. S. Lukin, A. Zocco, {\em Phys. Rev. Lett.}, 025003 (2008)} pair-plasmas simulations have demonstrated that fast magnetic reconnection is indeed possible, thus casting doubt on the accepted view. In this study, we develop an analytical fluid model for 2D reconnection in non-relativistic, large-guide-field, low-$\beta$ pair plasmas, including inertia, resistivity, and parallel viscosity.$^4$ We conclude that fast reconnection is possible in the collisionless (viscosity-dominated) regime, but not in the collisional (resistivity-dominated) one. [Preview Abstract] |
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GP6.00014: Scaling of Reconnection and Formation of Extended Current Sheets in Large Collisionless Systems B. Sullivan, A. Bhattacharjee, Q. Xin, H. Yang Recently, particle-in-cell simulations have shown the formation of extended current sheets in large collisionless systems. In order to determine whether such current sheets are realizable in Hall MHD models, we have carried out a sequence of simulations using the same initial conditions for large systems using a generalized Ohm's law that includes resistivity, hyperresistivity, and electron inertia as mechanisms that break field lines. In the resistive MHD model, the long thin current sheet spanning Y-points become near-explosively unstable to secondary tearing, producing plasmoids copiously. In resistive Hall MHD, the nonlinear dynamics changes qualitatively, as the Y-points contract spontaneously to form X-points thwarting the secondary tearing instabilities seen in the resistive MHD study. A steady state is then realized due to a balance between the spatial gradients of the current density and the velocity shear. Hyperresistive Hall MHD does not appear to produce extended current sheets, contrary to the suggestion in a recent theoretical study. However, in the presence of electron inertia, extended current sheets appear to form transiently before breaking up into secondary islands. Scaling studies that appear to account for simulation results under quasi-steady conditions are presented. [Preview Abstract] |
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GP6.00015: Stability of Extended Thin Current Sheets in High-Lundquist-Number Plasmas Lei Ni, A. Bhattacharjee, H. Yang In large systems such as the solar corona or the magnetotail, extended thin current sheets are formed in situations involving free or forced reconnection. These current sheets are characterized by large values of the tearing instability parameter delta- prime, which is strongly destabilizing, and shear in the outflow velocity along the current sheet, which is stabilizing [S. V. Bulanov et al. JETP Lett., 28, 177 (1978)]. We have carried out a systematic analytical and computational study of this problem in resistive MHD, including the effect of finite plasma compression. When the shear in the outflow velocity is weak, we obtain a super-Alfvenic instability in the high-S regime, predicted by N. F. Lourerio et al. [Phys. Plasmas 14, 100703 (2007)]. The nonlinear evolution of the system exhibits the copious generation of plasmoids and island coalescence. In the presence of strong velocity shear along the current sheet, the system tends to be more stable, and the wave number of the fastest growing instability itself evolves as a function of time. In the regime of small growth rates, it is possible to define a critical length below which the system is stable for a given value of S. The parametric dependence of this critical length is determined by analysis and simulations. [Preview Abstract] |
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GP6.00016: Particle-in-cell simulations of electron acceleration involving magnetic islands in hydrogen and pair plasmas Naoki Bessho, Li-Jen Chen, Amitava Bhattacharjee Motivated by Cluster observations of energetic electrons in magnetic islands (Chen et al., Nature Physics, 4, 19, 2008), we have studied electron acceleration in reconnection by means of 2D PIC simulations. In electron-ion plasmas, we have identified a number of possible acceleration mechanisms involving magnetic islands. These include acceleration due to (i) the out-of-plane electric field around X-lines, (ii) the in-plane electric field that energizes electrons trapped in the islands, and (iii) the coalescence of small islands producing larger islands. During process (iii), we demonstrate that despite the dominance of electron acceleration and heating, there can be cooling of electrons as well. We have found qualitative agreement between our simulations and certain aspects of the Cluster observations as far as density and magnetic field structures are concerned. We have also carried out a comparative study of electron acceleration in pair (electron-positron) and hydrogen plasmas. In large pair-plasma systems, the diffusion region exhibits significant stretching with more secondary instability, magnetic island generation, and coalescence. The commonality and differences between acceleration mechanisms in both types of plasmas will be discussed. [Preview Abstract] |
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GP6.00017: Ion velocity structures within the reconnection ion diffusion region Li-Jen Chen, Naoki Bessho Through space observations and particle-in-cell simulations of collisionless reconnection, we find that ion velocity distributions vary drastically from the inflow region to the electron-scale current sheet, and from boundaries of magnetic islands to the island center. Ions exhibit two counterstreaming beams along the current sheet normal within the electron-scale current sheet as well as near the separatrix region that is not too far away from the electron current sheet. The two counterstreaming beams are due to acceleration by the Hall electric field. Near the mid-plane of the reconnection exhaust, the ion outflow jet appears as the third component in addition to the two beam components. Further downstream into the exhaust toward center of magnetic islands, diffusion in the ion velocity space smears out the three components, leaving a half-ring shaped distribution that preserves the net outflow velocity. Near the center of ion-skin-depth-scale magnetic islands, the half-ring distributions mirror along the out-flow velocity axis and reflect a flow reversal. We discuss effects of these ion velocity structures within the ion diffusion region on the evolution of reconnection. [Preview Abstract] |
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GP6.00018: Investigation of the effects of guide field during magnetic reconnection B. McGeehan, M. Yamada, H. Ji, E. Oz, S. Dorfman, C. Jacobson In the Magnetic Reconnection eXperiment (MRX), external toroidal field coils have been installed to create a steady-state uniform guide field. In the MHD regime as the plasma is convected into the diffusion region, toroidal flux will also be pulled in resulting in a guide field pile-up. This concentration of guide field introduces extra magnetic pressure. Previous experiments with guide field [1,2] and the present research verified that the reconnection rate decreases with stronger guide field. Using magnetics data along with langmuir probe measurements across and along the current sheet, the reconnection rate and effective resistivity of the plasma is investigated with the addition of this extra toroidal field pressure by varying the drive and boundary conditions of the experiment. Because of the versatility of MRX both the MHD and two-fluid regimes of reconnection can be investigated. In the two-fluid regime, other experiments [3] have shown that in-plane hall currents can interact with the background guide field. The effect of this interaction is studied as a function of collisionality, effectively probing the boundary of the two-fluid and MHD regimes. [1] M. Yamada et al, Phys. Rev. Lett. \textbf{65}, 721 (1990). [2] T. Sato et al, Phys Fluids B \textbf{4}, 450 (1992). [3] S. Y. Bogdanov et al, Plasma Phys. Rep. \textbf{33}, 930 (2007). This work is supported by DOE, NSF, and NASA. [Preview Abstract] |
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GP6.00019: Laboratory Studies of Solar Flux Loops in MRX Erdem Oz, Masaaki Yamada, Brendan McGeehan, Seth Dorfman, Hantao Ji, Nate Williams, Stewart Zweben Energetic solar phenomena such as solar flares occur as a result of interplay between solar magnetic fields and plasma. The physics of these events is unexplained because of the scarcity of experimental data which can only be obtained using remote sensing. We will present an experimental study of the dynamics of half-toroidal plasma arcs relevant to solar coronal activities in the existing MRX facility with extensive measurements of magnetic topology. Two electrodes are used to generate a variety of plasma flux loops which contain variable toroidal guide field. The three dimensional evolution of the simulated flux loops is monitored by an ultra fast frame rate camera. Discharges of Ar, He and H show the time evolution of flux loops with variety of currents and reveal the stability condition for the plasma flux loop with the presence of line-tying. Our experimental results will contribute to the understanding of evolution of magnetic topology in the solar atmosphere and concepts such as current sheets, flow patterns, and line-tying, which are vitally important for understanding the Solar/Heliospheric and Interplanetary Environment. [Preview Abstract] |
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GP6.00020: Scaling of magnetic reconnection processes from MRX to astrophysical plasmas M. Yamada, R. Kulsrud, H. Ji, D. Uzdensky, E. Zweibel We discuss how the MRX (Magnetic Reconnection Experiment) physics results scale to space astrophysical plasmas. When the collisionality is reduced to satisfy the relationship $c/\omega_{pi}>\delta_{SP}$ between the ion skin depth ($c/\omega_{pi}$) and the Sweet-Parker width $\delta_{SP}$, a fast reconnection rate is observed in MRX [1], and the results are verified by numerical simulations. Since $(c/\omega_{pi})/\delta_{SP}$ is roughly equal to $5 (\lambda_{mfp}/L)^{1/2}$, this relationship suggests that two-fluid effects become dominant even when the electron mean free path is one order of magnitude smaller than the system size [1]. The reconnection rate is found to increase rapidly as the ratio of the electron mean free path to the scale length increases. This result is attributed primarily to the large Hall electric field in the reconnection layer except near the X point where dissipative processes caused by electron pressure gradients and high frequency turbulence take place. Finally, a fast local reconnection generally leads to an impulsive global topology change or global magnetic self-organization phenomena. We also discuss how our local analysis can be applied to variety of magnetic reconnection phenomena in space astrophysical plasmas [2]. [1]M. Yamada, Phys. Plasmas, v. 14, 058102 (2007)[2]D. Uzdensky, Ap. J v.671, 2139 (2007) [Preview Abstract] |
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GP6.00021: Electron Layer Dissipation Mechanisms in Driven Magnetic Reconnection S. Dorfman, H. Ji, M. Yamada, B. McGeehan, E. Oz, J. Schroeder, W. Daughton, V. Roytershteyn, Y. Ren An open question in magnetic reconnection is the nature of the dissipation mechanism(s) responsible for fast reconnection rates in laboratory and astrophysical plasmas. In 2-D collisionless particle in cell simulations, the off-diagonal terms in the electron pressure tensor provide the necessary force balance at the electron diffusion layer center [1]. Recent comparisons between the Magnetic Reconnection Experiment (MRX) and a well-matched PIC code have shown that this mechanism is insufficient to balance the reconnecting electric field in MRX [2,3]. Candidate mechanisms not present in the simulation are under investigation, including electromagnetic fluctuations and other 3-D effects such as layer distortions. The relationship between fluctuations and equilibrium parameters such as the outflow current and layer width is examined, and first investigations into the 3-D symmetry of the layer are presented. On the simulation side, analysis is underway to gain further insight into the nature of the off-diagonal pressure tensor terms, especially effects due to the driven nature of the simulation. [1] M. Hesse, et al., Phys. Plasmas, {\bf 6}:1781 (1999). [2] S. Dorfman, et al., submitted to Phys. Plasmas (2008). [3] H. Ji, et al., Geophys. Res. Lett., {\bf 35}, L13106 (2008). This work was supported by NDSEG, DOE, NASA, and NSF. [Preview Abstract] |
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GP6.00022: Nonlinear mode coupling calculation on the perpendicularly propagating electromagnetic instabilities in the MRX Yansong Wang, Russell Kulsrud, Hantao Ji Motivated by the observation of electromagnetic fluctuations in the current sheet of the MRX, a local theory has been developed.\footnote{Y. Wang {\it et al.}. submitted to PoP.} Because the {\it oblique} mode does not seem to be responsible for any enhanced resistivity, we have concentrated on {\it perpendicular} propagation mode. We found an unstable beam mode and two magneto-sonic modes. To calculate the mode saturation and anomalous resistivity, we develop a nonlinear theory using beat wave generated by the two coupling modes. Results show that it is easier for the damped magneto-sonic mode to get energy from the unstable beam mode but not other way around. The beam mode cannot be saturated by this nonlinear mode coupling process. Instead, this beam mode could be modified by the magneto-sonic mode through the mode coupling and have a larger group velocity across the current layer, then propagate out of the unstable region and be stabilized. Applying the quasi-linear theory,\footnote{R. Kulsrud {\it et al.}, PoP. {\bf12}, 082301 (2005)} we found the damping magneto-sonic mode could produce large amount of anomalous resistivity. Since this mode is linearly damped, the only way for it to grow is to get energy from unstable beam modes through nonlinear mode coupling. We show a plausible mechanism for the waves to produce anomalous resistivity. This work is supported by NASA. [Preview Abstract] |
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GP6.00023: The in-plane electric field and its role in the ion dynamics in collisionless magnetic reconnection Russell Kulsrud, Dmitri Uzdensky, Masaaki Yamada A potential in-plane electric field, observed in numerical simulations and detected by spacecraft in the Earth magnetosphere, plays an important role in the dynamics of collisionless magnetic reconnection. In particular, it pulls the ions into the layer, effectively resulting in ion heating. Building on our previous work,\footnote{D.~Uzdensky \& R.~Kulsrud, Phys. Plasmas, 13, 062305 (2006).} we discuss the physics of the in-plane electric field and its relation to the quadrupole magnetic field. We also present a formalism (within the electron-MHD approach) for calculating it analytically for a given structure of the in-plane magnetic field. We emphasize the role of this electric field on the ion dynamics and heating. [Preview Abstract] |
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GP6.00024: Driven Magnetic Reconnection in Semi-Collisional Parameter Regimes V. Roytershteyn, W. Daughton, B.J. Albright, K.J. Bowers, L. Yin, S. Dorfman, H. Ji, M. Yamada Recent kinetic simulations of driven magnetic reconnection with boundary conditions relevant to the Magnetic Reconnection eXperiment (MRX) have demonstrated that the electron diffusion layer is significantly thicker in the experiment\footnote{Ji et al., to appear in {\em GRL}, 2008} than in the 2D collisionless simulations.\footnote{Dorfman {\em et al.}, submitted to {\em Physics of Plasmas}, 2008} The two leading possibilities to explain this discrepancy are 3D effects such as current aligned instabilities and Coulomb collisions. In order to address both of these possibilities, we have implemented the new MRX relevant boundary conditions$^2$ within the 3D kinetic simulation code VPIC.\footnote{K. J. Bowers {\em et al.} Phys. Plasmas, v. {\bf 15}, p.~055703, 2008.} Coulomb collisions are treated using a well-known Monte-Carlo technique\footnote{T. Takizuka and H. Abe, J. Comput. Phys., v. {\bf 25}, p. 205, 1977} that models a full collision operator. This approach will allow us to systematically examine the influence of Coulomb collisions and plasma instabilities on the dynamical evolution of the reconnection layer using boundary conditions relevant to the actual experiment. Initial results illustrating the transition between collisionless and semi-collisional regimes are presented. [Preview Abstract] |
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GP6.00025: Magnetic reconnection with asymmetry in the outflow direction Nicholas Murphy, Carl Sovinec, Paul Cassak Magnetic reconnection with asymmetry in the outflow direction occurs in the Earth's magnetotail, spheromak merging experiments, coronal mass ejections, and astrophysical disks. We analyze the case of steady reconnection with asymmetric downstream pressure, using conservation of mass, momentum, and energy to derive the outflow velocities for both sides of the reconnection layer. As in reconnection with asymmetric inflow [1], the flow stagnation point and magnetic field null will not coincide, unless the pressure gradient is negligible at the flow stagnation point. When the two points are separated, there will be a Poynting flux across the flow stagnation point. We compare the derived properties of this model with resistive MHD simulations of driven reconnection. We perform a similar analysis for reconnection in toroidal geometry when the outflow is aligned with the radial direction. The toroidal geometry model is compared against simulations of push reconnection (similar to spheromak merging) using the geometry of the MRX device [2]. \\ {[1]} P.\ A.\ Cassak \& M.\ A.\ Shay, Phys.\ Plasmas 14, 102114 (2007)\\ {[2]} N.\ A.\ Murphy \& C.\ R.\ Sovinec, Phys.\ Plasmas 15, 042313 (2008) [Preview Abstract] |
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GP6.00026: Ion heating due to reconnection and turbulence in SSX M.R. Brown, T. Gray, B.S. Gerber-Siff, K.R. Labe, E.H. Dewey, L.D. Bookman, C.D. Cothran, M.J. Schaffer Ion heating is measured in the SSX device with a high resolution ion Doppler spectrometer. The SSX IDS instrument measures the width and Doppler shift of either the nascent $C_{III}$ impurity $229.7~nm$ line or a doped $He_{II}$ impurity $468.6~nm$ line to determine the temperature and line-averaged flow velocity. The velocity resolution of the instrument is $\le 5~km/s$. There is enough signal to resolve the full line within an MHD dynamical time (about $1~\mu s$ in SSX). Peak ion temperatures of $80~eV$ ($C_{III}$) have been recorded during reconnection events as well as flows up to $40~km/s$. Spheromak merging in a new oblate flux conserver ($R=0.25~m, L=0.4~m$) has resulted in some stable FRC configurations but often results in excitation of several unstable MHD modes. We plan to study the effect of reconnection and turbulence on ion heating for various ion masses (He, C, Xe). Preliminary results indicate that the temperature of helium and carbon ions is comparable but systematic studies are planned. Results from a new ion energy analyzer as well as a high purity gas delivery and mixing system will be presented. [Preview Abstract] |
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GP6.00027: Internal and external characterization of reconnection in SSX C.D. Cothran, M.R. Brown, T. Gray, M.J. Schaffer, E.V. Belova The internal and external properties of the reconnection process responsible for the merging of left- and right-handed spheromaks in the SSX device is analyzed using data from a variety of diagnostic instruments. Time resolved magnetic probe data permit the determination of the external field strength, an estimate of the external ideal inflow velocity, as well as the reconnection electric field from the rate of change of poloidal flux. At the internal scale, the outflow speed is known from time resolved spectroscopy of Doppler shifted line emission from carbon impurity ions; combined with interferometer density measurements and assuming Alfvenic outflow, the internal inflow (at the entrance to the reconnection layer) magnetic field and velocity is inferred. The aspect ratio of the layer is thus determined, and the length is found to be only a factor of 3 to 4 times the width. Purely 2D scaling based on ideal MHD to relate the external and internal structure does not seem to work, so numerical simulation accounting for the toroidal geometry seems necessary. [Preview Abstract] |
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GP6.00028: Simulation of Swarthmore Spheromak Reconnection Experiment Using Hybrid Code Y. Lin, X.Y. Wang, M.R. Brown, M.J. Schaffer, C.D. Cothran A 3-D hybrid-particle model is developed for investigation of magnetic reconnection in the Swarthmore Spheromak Experiment (SSX). In this numerical model, ions are treated as fully kinetic particles, and electrons are treated as a massless fluid. The plasma responds to the electromagnetic fields in a self-consistent manner. The simulation is performed in a cylindrical domain. Initially, a pair of counter-helicity spheromaks are assumed, in which the magnetic field and plasma pressure are set up according to the MHD equilibrium. The ion particles are loaded with a Maxwellian distribution function. Conducting boundary conditions are applied to all the boundaries. As the simulation proceeds, magnetic reconnection takes place at the current sheet between the pair of spheromak fields. Plasma is ejected away from the X line towards the central axis, where heating of the transmitted ion is found. Meanwhile, quadrupole out-of-plane magnetic field structure associated with the Hall effects is present around the reconnection site. The simulation results will be compared with the SSX experiment in various aspects. [Preview Abstract] |
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GP6.00029: Analytical Model for the Phase-Space Distribution of Electrons in Guide Field Magnetic Reconnection J. Egedal, W. Fox, N. Katz, A. Le, A. Vrublevskis, M. Porkolab Electron distributions measured in situ by the Wind spacecraft has revealed that electrons were trapped in the electromagnetic geometry of the reconnection event encountered in the deep magnetotail [1]. Here we present a new analytical theory that can account for the anisotropic features of the electron distributions observed by Wind [2]. The anisotropy is related to extensive trapping of electrons in parallel electric fields. Trapping is found to be generic in guide-field reconnection, as it is required in order to maintain the condition of quasi- neutrality. In addition to the spacecraft data, evidence of trapping in numerical simulations is also presented. Trapping is effective in controlling the free-streaming of electrons along magnetic fields. Its importance for fast reconnection is discussed and emphasized by observations in the VTF experiment. \\[1ex] [1] J.~Egedal, M.~Oieroset, W.~Fox, and R.~P.~Lin., Phys.~Rev.~Lett., {\bf 94}, 025006 (2005). \\[1ex] [2] J.~Egedal, W.~Fox, N.~Katz, et al., {\sl ``Evidence and theory for trapped electrons in guide field magnetotail reconnection''}, submitted to Journal of Geophysical Research, 2008. [Preview Abstract] |
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GP6.00030: Observation of electron phase space holes during magnetic reconnection experiments in the Versatile Toroidal Facility W. Fox, M. Porkolab, J. Egedal, N. Katz, A. Le, A. Vrublevskis We report recent observations of nonlinear electrostatic fluctuations, identified as electron phase space holes, excited during spontaneous reconnection events on the VTF experiment[1]. Electrostatic fluctuations are observed by small, high-bandwidth, impedance-matched Langmuir probes. Among a large number of wave phenomena, we observe narrow, large-amplitude, positive potential spikes, identified as electron holes. With arrays of such probes we have observed the speed and shape of these propagating structures. The parallel and perpendicular sizes are roughly equal, approximately 1-2~mm (50-100~$\lambda_D$, or 5-10~$\rho_e$). Finally, we will present studies of the relationship between the holes, reconnection electric fields, and creation of energetic particles by the reconnection process. The latter are studied with a novel electron energy analyzer which integrates 7 grid/collector pairs into a small, 1.5~cm area. Our interpretation is that the holes arise from velocity-space instability in the energetic electron population. \\[0pt] [1] J. Egedal, \textit{et al}., PRL \textbf{98}, 015003 (2007). [Preview Abstract] |
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GP6.00031: A New Kinetic-Based Fluid Approach to Collisionless Magnetic Reconnection A. Le, J. Egedal, W. Fox, N. Katz, A. Vrublevskis, M. Porkolab We present a new fluid model for the electrons in a current sheet undergoing collisionless magnetic reconnection. Analytical expressions for the electron phase space density were recently derived for general reconnection geometries in the limit of fast electron transit time [1]. This model accounts for kinetic effects associated with extensive trapping of electrons in the reconnection region. First, the electron density and equations of state for the parallel and perpendicular pressures are obtained by taking moments of this distribution function, which is exact for magnetized electrons of negligible mass and takes into account parallel electric fields and the presence of trapped and passing electrons. The model is compared to kinetic simulations. Among other results, excellent agreement is found between the predicted profiles of $p_{\parallel}$ and $p_{\perp}$ and those observed in the simulation. \\[1ex] [1] J.~Egedal, W.~Fox, N.~Katz, et al., ``Evidence and theory for trapped electrons in guide field magnetotail reconnection,'' submitted to {\sl Journal of Geophysical Research}, 2008. [Preview Abstract] |
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GP6.00032: Washer-Gun Plasma Source for Magnetic Reconnection Experiments on VTF A. Vrublevskis, J. Egedal, W. Fox, N. Katz, A. Le, M. Porkolab We present an electrostatic washer-gun plasma source for the Versatile Toroidal Facility (VTF). The gun will produce plasmas with densities on the order of $10^{18}$ m$^{-3}$ and electron temperatures on the order of 10-20 eV. It will extend the range of configurations achievable on VTF since the present plasma production method is limited to configurations with strong toroidal magnetic fields, which are required for microwave-induced electron cyclotron resonant breakdown. The gun is based on the design developed by Sterling Scientific [1] with detailed operation described in [1, 2]. During the gun's operation gas is injected into a channel formed by a stack of alternating molybdenum and boron nitride washers with a molybdenum electrode washer at each end. A voltage from a capacitor bank is applied to these electrodes and breaks down the gas in the channel. The resulting plasma escapes the channel into the main chamber of the experiment. If available we will present data characterizing the argon plasma produced by the device.\\[1ex] [1] Fiksel G et al. Plasma Sources Sci. Technol. {\bf 5} (1996) 78\\[1ex] [2] Den Hartog D et al. Plasma Sources Sci. Technol. {\bf 6} (1997) 492 [Preview Abstract] |
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GP6.00033: Experiments on 3D Evolution of Spontaneous Magnetic Reconnection Noam Katz, Jan Egedal, Will Fox, Ari Le, Arturs Vrublevskis, Miklos Porkolab Magnetic reconnection is a fundamental process in plasmas that results in the often explosive release of stored magnetic energy. We study this process experimentally in the Versatile Toroidal Facility (VTF) at MIT, where we have observed 3D effects during spontaneous reconnection. A set of coils inside the vacuum chamber is used to drive reconnection. After the drive is applied, the reconnection rate remains low for about 100 $\mu s$ and then a sudden burst of spontaneous fast reconnection is observed [1]. Although the experiment is toroidally symmetric, the onset and development of the spontaneous reconnection is not symmetric: we observe that it starts at one toroidal angle and then propagates around the machine in roughly 10 $\mu s$, the Alfvenic time. We explore the 3D properties of this collisionless reconnection by considering the global modes in the plasma, and the current flow patterns.\\[0pt] [1] J. Egedal et al, Phys. Rev. Lett. 98, 015003 (2007) [Preview Abstract] |
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GP6.00034: Progress of Multi-Hierarchy Simulation for the Full Understanding of Magnetic Reconnection Shunsuke Usami, Hiroaki Ohtani, Ritoku Horiuchi, Mitsue Den Magnetic reconnection is controlled by multi-hierarchy physics. The change in magnetic topology is global phenomenon, while microscopic kinetic process is needed to trigger magnetic reconnection. For the full understanding of magnetic reconnection, a multi-hierarchy model which deals with both microscopic and macroscopic physics consistently and simultaneously are developed. Our multi-hierarchy model is based on the domain division method, and thus is composed of two hierarchies: micro and macro hierarchies. The neighborhood of reconnection points is micro hierarchy, where microscopic kinetic effects play important roles. Dynamics in this system are solved by particle (PIC) simulation. On the other hand, the surrounding of PIC domain is macro hierarchy, and is described by MHD simulation. Between MHD and PIC domains, an Interface domain with a finite width is inserted. The applicability of our model has been examined with simulations of linear waves. Furthermore, as a first step of magnetic reconnection studies, propagation of plasma flow from MHD to PIC domains is simulated. In our presentation, these simulation results will be demonstrated. [Preview Abstract] |
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GP6.00035: Formation of high energy particles through collisionless driven reconnection Ritoku Horiuchi The energy conversion process in collisionless driven reconnection is studied by using an electromagnetic particle simulation code in a microscopic open system (PASMO). Kinetic regime appears in the central current layer, in which frozen-in condition is broken due to various kinetic effects, e.g., the inertia effect and the effect of nongyrotopic thermal motion. The kinetic regime consists of two dissipation regions with different spatial scales, i.e., ion dissipation region (IDR) and electron dissipation region (EDR). The out-of-plane component of electron average velocity at the center of EDR is found to reach the electron Alfven velocity evaluated at the edge of EDR, suggesting that most of magnetic energy carried into EDR is effectively converted into the electron kinetic energy. On the other hand, the energy conversion to ions takes place strongly inside the slow shock region in the downstream. Futhermore, strong in-plane electrostatic field accelerates unmagnetized meandering ions inside IDR and creates non-Maxwellian distribution with a hole structure in which distribution becomes two-peaked and only a few ions exist in the low energy region. Thus, the particle energy spectrum changes from initial thermal profile to non-thermal one with a high energy tail. Especially, this change happens strikingly in the ion energy spectrum. [Preview Abstract] |
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GP6.00036: Role of Chaotic Orbits of Meandering Particles in Magnetic Reconnection H. Ohtani, W. Horton, T. Petrosky, R. Horiuchi Ions become un-magnetized and execute a complex thermal motion called meandering motion in the ion dissipation region of magnetic reconnection. The complex meandering (chaotic) motion leads to the growth of off-diagonal components of pressure tensor term, which is one of main causes to break ion frozen-in condition in the vicinity of magnetic neutral sheet. In this paper we investigate the role of the meandering motion in the formation of ion dissipation region by examining particle simulation results of collisionless driven reconnection based on the simple model. Because the meandering motion is relevant to the mechanism of dissipation, the size of the dissipation region is given by the orbit amplitude of the meandering motion. Since the average velocities outside the dissipation region are approximately given by ExB drift in inflow direction and thermal velocity in out-of-plane direction respectively, the pressure tensor term in the force balance equation is written down analytically. The tendency of this analytic solution is in agreement with that of the simulation result. Moreover, we will report the effect of the chaotic orbits on the energy gain of the particles passing through the dissipation region. [Preview Abstract] |
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GP6.00037: Reconnection rate in collisionless magnetic reconnection Zhiwei Ma, Jun Huang Collisionless magnetic reconnection is studied by using two dimensional Darwin particle-in-cell simulations with different types of open boundary conditions. Simulation results indicate that reconnection rates are strongly dependent on the imposed boundary conditions of magnetic field in the inward side. With the zero-gradient boundary condition, the reconnection rate quickly decreases after reaching its maximum and no steady-state is found. For both electromagnetic and magnetosonic boundary conditions, the system can reach a quasi- steady state. But the reconnection rate $E_{\rm \vert\vert} \approx$ 0.08 in the electromagnetic boundary is weaker than $E_{\rm \vert\vert} \approx$ 0.13 in the magnetosonic boundary. The dependence of the reconnection rate on the length and thickness of the initial current sheet is also studied under the magnetosonic open boundary. It is found that the reconnection rate decreases with the increase of the length or thickness. [Preview Abstract] |
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GP6.00038: WAVES, OSCILLATIONS, AND INSTABILITIES |
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GP6.00039: Stability of the ion-acoustic surface waves in a Lorentzian plasma Taejoon Kim, Myoung-Jae Lee We investigated the stability of ion-acoustic surface waves propagating on a boundary of semi-infinite Lorentzian (kappa) plasma. The real and imaginary parts of the wave frequency $\omega =\omega _r +i\gamma$ are obtained as functions of the normalized wave number $k_x \lambda _e$ where $k_{x}$ is the x-component of the wave number and $\lambda _e$ is the electron Debye length. The phase speed of the wave is found be decreased as the spectral index of the Lorentzian distribution function is decreased. In the long wavelength limit, the scaled phase velocity ${\left( {{\omega _r} \mathord{\left/ {\vphantom {{\omega _r } {\omega _{pi} }}} \right. \kern-\nulldelimiterspace} {\omega _{pi} }} \right)} \mathord{\left/ {\vphantom {{\left( {{\omega _r } \mathord{\left/ {\vphantom {{\omega _r } {\omega _{pi} }}} \right. \kern-\nulldelimiterspace} {\omega _{pi} }} \right)} {k_x \lambda _e }}} \right. \kern-\nulldelimiterspace} {k_x \lambda _e }$ becomes $\sqrt {\mu _K } $ where $\omega _{pi}$ is the ion plasma frequency and $\sqrt {\mu _\kappa } $ is a constant. The wave displays the resonance at ${\omega _r } \mathord{\left/ {\vphantom {{\omega _r } {\omega _{pi} }}} \right. \kern-\nulldelimiterspace} {\omega _{pi} }=1 \mathord{\left/ {\vphantom {1 {\sqrt 2 }}} \right. \kern-\nulldelimiterspace} {\sqrt 2 }$ as expected as the case of Maxwellian plasma. The imaginary part of the wave frequency appears to be negative which exhibits the linear wave dissipation in a collisionless plasma called Landau damping. The maximum damping rate is obtained as $\lambda _{Max} =0.14M_\kappa \sqrt {m \mathord{\left/ {\vphantom {m M}} \right. \kern-\nulldelimiterspace} M} $ at $k_x \lambda _e ={0.44} \mathord{\left/ {\vphantom {{0.44} {\sqrt {\mu _\kappa } }}} \right. \kern-\nulldelimiterspace} {\sqrt {\mu _\kappa } }$ where $M_\kappa$ is a kappa-dependent function, and $m$/$M$ is the electrion-ion mass ratio. The damping of the wave disappears as $k_x \lambda _e \to \infty $. [Preview Abstract] |
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GP6.00040: Landau damping of the dust ion-acoustic surface waves in a Lorentzian plasma Myoung-Jae Lee, Taejoon Kim The Lorentzian (kappa) velocity distribution function is employed to study the stability of dust ion-acoustic surface waves propagating on a boundary of semi-infinite plasma. We allow that the electrons and ions are Lorentzian but the dusts are cold. Then the dust ion-acoustic surface wave can be excited: the real and imaginary parts of the wave frequency $\omega =\omega _r +i\gamma$ are obtained as functions of \textit{$\delta $} = $n_{i}$/$n_{e}$ (ion-to-electron mass ratio) as well as the normalized wave number $k_x \lambda _e $ where $k_{x}$ is the x-component of the wave number and $\lambda _e $ is the electron Debye length. The wave exhibits resonances and the resonant frequency is strongly dependent on the value of \textit{$\delta $}. For a negatively (positively) charged dust particles, the phase velocity of the wave increases as \textit{$\delta $} increases (decreases). When \textit{$\delta $ }=1$,$ the result displays the phase velocity of a Maxwellian wave. The imaginary part of the wave frequency appears to be negative always regardless of the value of \textit{$\delta $}. Such collisionless dissipation of the wave is known as the Landau damping. We also found that the damping is enhanced as \textit{$\delta $} increases (decreases) for a negatively (positively) charged dust particles. [Preview Abstract] |
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GP6.00041: Electron acoustic wave propagation in a two-electron-temperature plasma layer applied to the problem of hypersonic vehicle communication Vladimir Sotnikov, David Rose Problem of electromagnetic wave propagation through a plasma sheath surrounding reentry vehicles and vehicles traveling at hypersonic velocities at high altitudes attracts the attention of many researchers. High plasma density inside a plasma sheath around a hypersonic vehicle prevents propagation of electromagnetic waves with the frequencies below the local plasma frequency. This results in RF frequency communication problems. One possibility to mitigate this problem is to induce a two-temperature electron distribution inside the plasma sheath. This allows electron acoustic waves (EAWs) with frequencies well below the local plasma frequency (fp $\sim $ 9 GHz) to propagate through a plasma layer, enabling communication. A small hot electron population is produced in the sheath by injection of an energetic electron beam in the sheath from the vehicle. Excitation, propagation, and attenuation of EAWs inside a plasma sheath in the presence of an electron beam has been investigated as well as efficiency of transformation of EAWs into electromagnetic waves on the sheath boundary. [Preview Abstract] |
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GP6.00042: Experiments on Alfv\'{e}n waves in high beta plasmas Walter Gekelman, Patrick Pribyl, Chris Cooper, Stephen Vincena The propagation of Alfv'{e}n waves in high beta plasmas is of great interest in solar wind studies as well as in astrophysical plasmas. Alfv\'{e}n wave propagation in a high beta plasma is studied on the axis of a toroidal device at UCLA. The vacuum vessel is 30 meters in circumference, 2 meters wide and 3 meters tall. The plasma has a cross sectional area of 20 cm$^{2}$ and can be as long as 120 m which is hundreds of parallel Alfv\'{e}n wavelengths. The waves are launched using two orthogonal 5-turn , 5.7 cm diameter loops. The AC currents (10 kHz $<$ f $<$ 250 kHz) to the loops are as high as 2 kA p-p, producing fields of 1 kG on the axis of the antenna. The antenna coils are independently driven such that waves with arbitrary polarization can be launched. Movable three axis magnetic pickup loops detect the wave and are used to construct field maps in the machine. Wave propagation results as a function of plasma beta and input wave energy will be presented. [Preview Abstract] |
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GP6.00043: Variational formulation of kinetic-bulk multiple-wave conversion in fusion plasmas A.N. Kaufman, A.J. Brizard, E.R. Tracy We present a variational formulation of a generic multiple-wave conversion process [1] between a kinetic-ion plasma wave, supported by a non-Maxwellian distribution of energetic particles, and one or two plasma waves supported by bulk-ions with Maxwellian distributions. The variational approach yields important conservation laws (e.g., action), that help us understand how rf power is exchanged between the bulk-ion species and the kinetic-ion species. We also discuss the implications of a kinetic-ion plasma wave with negative energy on the multiple-wave conversion process. [1] A.J. Brizard, A.N. Kaufman, and E.R. Tracy, \textit{Recirculation in multiple wave conversions}, to appear in Physics of Plasmas (2008). [Preview Abstract] |
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GP6.00044: Reconstructing RF Fields from Ray Tracing Data A.S. Richardson, P. Bonoli, J. Wright Ray tracing techniques can be used to estimate the propagation of RF fields in plasmas. However, there are discrepancies between the power density profiles calculated using full-wave codes and ray tracing simulations [1]. We want to examine the impact of diffraction and interference on these results. In order to do this, we have been examining the possibility of reconstructing RF fields from ray tracing data, using tools developed for semi-classical quantum approximations. By retaining the next order terms in the derivation of the ray tracing approximation, it is possible to obtain a set of ODE's which describe the dynamics of a wave packet centered on the ray [2]. From this information, an approximate solution for the fields can be obtained. These fields can then be used to calculate power deposition profiles for comparison with the full-wave solutions. In this poster, we describe the semi-classical wave packet approximation, and show preliminary results for the reconstruction of LH fields from ray data. [1] P. Bonoli, et al., Phys. Plasmas 15, 056117 (2008) [2] R. Littlejohn, Phys. Rep. 138 (4) p. 193-291 (1986) [Preview Abstract] |
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GP6.00045: A Study of Full-wave and Ray-tracing Methods for Two Simple Models of Multi-dimensional Mode Conversion Y. Xiao Here we report on our efforts to carry out a direct comparison of ray-based and full-wave methods for coupled wave equations in non-uniform plasmas exhibiting mode conversion. We consider two different types of model wave equations in two spatial dimensions. The first model is taken to be two coupled wave equations with spatially varying wave speeds. These wave speeds are distinct for almost all space, and are equal only along a line where conversion occurs. We launch a WKB-type wave packet in one channel and study its behavior as it crosses the line of conversion. The second case studied uses the model developed by Cook et al. to treat a fast wave crossing a finite temperature minority-ion gyroresonance layer, though here we consider only the case of a cold minority-ion species. In both models, outgoing WKB boundary conditions are used so we can perform a direct comparison with ray tracing results for the same models. [1] D.R. Cook, A.N. Kaufman, E.R. Tracy, T.Fl{\aa}, Phys. Lett. A. 175 (1993) 326-333 [Preview Abstract] |
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GP6.00046: A new iterative method to compute the dielectric tensor in the low-frequency wave code LEMan N. Mellet, W.A. Cooper, M. Jucker, J.P. Graves, L. Villard The full-wave code LEMan is designed to perform computation in both 2D and 3D geometries. It uses a warm formulation in order to model, for example, the Kinetic Alfv\'en Wave and the electron Landau damping. One of the main difficulties emerging with this modelisation is the determination of the parallel wave vector. Thus a method based on a polynomial matrix inversion has been implemented. Results were obtained in the Alfv\'en domain for both tokamak and stellarator configurations, including LHD. On the other hand, for ICRH cases, the parallel wave vector has to be approximated because the matrix is ill-conditioned. A new iterative method has been consequently developed to preclude imposing approximations. The first results obtained with it are in good agreement with those computed with the method previously implemented. Another important advancement is the introduction of a bi-Maxwellian distribution function to model anisotropy in the velocity space for fast ions. Furthermore, the LEMan code is now coupled with the single particle code VENUS, in order to model RF heating in a self-consistent way. [Preview Abstract] |
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GP6.00047: Laboratory Studies Of Whistler And Lower Hybrid Wave Propagation Bill Amatucci, Dave Blackwell, George Gatling, Guru Ganguli, Chris Cothran, Dave Walker An experimental investigation of the generation and propagation of whistler and lower hybrid waves is underway in the NRL Space Physics Simulation Chamber. These studies are carried out in both homogeneous plasma and plasma containing density structures. In homogeneous plasma, resonance cone propagation of the waves is observed, consistent with theoretical predictions. In plasma containing a density depletion layer, wave ducting within the layer has been observed. Experimental results related to the propagation characteristics of whistler/lower hybrid waves under these conditions will be presented. [Preview Abstract] |
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GP6.00048: Antenna Coupling and Whistler Wave Propagation Experiments at the NRL SPSC D.D. Blackwell, W.E. Amatucci, G.I. Ganguli, G.R. Gatling, C.S. Compton, C.D. Cothran, E.M. Tejero, D.N. Walker We present results of recent whistler wave propagation experiments in the Space Physics Simulation Chamber facility at the Naval Research Lab. The waves are driven and detected using balanced dipole and loop antennas connected to a network analyzer which measures the amplitude and phase of the wave in two dimensions (\textit{r} and \textit{z}). In addition the frequency of the signals is also swept over a range of several hundred megahertz, providing a very comprehensive picture of the near and far field antenna radiation patterns over a variety of plasma conditions. The magnetic field is varied from a few Gauss to 200 Gauss, with the density variable over at least three decades from 10$^{7}$-10$^{10}$cm$^{-3}$. Observations to be presented are the efficacy of resonant vs non-resonant antenna coupling in driving large amplitude waves, ducting of waves with density channels, and wave propagation in linear and non-linear regimes. [Preview Abstract] |
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GP6.00049: Analytic study on localized wave modes driven by relativistic ion cyclotron in nonuniform magnetic field Tsung-Hua Tsai, Kuan-Ren Chen, Liu Chen A systematic perturbation theory is developed to study in-depth the localized ion cyclotron modes observed in our simulation. The parabolic magnetic field profile studied in the theory is an approximation of the magnetic field at the minimum of the sinusoidal profile considered in the simulation. The theory is based on an absolute instability condition and the assumption of local homogeneity. It reveals the mechanism for driving the localized modes by fusion-produced alpha particles. The analytical results indicate that the localized modes are corresponding to the eigenmodes excited by the relativistic alpha-driven ion cyclotron instabilities at a specific eigen-frequency. The frequency, growth rate and spatial profile of the wave modes obtained from the analytical theory are in a good agreement with the simulation results. Moreover, both our analytical and simulation results show that the wave modes can exist at where the wave eigen-frequency is lower than the local harmonic cyclotron frequency; even this violates the resonance condition required for the relativistic cyclotron instabilities as generally believed. [Preview Abstract] |
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GP6.00050: Simulation study on alpha-driven localized cyclotron modes in nonuniform magnetic field Kuan-Ren Chen, Tsung-Hua Tsai, Liu Chen Resonance is a fundamental issue in science and requires precise synchronization. As an ion version of cyclotron maser, relativistic ion cyclotron instability is driven by fusion produced MeV ions whose Lorentz factor is very close to unity. Cyclotron maser requires a small positive frequency mismatch between the wave and the harmonic cyclotron motion of fast particles. Thus, it is generally believed that it can not survive the nonuniformity of magnetic field such as in realistic devices. However, our simulations have shown that localized cyclotron waves are excited when the magnetic field is with a sinusoidal nonuniformity much larger than the frequency mismatch required. This indicates that resonance is a consequence of the need to drive instability for dissipating free energy and increasing the entropy. When a favorable wave eigen-frequency is collectively decided in a coherent means, a special wave form in real space is created for this purpose, even without boundary. Furthermore, the results also indicate that the wave eigen-frequency found can be lower than the local harmonic cyclotron frequency. The simulation results are compared with the analytical results from a perturbation theory. [Preview Abstract] |
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GP6.00051: Kinetic Damping of Alfven Eigenmodes in General Tokamak Geometry Limin Yu, Guoyong Fu A non-perturbative kinetic/MHD eigenvalue code has been constructed for calculation of kinetic damping of shear Alfven eigenmodes in general tokamak geometry. The model describes shear Alfven waves with kinetic effects from both thermal species and energetic particles including thermal ion FLR, parallel electric field and energetic particle destabilization. The model generalizes the previous work [1] to general tokamak equilibria with finite aspect ratio, finite beta and non- circular shape. The code has been benchmarked against known analytic and numerical results for Alfven eigenmodes such as GAE, TAE, and RSAE (or Alfven Cascades). The code is being used to investigate the existence and kinetic damping of various Alfven eigenmodes with non-perturbative kinetic effects from thermal species. The results will be presented. [1] G. Y. Fu, H. L. Berk and A. Pletzer, Phys. Plasmas 12, 082505 (2005) [Preview Abstract] |
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GP6.00052: Ion Beam Plasma Interactions in the ASTRAL Helicon Plasma Source. R.F. Boivin, A. Kesterson, O. Kamar, Y. Lin, J. Munoz, X. Wang A 100 KeV NEC duoplasmatron is used to produce an energetic ion beam (10 KeV $<$ E $<$ 100 KeV). The beam is sent through plasmas produced by the ASTRAL helicon plasma source. The beam current and beam size are measured by a device combining Retarding Field Analyzer (RFA) and Faraday Cup (FC) features. ASTRAL produces bright intense He/Ne/Ar plasmas with the following parameters: ne = 1E11 -- 1E13 cm-3 and Te = 2 - 10 eV, B-field $<$ 1.3 kGauss, rf power $\le $ 2 kWatt. RF compensated Langmuir probes are used to measure Te and ne. Depending on the ion beam energy and the ratio of beam density over plasma density different wave instabilities will be generated within the plasmas. A real-time spectrum analyzer will be used to identify the wave instabilities and their evolution in the plasma. We will present early experimental results together with some preliminary theoretical simulation using 2D and 3D hybrid simulation codes. In these codes, ions are treated as fully kinetic particles while electrons are treated as a fluid. Both species are moving in a self-consistent electromagnetic field. [Preview Abstract] |
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GP6.00053: Alfv\'{e}n wave Measurements in Helium using the HelCat Device at UNM Ralph Kelly, Christopher Watts, Yue Zhang, Rich Compeau, Mark Gilmore, Alan G. Lynn Research is being conducted at UNM to verify the theoretical relationship between Alfv\'{e}n waves and neutral density in Helium plasmas. We are particularly interested in the role collisionality plays in wave damping and mode structure development. The non-axisymmetric waves are launched using a hand wound emitter coil driven by a differential current amplifier circuit. The receiver consists of a hand wound B-dot coil and an amplifier circuit. Construction of the emitter, driver circuit, detector, and amplifier circuit is described. The plasma is generated using the helicon source of the HelCat plasma device at UNM. HelCat is a 4 meter long, 50 cm diameter machine with a helicon source on one end and hot cathode source on the other. Initial data indicating the presence of Alfv\'{e}n waves in Helium plasma is presented. [Preview Abstract] |
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GP6.00054: Pulsed Alfv\'{e}n Wave Experiments in a Helicon Plasma Source Alexander Hansen, Saeid Houshmandyar, Earl Scime Experiments to test a model for ion heating in the fast solar wind based on ion cyclotron damping of MHD turbulence driven by nonlinearly interacting, low frequency Alfv\'{e}n waves [Matthaeus et. al.,1999], are being conducted in the West Virginia University HELIX (Hot hELIcon eXperiment) device in argon and helium plasmas. It is argued that counter-propagating waves arise from reflection of the waves off of a gradient in the Alfv\'{e}n speed. The HELIX device has a similar speed gradient profile to that found in the solar corona: a short region of high Alfv\'{e}n speed followed by an expansion region of lower Alfv\'{e}n speed. Here we present measurements of pulsed Alfv\'{e}n waves that have been launched via an exciter probe inserted into the helicon source near to the primary RF antenna. The pulsed scheme makes it easier to detect changes in the plasma parameters in time than the CW method and should produce a broader frequency spectrum Measurements of the wave magnetic field structure, wave phase speed, the radial profile of the wave amplitude, along with time-dependent electrostatic probe measurements will be presented as functions of the plasma density and magnetic field strength in the helicon source. [Preview Abstract] |
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GP6.00055: The effects of electron and ion temperature anisotropy on stationary Alfv\'en waves in the kinetic and inertial regimes. S.M. Finnegan, M.E. Koepke, D.J. Knudsen The two-fluid model describing nonlinear stationary Alfv\'en
(StA) waves [Finnegan \textit{et al.}, Phys. Plasmas \textbf
{15}, 052108 (2008)] has been extended to include electron and
ion temperature anisotropy. A StA wave is a nonfluctuating,
nontraveling spatial pattern in plasma fluid variables and
electric and magnetic fields. Electron inertia and parallel
gradients in electron pressure balance a magnetic-field-aligned
component of electric field capable of accelerating electrons
along magnetic field lines. Electron temperature anisotropy is
shown to either reduce or enhance the parallel component of
electric field depending on the sign of the anisotropy
parameter $a_e=1-T_{e\parallel}/T_{e\perp}$. In the small
amplitude limit, electron temperature anisotropy is predicted
to reduce the parallel wave phase speed for $T_{e\parallel} |
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GP6.00056: Simulation of Self-Induced Mode Transitions of a Spatially-Localized Langmuir Eigenmode in a Cathode Sheath M.I. Zimmerman, S.M. Finnegan, H. Gunell, M.E. Koepke, N. Brenning Laboratory experiments and computer simulations have previously been carried out to study situations in which an electron beam was accelerated from low electric potential, low density, and high magnetic field to high potential, high density, and low magnetic field in the sheath of a hot cathode discharge. Beam-driven Langmuir waves at the top of the density `ramp' form an eigenmode in a cylinder bounded by the cathode and aligned with the axial magnetic field. The E-field envelope develops a localized shape at its maximum, referred to as a spike, where the eigenmode frequency matches the local plasma frequency. We perform computer simulations on the sensitivity of the location and frequency of the spike to the density gradient. The previously-used 1D PIC code is employed to investigate the relaxation of the eigenmode's spike. The hypotheses are that (a) the initially-excited eigenmode is determined by the slope of the density ramp and (b) the spike jumps discontinuously among different eigenmodes as it evolves in time. The spike envelope exhibits a bursty time-dependence which may explain the spectrum of whistler-mode waves observed in recent laboratory experiments. [Preview Abstract] |
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GP6.00057: Simulation of a plasmoid penetrating a magnetic barrier Herbert Gunell, J.J. Walker, T. Hurtig, M.E. Koepke, N. Brenning, H. Nilsson Perturbed currents perpendicular to the magnetic field 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 cannon to shoot a plasma at a magnetic barrier (Brenning, et al., PoP, 2005). Simulations of the above scenario for different values of the plasma density have reproduced experimentally observed lower hybrid frequency oscillations (Gunell, et al., Plasma Phys. Control. Fusion, 2008). We present simulations of plasmoids that are longer than those previously published and run over longer periods of time. New findings are waves propagating upstream from the barrier, and also that the penetration process causes the part of the plasmoid that is upstream of the barrier to rotate. [Preview Abstract] |
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GP6.00058: Finite ion gyroradius effects in exciting low frequency electrostatic waves in a {\it Q}-machine P.M. Miller, M.E. Koepke, E.W. Reynolds, H. Gunell Low-frequency ($\sim 3~f_{ci}$) electrostatic waves appear in a magnetized, cylindrical barium plasma in a double-ended {\it Q}-machine. Coaxial plasmas with different potential were formed, yielding a strong (up to $25~V/cm$), narrow ($HWHM \sim \rho_{i}/2$) radial electric field at the cylindrical interface. We evaluate the plausibility of three free energy sources. (1) Cross-field electron current (2) ${E \times B}$-drift shear, and (3) ion-temperature anisotropy. We characterize the dependence of the wave frequency and wave-vector on the magnetic field strength and neutral pressure. We eliminate drift, cyclotron, and ion-acoustic mechanisms and are left with shear-modified ion acoustic, velocity space, and Farley-Buneman instability mechanisms. [Preview Abstract] |
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GP6.00059: Mirror and Weibel Instability: Similarities and Nonlinear Dynamics Galina Dudnikova, Roald Sagdeev, Mikhael Balikhin, Oleg Pokhotelov A commonality in nonlinear saturation mechanisms of Mirror and Weibel instabilities near threshold is demonstrated. In both cases the major contribution is provided by modification of the velocity distribution function in the vicinity of small parallel particle velocities (ions in Mirror case and electrons in Weibel). The final relaxation scenario is based on almost resonant particle interaction with Mirror/ Weibel modes. This scenario differs from that in quasilinear plateau formation (or equivalent trapping effects). The analogy between nonlinear regimes of those instabilities developing far from thresholds becomes muted. The saturated plasma state can be considered as a magnetic counterpart to electrostatic BGK modes. Our analytical model is verified by relevant numerical simulations. Test particle and PIC simulations indeed show that it is a modification of distribution function at small parallel velocities that results in fading away of free energy driving Mirror/Weibel modes. The multipoint measurements in space plasma are used to validate a proposed scenario. [Preview Abstract] |
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GP6.00060: Low Frequency Electrostatic Waves in Inhomogeneous Plasma Modeled by Kappa Distributions Bamandas Basu Linear dispersion relations for electrostatic waves in inhomogeneous, current-carrying, anisotropic plasma, where the equilibrium particle velocity distributions are modeled by various Lorentzian (kappa) distributions, are presented. Spatial inhomogeneity includes density gradient, temperature gradients and gradient (shear) in the parallel flow velocity associated with the current. Special attention is given to the low frequency (lower than ion cyclotron frequency) and long perpendicular wavelength (longer than ion gyroradius) modes. Specifically, stability properties of drift waves, current-driven ion-acoustic waves in the presence of velocity shear, velocity shear-driven ion-acoustic modes, and ion temperature gradient-driven modes are studied in details. Growth rates of drift waves and current-driven ion-acoustic waves in the presence of velocity shear are reduced from their values for bi-Maxwellian distribution due to larger ion damping rates associated with kappa distributions. Consequently, the excitation conditions for these two instabilities become more stringent in the case of kappa distributions. Growth rates of velocity shear-driven ion-acoustic modes and ion temperature gradient-driven modes are also reduced from their values for bi-Maxwellian distribution as a consequence of the reduced adiabatic response of the electrons to the perturbed electrostatic potential. [Preview Abstract] |
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GP6.00061: Averaged variational principle for autoresonant BGK modes Pavel Khain, Lazar Friedland Whitham's averaged variational principle is applied in describing the dynamics of the excitation process of Bernstein- Greene-Kruskal (BGK) modes driven by a chirped frequency wave [1]. A flat-top electron velocity distribution is assumed as a model allowing Lagrangian formulation within the water bag theory [2]. The corresponding Lagrangian, averaged over the fast phase variable, yields variational equations for the slow fields in the problem. In the quasi-linear limit, these equations yield a system characteristic of autoresonance in many nonlinear dynamical problems. Numerical solutions of the full nonlinear set of the variational equations are in a good agreement with Vlassov-Poisson simulations. [1] L. Friedland, P. Khain, and A.G. Shagalov, Phys. Rev. Lett. 96, 225001 (2006). [2] P. Khain and L. Friedland, Phys. Plasmas 14, 082110 (2007). [Preview Abstract] |
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GP6.00062: Cross-field current rotating around a linear magnetized laboratory plasma column as a new mechanism of the low-frequency instability Steve Jaeger, Thiery Pierre A new mechanism leading to low-frequency instability of a linear magnetized plasma column is identified. in the MiSTRAL device (Institute for Fusion Physics, CNRS and Aix-Marseille Univ.), the injection of negative charges along the plasma column with no complete axial collection by the end plate leads to the intermittent relaxation of the plasma potential evolving most often in a rotating plasma channel across the B-field. The negative charges are evacuated along the transverse rotating plasma leading to the recorded cross-field current. The polarization of the collecting tube around the plasma column is the control parameter of this instability which had formerly incorrectly been analyzed as diamagnetic drift waves or cyclotron waves. [Preview Abstract] |
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GP6.00063: Alpha-Channeling with Plasma Waves in Mirror Plasmas Andrey Zhmoginov, Nathaniel Fisch The alpha-channeling effect can be obtained by coupling waves resonating with alpha particles in mirror machines. Modes suitable for alpha-channeling implementation in mirror machines are identified and the achievable efficiency of alpha particle energy extraction is estimated. Methods of coupling external energy sources to the identified plasma modes are proposed. The feasibility of implementing alpha-channeling concept in existing mirror machines including Gamma 10 and AMBAL-M is discussed. [Preview Abstract] |
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GP6.00064: Fast Particle driven Alfven Quasimodes in Tokamaks Ian Abel, Boris Breizman, Sergei Sharapov Upward sweeping Alfv\'en cascade eigenmodes have long been observed in tokamak reverse shear discharges. These upward sweeping modes have been explained as eigenmodes, localized around $q_{\mbox{min}}$ driven by fast ion inhomogeneities in the plasma~[Physics of Plasmas 10 3649]. However in flat $q$-profile discharges an accompanying downward sweeping mode, not provided for by the previous work, are occaisionally observed[Physics of Plasmas 12 112506]. We explain these modes as quasimodes supported by energetic particle drive against their weak radiative decay. The computation of the fast particle drive is simplified using an action/angle variable drift kinetic formalism to treat the trapped fast particle population. [Preview Abstract] |
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GP6.00065: Energetic Particle-induced Geodesic Acoustic Mode Guoyong Fu A new energetic particle-induced Geodesic Acoustic Mode (EGAM) is shown to exist. The mode frequency, mode structure, and mode destabilization are determined non-perturbatively by energetic particle kinetic effects. In particular we find the EGAM frequency is substantially lower than the standard GAM frequency. The radial mode width is determined by the energetic particle drift orbit width and can be fairly large for high energetic particle pressure and large safety factor. These results are consistent with the recent experimental observation of the beam-driven n=0 mode in DIII-D. The new mode is important since it can degrade energetic particle confinement as shown in the DIII-D experiments. The new mode may also affect the thermal plasma confinement via its interaction with p lasma micro-turbulence. [Preview Abstract] |
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GP6.00066: The Effect of Weak Collisions on the Plasma Wave Echo Carrie Black, Kai Germaschewski, C.S. Ng, Amitava Bhattacharjee It has been shown recently that weak collisions, which are a singular perturbation on the collisionless Vlasov equation, have a profound effect on the underlying spectrum for linear plasma waves by eliminating the Case-Van Kampen continuous spectrum and replacing it with a complete class of discrete eigenmodes [C.S. Ng, A. Bhattacharjee, F. Skiff, Phys. Rev. Lett. {\bf 83}, 1974 (1999); {\bf 92}, 065002 (2004).]. This discovery has important consequences for the regime of validity of C. H. Su and C. Oberman's classical theory [Phys. Rev. Lett. {\bf 20}, 427 (1968)] on the collisional decay of plasma wave echoes. Using a fully nonlinear one-dimensional Vlasov-Poisson system solver including the Lenard-Bernstein collision operator, we have studied the effects of collisions on the echoes. We have identified the Su-Oberman regime on intermediate time scales. The long-time asymptotics of the system and its relation to the complete set of discrete eigenmodes found by Ng, Bhattacharjee and Skiff will be discussed. [Preview Abstract] |
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GP6.00067: Electromagnetic effects on geodesic acoustic modes A. Smolyakov, X. Garbet, C. Nguyen, G. Falchetto, M. Ottaviani A new type of electromagnetic modes induced by geodesic compressibility is predicted. The modes are related to\ Alfven and geodesic acoustic modes. While a standard geodesic acoustic mode involves poloidally and toroidally symmetric perturbations of electrostatic potential ($m=n=0)$ and the first poloidal side-bands of plasma pressure, new modes involve side-bands of the electrostatic and vector potential as well as pressure perturbations at zeroth and second harmonics. Both standard (electrostatic) geodesic acoustic modes and new electromagnetic modes involve finite perturbations of parallel viscosity, which modify an effective adiabatic (compressibility) index for a toroidal plasma. Dispersion relations are derived by using the Grad hydrodynamic equations, which thereby reconcile long known but not previously explained discrepancy between the results of kinetic and fluid calculations. The electromagnetic effects on geodesic acoustic modes due to electron parallel motion are also investigated by employing the kinetic theory and appropriate expansion of the electron distribution function. The dispersive corrections to the mode frequency are calculated in two different regimes. [Preview Abstract] |
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GP6.00068: Global Magnetorotational Instability with Density Gradients Jesse Pino, Swadesh Mahajan The magnetorotational instability (MRI) is an important mechanism for the transfer of angular momentum in rotating astrophysical systems such as accretion disks and proto-neutron stars (PNS). The standard MRI dispersion relation is drawn from a local analysis, but can be misleading when the radial wavelength is comparable to the equilibrium scale size. We examine global perturbations of a differentially rotating MHD plasma with radial density gradients. If the equilibrium magnetic field is either purely axial or purely toroidal, axisymmetric modes can be found as global radial eigenvalues of an effective potential. A class of unstable `cavity' modes are found to be localized by the form of the rotation and density profiles, with reduced dependence on boundary conditions. For equilibria in which rotation shear is restricted to a finite area, a well-defined boundary-condition problem is solved and the results are compared with the local theory, and the importance of these MRI modes to core collapse supernova is analyzed. Non-axisymmetric perturbations and helical equilibrium field profiles are investigated, as well as consequences for the nonlinear regime. [Preview Abstract] |
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GP6.00069: Magnetorotational instability with Hall effect in nonuniform magnetic field Victor Ilgisonis, Ivan Khalzov, Dalton Schnack, Fatima Ebrahimi Magnetorotational instability (MRI) is a possible mechanism of turbulence in rotating plasma, e.g., in accretion disks. Originally MRI was found as an axisymmetric magnetofluid instability. It appears if plasma angular velocity exceeds a threshold value determined by imposed magnetic field. There are two physical effects able to modify MRI threshold value. The first is the Hall effect which makes MRI threshold depending on direction of fluid rotation.\footnote{S. A. Balbus, C. Terquem, ApJ, \textbf{552}, 235 (2001)} The second is magnetic field inhomogeneity which can decrease MRI threshold.\footnote{V.I.Ilgisonis, I.V.Khalzov, JETP Lett., \textbf{86}, 705 (2007)} Here we study the combined influence of these two effects on a linear stability of incompressible Keplerian rotating fluid. Both axisymmetric and non-axisymmetric modes which have quite different instability thresholds are considered. [Preview Abstract] |
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GP6.00070: Sheared flow measurements in a magnetized plasma column A. Eadon, E. Tejero, E. Thomas Transverse and parallel sheared flows are important topics in both space and fusion plasmas, and have been the subjects of extensive study. The Auburn Linear EXperiment for Instability Studies (ALEXIS) is a 170 cm long, 10 cm diameter linear magnetized plasma column. Previous investigations [E. Thomas, et al., Phys. Plasmas, 10, 1191 (2003)] on the ALEXIS device have focused on using modifications of the radial electric field to generate spatially inhomogeneous flows, which in turn led to the generation of ion cyclotron instabilities. To better classify and study these low frequency instabilities, a Laser Induced Fluorescence (LIF) system was recently installed to characterize the plasma flow. Initial measurements of flows in radio frequency (rf) generated plasmas will be presented, along with evidence of flow modification resulting from varying the radial electric field. [Preview Abstract] |
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GP6.00071: Laboratory Investigations of Electromagnetic Shear-Driven Instabilities Erik Tejero, Bill Amatucci, Guru Ganguli, Edward Thomas Observations of low frequency, electromagnetic ion cyclotron waves have been made in many regions of the space environment. These waves are thought to be generated by an electron beam-driven instability, but problems have arisen when trying to match observations with theory. Sheared flows produced by localized electric fields coupled with a perpendicular magnetic field are a potentially important energy source that can create waves of this type. In situ observations have led to a laboratory effort to investigate the impact of electromagnetic, velocity shear-driven instabilities on the near-Earth space plasma dynamics. Preliminary results from experiments on the Space Physics Simulation Chamber at NRL are presented. [Preview Abstract] |
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GP6.00072: Status and future plans for studies of nonlinear Alfv\'{e}n waves and Alfv\'{e}nic turbulence in a laboratory plasma T.A. Carter, D.W. Auerbach, S. Vincena, W. Gekelman, C. Cooper, P. Pribyl A program of studying processes associated with large amplitude kinetic Alfv\'{e}n waves is ongoing on the LAPD at UCLA. The goal of this research is to investigate fully-developed Alfv\'{e}nic turbulence, driven by injection of waves and a nonlinear cascade. Primary results of the study to date have concerned three-wave interactions and nonlinear processes associated with single large amplitude waves. With single large amplitude waves, substantial electron heating is observed which creates filamentary structure in the plasma temperature, density and potential. The structuring of the background plasma results in the excitation of drift-Alfv\'{e}n waves. These drift waves then interact with the incident Alfv\'{e}n wave, causing sideband generation which results in a nearly broadband state at high wave power. The strong damping of Alfv\'{e}n waves in LAPD that gives rise to the observed heating is problematic for the observation of a turbulent cascade: the damping time competes with the nonlinear energy transfer time. A new toroidal facility at UCLA is being developed which will have much longer plasma length, lower collisionality and higher plasma $\beta$ than LAPD. A discussion of the possibility of developing a turbulent cascade in this machine will be presented, including mention of new physics studies made possible by higher plasma $\beta$. [Preview Abstract] |
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GP6.00073: Experimental studies of interactions between Alfv\'{e}n waves and striated density depletions in the LAPD D.W. Auerbach, T.A. Carter, S. Vincena Satellite measurements in the earth's magnetosphere have associated Alfv\'en frequency fluctuations with density depletions striated along the geomagnetic field. This poster presents laboratory studies in the LADP experiment at UCLA modeling this phenomena. Density depletions are pre-formed in the plasma column by selectively blocking a portion of the drive beam, and Alfv\'en waves are driven in the cavity by means of an inserted antenna. Relevant experimental parameters include an ion cyclotron radius around a mm, alfven parallel wavelength several meters, electron inertial length around 6 mm, and electron thermal speeds about a third of the alfv\'en speed. We report here on modifications to the wave propagation due to the density depletion. We also report on the details of the interactions between the driven wave and the secondary drift-alfv\'en wave instabilities that arise on the density boundary, including wave-wave interactions and possible turbulent broadening effects on the main wave. [Preview Abstract] |
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GP6.00074: Penetration of a Laser-Produced Plasma Across an Externally Applied Magnetic Field Christopher Plechaty, Radu Presura, Sandra Wright, David Martinez, Stephan Neff, Vladmir Ivanov, Yuriy Stepanenko Plasma flow across a magnetic field is an important topic in laboratory plasmas. In recent experiments performed at the Nevada Terawatt Facility, a plasma flow created by ablating a polyethylene target with a high-intensity laser (10 joules in 1 ps), was allowed to interact with a magnetic field produced by passing a 0.6 MA current through a straight cylindrical electrode. In experiment, the laser-produced plasma was diagnosed with interferometry and Schlieren diagnostics in the plane perpendicular to the magnetic field. The laser-produced plasma was observed to be collimated by, and penetrate, the externally applied magnetic field. In previous experiments performed with a ns laser (Mostovych (1988), Peyser (1992) and Burneteau (1970)), a similar effect was observed, and was attributed to \textbf{E} X \textbf{B} drift produced by the polarization of the plasma flow. All current progress will be presented. Work supported by DOE/NNSA grant DE-FC52-06NA27616. [Preview Abstract] |
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GP6.00075: Gyrokinetic analysis of shear flow instability in torodial geometry Eisung Yoon, T.S. Hahm Motivated by recent observation of intrinsic rotation in tokamak plasmas, we study linear stability of ion gyroradius scale short wavelength fluctuations in the presence of sheared parallel flow, ion temperature gradient, and toroidal mode coupling. Our gyrokinetic approach in toroidal geometry is an extension of previous studies including those by Catto et al., [Phys. Fluids \textbf{16} 1719 (1973)] Mattor and Diamond [Phys. Fluids \textbf{31} 1180 (1988)], and Artun and Tang [Phys. Fluids B \textbf{4} 1102 (1992)]. [Preview Abstract] |
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GP6.00076: NONLINEAR PHENOMENA, TURBULENCE AND TRANSPORT |
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GP6.00077: Autoresonant backward Raman scattering in inhomogeneous plasmas Oded Yaakobi, Lazar Friedland, Ryan Lindberg, Andrew Charman, Gregory Penn, Jonathan Wurtele New solutions to the coupled three-wave equations in a nonuniform plasma medium are presented that include both space and time dependence of the waves. By including the dominant nonlinear frequency shift of the material wave, it is shown that if the driving waves are sufficiently strong (in relation to the medium gradient), a nonlinearly phase-locked solution develops that is characteristic of autoresonance. In this case, the material (electrostatic) wave develops into a front starting at the linear resonance point and moving with the wave group velocity in a manner such that the intensity increases linearly with the propagation distance. The forms of the other two (electromagnetic) waves follow naturally from the Manley-Rowe relations. [Preview Abstract] |
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GP6.00078: Whistler emissions from nonlinear EMHD fields J.M. Urrutia, R.L. Stenzel, K.D. Strohmaeir Time-varying magnetic fields comparable to the ambient dc magnetic field are produced in the parameter regime of electron MHD (EMHD) in a large laboratory plasma. These are excited with magnetic loop antennas driven by a large low frequency ac current ($f=$ 200~kHz~$\ll$ $f_{ce}=$ 14~MHz~$\ll$ $f_{pe}=$ 4~GHz). Weak whistler emissions are observed from current layers which may coincide with magnetic null lines. The waves are transient and propagate generally oblique to the total magnetic field which is nonuniform and time-varying. The waves can be produced by whistler instabilities due to temperature anisotropies or currents. Magnetic null lines are frequently sources of whistler emissions, but cross-field currents have also been identified as producing transient waves. When the loop antenna axis is parallel to the ambient field, the EMHD field itself propagates and becomes a moving wave source. The topology of fields and waves requires 3D measurements to unravel when the loop antenna axis is orthogonal to $B_{0}$. A strong antenna field can magnetize the electrons even in the absence of the ambient field and support locally produced whistler emissions. [Preview Abstract] |
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GP6.00079: Efficient whistler wave excitation in laboratory and space plasmas K.D. Strohmaier, J.M. Urrutia, R.L. Stenzel In a dense, weakly magnetized laboratory plasma, strong low frequency whistler modes are excited with magnetic loop antennas. Wave amplitudes in excess of the ambient magnetic field have been produced. When the loop axis is along the ambient field, the wave creates field reversals or field enhancements which propagate differently. Three-dimensional fields arise when the loop axis is orthogonal to $B_{0}$. In both cases, typically 10 kW of radiated power is obtained, as derived from internal magnetic field measurements as well as from external antenna current and voltage measurements. Such power levels are desirable to eject from spacecraft to control the population of trapped energetic electrons. Since it is difficult to deploy a properly scaled loop antenna in space, it is suggested to reproduce the laboratory plasma parameters with a dense plasma source on the spacecraft and to expand the plasma and field to the ambient conditions. The injected power will disperse since there is no wave cutoff. The expansion process can also be demonstrated in a nonuniform laboratory plasma. [Preview Abstract] |
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GP6.00080: Magnetic bubbles in unmagnetized plasmas R.L. Stenzel, J.M. Urrutia, K.D. Strohmaier A strong dipole magnetic field is produced with a loop antenna in a large unmagnetized laboratory plasma. The field oscillates well below the electron plasma frequency ($f=$ 200~kHz~$\ll$ $f_{pe}=$ 4~GHz) such that all electromagnetic modes are evanescent. However, the field strength ($B_{\mathrm{max}} \simeq$ 50~G) is sufficiently large to locally and temporally magnetize the electrons such that wave propagation in the whistler mode is possible. The space-time behavior of the magnetic field is measured at large and small amplitudes. The field topology forms field-reversed configurations when the antenna field reverses sign while the field from previous cycles is still frozen into the plasma. Magnetic helicity consistent with whistler modes is created. Convection dominates in the center, while diffusion dominates at the boundary of the magnetic bubble. Rapidly varying transient magnetic fields are created near magnetic null points. [Preview Abstract] |
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GP6.00081: A PIC simulation study of non linear frequency shift and damping rate of 1D plasma waves Thomas Grismayer, Benjamin Winjum, Jay Fahlen, Frank Tsung, George Morales, Warren Mori Recently much interest has been devoted to non linear plasma waves. Here we address the 1D electrostatic PIC simulations of the non linear effects associated with the evolution of impulse excited plasma waves in the kinetic regime ($k\lambda_D \ge 0.3$). The simulation results are in reasonable agreement with the transient and asymptotic theoretical predictions (non linear Landau damping and frequency shift) of Morales and O'Neil for parameters where their theory is appropriate. Furthermore, the strength in which the theory holds is tested by varying the parameters outside of the range of validity. Classical non linear effects such as sideband instabilities, damping rate of high amplitude plasma waves and BGK modes will also be discussed. In addition, we will consider the effects of small numbers of resonant particles. [Preview Abstract] |
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GP6.00082: On the Fate of Potential Vorticity Homogenization with Magnetic Linkage and Suppression of Zonal Flows in Beta-Plane MHD T. Brown, S.R. Keating, P.H. Diamond, S.M. Tobias We investigate the effect of external linkage by a large scale magnetic field upon the dynamics of (potential) vorticity transport and homogenization in 2D and beta plane MHD. The motivation for this study is to understand the mechanism whereby even a relatively weak mean magnetic field can suppress jet formation \textit{at levels well below that required for simple linear ``Alfvenization'' of the Rossby Wave Turbulence}. In particular, we aim to elucidate the magnetic Prandtl number dependence of the vortex or jet PV gradient homogenization, thought to be required for jet formation. Analytical and computation studies of the residual PV gradient dependence on Reynolds number, Alfvenic Mach number and magnetic Prandtl number will be presented. Applications to the problem of jet formation in the solar tachocline will be discussed. This research is supported by DoE Grant Numbers FG02-04ER54738 and FC02-08ER54959. [Preview Abstract] |
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GP6.00083: Studies of Strong Langmuir Turbulence at the HAARP Ionospheric Observatory J.P. Sheerin, M.E. Bacon, J.M. Gerres, B.J. Watkins, W.A. Bristow, S.I. Oyama, C.J. Heinselman High power HF transmitters have induced a number of plasma instabilities in the interaction region of overdense ionospheric plasma. We report results from a series of such experiments using over one gigawatt of HF power (ERP) in comprehensive studies of strong Langmuir turbulence (SLT) and particle acceleration at the HAARP Observatory, Gakona, Alaska. 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, collapse, cascade and co-existing spectra, control of artificial field-aligned irregularities (AFAI), the aspect angle dependence 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. We have discovered a second region of strong interaction displaced southward of the primary HF interaction region. Experimental results are compared to previous high latitude experiments and predictions from recent modeling efforts. [Preview Abstract] |
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GP6.00084: Low Collisionality Neoclassical Toroidal Viscosity in Tokamaks and Quasi-symmetric Stellarators A.J. Cole, C.C. Hegna, J.D. Callen Non-resonant magnetic perturbations can affect plasma rotation in toroidally confined plasmas through their modification to $|B|$. Variations along a field line induce nonambipolar radial transport and produce a global neoclassical toroidal viscous force [NTV]. In this work, previously calculated radial particle fluxes for the low-collisionality ``$\nu$'' and ``$1/\nu$'' regimes [1] are unified into a single particle flux (or toroidal viscous force). Provided pitch-angle scattering dominates over collisional energy exchange, the energy component of phase space can be decoupled into independent regions $\left(E >E_c \right.$ for $\nu$ regime, $E < E_c$ for $1/\nu$ regime, with $E_c$ determined by $\left.\nu_i(E_c) =\epsilon \, \omega_E\right)$ within which the perturbed distribution function can be calculated similar to [1]. Using a technique first employed in axisymmetric neoclassical theory [2], the smoothed particle flux is constructed by summing the partial contributions from $\nu$ and $1/\nu$ banana drift effects respectively. The complete NTV force is expressed in terms of the equilibrium flows and a temperature-gradient-determined ``intrinsic'' flow. [1] K.C.~Shaing, Phys.~Plasmas, \textbf{10}, 1443 (2003). [2] K.T.~Tsang, and J.D.~Callen, Phys.~Fluids \textbf{19}, 667 (1976). [Preview Abstract] |
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GP6.00085: Fluid Moment Transport Equations In Tokamak Plasmas J.D. Callen, C.C. Hegna, A.J. Cole Transport equations for tokamak plasmas are usually obtained by first taking flux-surface averages of the collisional Braginskii equations. Then, {\it ad hoc} terms are added for: neoclassical effects on the parallel Ohm's law (trapped particle effects on resistivity, bootstrap current); fluctuation-induced transport; heating, current-drive \& flow sources and sinks; small non-axisymmetries; etc. However, tokamak plasmas are usually not in collisional regimes. We have begun developing self-consistent second order in gyroradius fluid-moment-based transport equations, including poloidal and toroidal mass flow equations, in nearly axisymmetric single-ion-species tokamak plasmas. The derivation begins from fluid moments of the plasma kinetic equation, incorporates constraints from faster processes (compressional Alfven waves, sound waves, poloidal flow damping) and includes: neoclassical effects through kinetically-determined parallel viscosity and heat flux closures, fluctuation-induced transport through ensemble averages, paleoclassical effects through transforming from laboratory to poloidal flux coordinates, neoclassical toroidal viscosity (NTV) induced by slight magnetic field non-axisymmetries, and the effects of sources and sinks. [Preview Abstract] |
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GP6.00086: Drift-kinetic theory for electron fluid closures in extended-MHD with applied RF wave sources J.J. Ramos, J.D. Callen, C.C. Hegna A form of the electron drift-kinetic equation is derived, suitable to evaluate the pressure anisotropy and the parallel heat flux needed to close the extended-MHD electron fluid equations that describe the interaction of slow macroscopic instabilities with externally applied ECCD waves. The effect of the RF source is represented by a quasilineardiffusion operator and the collisional terms are evaluated under a realistic ordering of the collisionality for fusion-relevant conditions. The analysis is carried out following a systematic expansion in the ratio between the ion sound gyroradius and the macroscopic lengths, assuming first-order distortions from Maxwellian distribution functions and a second-order electron to ion mass ratio. Important physical effects taken into account are the precise contribution of the parallel electric field, inhomogeneous and compressible macroscopic flows, finite ion sound gyroradius effects that contribute to the diamagnetic perpendicular heat flux even in the limit of vanishing electron mass, and independent density and temperature gradients. [Preview Abstract] |
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GP6.00087: Whistlers of ultra-low frequency in tokamak discharges with Internal Transport Barriers Boris Breizman, Sergei Sharapov Conventional frequencies of whistlers are above the ion gyrofrequency, so that ions are not magnetized in these waves. However, in perturbations with very short radial wavelength, $k_r \rho_i \geq 1$, the ions are not magnetized even at lower frequencies, which creates a parameter window for ultra-low-frequency whistlers. We present a theoretical description of such waves in inhomogeneous toroidal plasmas. The electron density gradient is shown to cause strong currents parallel to the equilibrium magnetic field and to form radial eigenmodes. The characteristic features of these modes are similar to the experimentally observed magnetic turbulence in ITB discharges on JET [1]. The relation between these modes and short-wavelength zero-frequency magnetic islands [2, 3] is also discussed. [1] S.E. Sharapov, F.M. Poli and JET-EFDA Contributors, EPS (2008). [2] B.B. Kadomtsev, Nuclear Fusion, v.31, p.1301 (1991). [3] P.H. Rebut and M. Hugon, PPCF, v.33, p.1085 (1991). [Preview Abstract] |
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GP6.00088: Heat Flux Suppression by Zonal Flow Shear Mikhail Malkov, Patrick Diamond The capability of thermal Rossby and drift wave turbulence, driven by a temperature gradient, to suppress the driving flux is well known. The suppression mechanism is based on zonal flow generation. Several transport bifurcation models for the L-H transition were built upon this idea. However, the microphysics of the relation between the nonlinear flux reduction and turbulence is not well understood. The models remain ambiguous in prescribing the exact location of the L-H transition. To further our understanding of the phenomenon we consider a standard 2D system of equations for the Boussinesq fluid with a temperature gradient across the fluid layer. We suggest an exact solution of this system in the limit of zero viscosity and thermoconductivity. The solution generalizes hydrodynamic solution for a traveling wave in a shear flow. The temperature gradient jumps across the critical layer. The perturbative, time dependent extensions of this solution will be discussed to elucidate the flux dependence on the cross-phase of the transport driving perturbations. [Preview Abstract] |
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GP6.00089: Investigation of Finite Amplitude Growth Rates in Microturbulence Simulations David Hatch, Paul Terry, Varun Tangri, Bill Nevins Microturbulence properties are generally framed in terms of the linear growth rate of the most unstable mode. However, the nonlinear growth rate (the rate of energy input or damping at \textit{finite} amplitude) can be quite different from the linear growth rate in both amplitude and wavenumber dependence. This is because the nonlinear growth rate can be constructed from a superposition of the effects of all (stable and unstable) accessible linear eigenmodes. It has been shown that stable eigenmodes are excited to levels sufficient to significantly affect saturation and transport in both simple fluid models and gyrokinetic models. We describe a nonlinear growth rate diagnostic for GYRO. This will be used to better understand the dynamics of finite amplitude energy input and the effects of damped eigenmodes in gyrokinetic simulations. Comparisons of linear and nonlinear growth rates will be presented. In addition, special attention is given to zonal (k$_{y}$=0) fluctuations, which are comprised of linearly undamped zonal flows and other modes which are linearly damped and are potentially a potent energy sink. [Preview Abstract] |
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GP6.00090: Survey of Damped Eigenmode Effects in Fluid Models of Plasma Turbulence P.W. Terry, D.R. Hatch, J.-H. Kim Eight distinct instability-driven plasma turbulence systems of relevance to fusion are surveyed for damped eigenmode effects in saturation using an analytic criterion based on general saturation balances.\footnote{P.W. Terry, et al., Phys. Plasmas \textbf{13}, 022307 (2006).} All are found to have regimes in which damped eigenmodes provide a finite-amplitude-induced energy sink that plays a significant role in saturation. Analysis of these models gives constraints on the linear and nonlinear coupling between fields conducive to damped eigenmodes affecting saturation. The most critical condition is that the damping rate of the damped eigenmode not greatly exceed the growth rate. This tends to hold in regimes of stronger instability, not right at threshold. However, nonlinearities also matter. Certain linear couplings that hinder damped eignemode effects in electrostatic turbulence are overcome by magnetic nonlinearities. This study indicates that damped eigenmodes are a robust and ubiquitous mechanism for the saturation of plasma instability in local fluid descriptions. Supported by USDOE. [Preview Abstract] |
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GP6.00091: Drift-Wave Eigenmodes and Spectral Gaps in Tandem Mirrors J. Pratt, H.L. Berk, W. Horton The GAMMA-10 tandem mirror system has achieved long energy confinement times ($70-90$ ms) with radial losses occurring faster than the electrostatically-plugged end-loss time ($100$ ms). This high confinement regime establishes a proof of principle that the combination of electrostatic and mirror confinement can successfully insulate electrons from thermal end losses. Scaling laws derived by Pratt and Horton [J. Pratt and W. Horton, Phys. Plasmas (13), 2006] provide a key prediction that drift wave motion in the tandem mirror geometry is qualitatively different from toroidal systems. With a discrete eigenmode solver, a shooting method, and by evaluating reflection coefficients, we calculate drift-wave frequencies in three model tandem mirror machines: the GAMMA-10, the kinetically stabilized tandem mirror (KSTM), and the LAPD machine where the magnetic field has been modulated to look like a tandem mirror. We examine the nature of the spectral gaps in these machines and their relation to instabilities. We validate the results of these calculations by comparing precisely with the Mathieu equation as well as with magnetic probe signals recorded in the LAPD as a function of axial position and of frequency of the antenna. We present 3D visualizations of the energetic ion orbits in each of these devices using a highly-parallel particle transport code. Work supported by DOE Grant No. DE-FG02-04ER54742. [Preview Abstract] |
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GP6.00092: Geodesic Acoustic Mode Induced by Trapped Energetic Paticles Tianchun Zhou, Herbert L. Berk Energetic particle driven global geodesic acoustic mode(GGAM) has been observed in JET[1, 2] and DIII-D[3, 4]. The mode is to be treated in fully kinetically. The descriptions of the background electrons and ions are based on standard low and high frequency expansions respectively. However the energetic ions must be treated without any inherent expansion of its bounce frequency to it mode frequency. For the distribution functions, we take the background ions and electrons to be Maxwellian at different temperatures, while for hot ions more general distributions are taken. We construct the quadratic form for the wave amplitude, from which an integro-differential equation is derived. We then investigate the limits, where the mode width is of the same order as the thickness of the energetic particle orbit thickness, and where the mode width is much larger than the orbit thickness. Numerical and analytic results will be presented. The study of passing energetic particles will be compared to the work recently presented by Fu [4]. 1 C.J. Boswell, et al., PHYSICS LETTERS A 358 154-158 (2006) 2 H.L. Berk, et al., Nucl. Fusion 46 S888-S897 (2006) 3 R. Nazikian, et al., submitted to Phys. Rev. Lett. (2008) 4 G.Y. Fu, submitted to Phys. [Preview Abstract] |
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GP6.00093: Ejection of Toroidal Angular Momentum by Plasma Edge Modes M. Landreman, L. Sugiyama, B. Coppi The ejection of angular momentum by modes in the plasma edge [1] can affect plasma rotation. Large ELM crashes are capable of ejecting significant amounts of toroidal angular momentum from a rotating plasma, depending on the rotation profile. Numerical studies have been carried out using the extended MHD code M3D for DIII-D-like plasmas. The initial work considers a toroidally rotating, axisymmetric MHD plasma that is ideally unstable to ELMs in the absence of rotation. It compares the effects of different rotation profiles $v_{\phi}=R\Omega(\psi)$ on the ELM and the momentum ejection. During the strong nonlinear phase, a ballooning-type ELM instability carries plasma across the magnetic field into the open field line region, while the initial closed plasma magnetic field structure remains relatively well contained. Angular momentum is carried with the plasma and the loss can be relatively large if the edge plasma rotation is large. The smaller change in the edge temperature gradient, also observed in experiment, is consistent with the relatively good confinement of the magnetic field lines. [1] B. Coppi, Nucl. Fusion 42, 1 (2002) [Preview Abstract] |
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GP6.00094: Plasma Angular Momentum Loss Processes: Recoil Involving Mode-particle Resonances with Traveling Modes and Recoil Associated Particle-Ejection by Collisional Ballooning Modes* O. Ohia, B. Coppi The most immediate interpretation [1] of the spontaneous rotation phenomenon in axisymmetric toroidal plasmas involves processes providing ejection of angular momentum from the edge of the plasma column and the consequent recoil of this. One process considered to prevail in the H-regime, where steep pressure gradients can form at the edge, involves a collisional ballooning mode with a toroidal phase velocity in the direction of the electron diamagnetic velocity. The mode has a considerable angular momentum and particles are expelled by it with a toroidal velocity component in the same direction as the mode phase velocity. The other process, assumed prevalent in the L-regime, depends on the presence of a cold particle population at the edge, in addition to the ``hot'' particle population, and on the excitation of an ``impurity-like'' mode with phase velocity in the ion diamagnetic velocity direction. The mode is of the traveling type along the magnetic field, unlike ballooning modes, with relatively short wavelengths (collisionless). The mode serves as a catalyst providing toroidal acceleration and ejection of the hot particle population, and toroidal deceleration and inward transport of the cold population. {*}U.S. D.O.E. partially sponsored.\\ {[1] B. Coppi, \textit{Nucl. Fus.} \textbf{42}, 1 (2002)}.\\ [Preview Abstract] |
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GP6.00095: Radial transport in tokamaks in the gyrokinetic ordering Felix I. Parra, Peter J. Catto We analyze the transport of particles, energy and momentum in the gyrokinetic formalism, retaining both the turbulent shorter wavelengths and the collisional longer length scales (on the order of the minor radius of the tokamak). In particular, we compare the radial transport obtained by using moments of the full Fokker-Planck equation and the radial transport calculated from moments of a gyrokinetic equation correct to first order in ion Larmor radius over minor radius. This comparison is especially interesting because several full f codes are being built that assume that the first order gyrokinetic equation is enough to evolve the full distribution function (and thereby evolve radial profiles). We expect that our analysis will provide insight into whether the missing second order terms in the gyrokinetic equation are important for the evolution of the long wavelength, Maxwellian piece of the distribution function. In addition, our analysis will include momentum transport, needed to determine the long wavelength axisymmetric radial electric field and the toroidal velocity shear. [Preview Abstract] |
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GP6.00096: Gyrokinetic simulations at finite beta William Nevins, Jeff Candy, Yang Chen, William Dorland, Darin Ernst, Frank Jenko, Greg Hammett, Scott Parker, Ron Waltz, Eric Wang Understanding anomalous transport in plasmas at pressures approaching the ideal MHD balloon limit is of great importance to projections of ITER operations. Past efforts to simulate plasma microturbulence as beta is increased toward the ideal limit have met with mixed success.\footnote{ see, e.g., J. Candy et al, Phys. Plasmas \textbf{12}, 072307 (2005), and references therein.} We investigate this problem by comparing results from the GYRO, GS2, GEM, and GENE codes over a sequence of runs in which beta is increased toward the ideal ballooning limit. We will also comment on finite-beta effects to trapped electron modes. [Preview Abstract] |
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GP6.00097: Global gyrokinetic calculations for experimental cases G. Rewoldt, Y. Chen, S.E. Parker The GEM (gyrokinetic electromagnetic) code[1] is a global particle-in-cell code with relatively complete physics, including non-circular cross section via the Miller MHD equilibrium, multiple ion species, electron collisions, and perpendicular ($\mathbf{E}\times\mathbf{B}$) and parallel equilibrium flows, and now can access experimentally-derived profile information from the TRANSP system. A coarse-graining procedure for the electrons has recently been implemented to limit particle weight growth [2]. For a DIII-D case with moderate ion temperatures and moderate rotation, a nonlinear GEM calculation for the core region ($0.1 < r/a < 0.8$) yields maximum ion heat fluxes comparable to experimental values, for the experimental levels of equilibrium flow. Sensitivity of the fluxes to changes in flow and in density and temperature gradients will be discussed. Also, an NSTX case known from flux-tube calculations to be linearly unstable to microtearing modes will be considered in a global nonlinear GEM simulation. [1] Y. Chen and S.E. Parker, J. Comput. Phys. 220, 839 (2007) [2] Y. Chen and S.E. Parker Phys. Plasmas 14, 082301 (2007) [Preview Abstract] |
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GP6.00098: Gyrokinetic Simulation of Energetic Particle Turbulence and Transport Zhihong Lin The confinement of energetic particles (EP) is a critical issue in ITER burning plasmas. The fully self-consistent simulation of the EP turbulence and transport in ITER must incorporate three new physics elements: kinetic effects of thermal particles at the thermal ion gyroradius (\textit{micro} scale), nonlinear interactions of many \textit{meso} scale (energetic particle gyroradius) shear Alfven waves (SAW) induced by the kinetic effects at the \textit{micro} scale, and \textit{meso-micro} couplings of the microturbulence and SAW turbulence. The large dynamical ranges of spatial-temporal processes further require global simulation codes that are efficient in utilizing massively parallel computers at the petascale level and beyond. This paper reports the progress of the gyrokinetic simulation of the EP turbulence and transport in tokamaks using GTC code. Work supported by DOE fusion SciDAC GSEP Center. [Preview Abstract] |
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GP6.00099: Multiscale Finite-Beta Gyrokinetics W.W. Lee, E.A. Startsev, R. Kolesnikov, W.X. Wang Finite-$\beta$ effects on microinstabilities have been investigated by gyrokinetic particles codes using the new double split scheme [Startsev, Lee and Wang, manuscript in preparation]. The scheme requires that $ F=(1 + \psi) F_0+\int dx_{||} {\kappa}_e\cdot (\nabla A_{||}\times {\bf b}_0)+\delta g,\label{eq1} $ so that a new full density gradient, which is set up by the fast electrons transverse to the direction of the full field, is consistent with the condition of $ {\bf b} \cdot \nabla (F_0+\int dx_{||}{\kappa}_e\cdot (\nabla A_{||}\times {\bf b}_0))=0$, where ${\bf b}={\bf b}_0+\delta {\bf B}/B_0$. Here $\phi$ is normalized by $T_e/e$, $A_\parallel$ by $cT_e/e c_s$, $F$ is the total distribution function, $F_0$ is the background distribution function, $\psi = \phi + \int (\partial A_\parallel / \partial t) d x_\parallel/c$, and $\phi$ and $A_\parallel$ are the perturbed potentials. The finite-$\beta$ effects on microinstabilities are found to be related to the multiscale equation of the form, $ ({\rho_s / \lambda_D})^2 \left [\nabla^2 \psi - {\psi / \delta_e^2} \right ]= -4 \pi q {v} \langle \delta({\bf r}) \rangle_\varphi $ where $\varphi$ is the gyro angle based on the ion gyroradius, $\rho_i$, and $\delta_e$ is the electron skin depth, which can be an order of magnitude smaller in the tokamak core. To the lowest order, this equation gives rise to the shielding effect as $ \psi = (q / r) (\rho_s / \lambda_D) exp({-r/\delta_e}). $ Thus, we have the presence of two distinct spatial scales in the problem. A numerical scheme based on the concept of singular perturbation methods is used and the resulting finite-$\beta$ effects on drift instabilities, ion temperature gradient drift modes and electron temperature gradient drift modes will be reported. [Preview Abstract] |
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GP6.00100: Developing Experimentally Relevant Benchmarks for Gyrokinetic Microstability Codes R. Bravenec, J. Candy, W. Dorland, D. Ernst, G. Staebler, R. Waltz A few nonlinear gyrokinetic microstability codes are now capable of simulating tokamak plasmas to an unprecedented level of complexity. Verification of these ``experimentally relevant'' simulations is difficult, however, because no benchmarks exist with which the codes can compare. This work describes the development of such benchmarks through ``apples-to-apples'' comparisons among codes, i.e., comparisons for the same plasma containing the same physics and having sufficient temporal, spatial, pitch-angle, and energy resolutions. A single utility code is used to extract experimental data from analysis by TRANSP, ONETWO, etc., and to produce input files for all the codes. The codes are first run linearly and, if differences in the mode frequencies are found, the computations are simplified by removing shaping, collisions, etc., one at a time, until agreement is reached. This process pinpoints the source(s) of the disagreement which the code developers attempt to resolve. Next, nonlinear runs are undertaken for the same cases and the procedure is repeated. The final results are both linear and nonlinear benchmarks at various levels of complexity by which other codes may be verified. [Preview Abstract] |
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GP6.00101: Numerical Simulation of Drift Wave Turbulent in Magnetized Plasmas Naohiro Kasuya, Masatoshi Yagi, Kimitaka Itoh, Sanae Itoh Drift wave instability is one of the candidates to drive anomalous transport in toroidal plasmas. We have developed a 3- dimensional numerical simulation code called Numerical Linear Device (NLD), which models drift wave turbulence in a simple cylindrical plasma configuration. Using this code, turbulent structural formation mechanisms have been studied, and selective formation of the turbulent structures, zonal flow and streamer, has been clarified by changing a damping parameter of the zonal flow (ion-neutral collision frequency). We have been extending the model to include a toroidal effect. A model helical magnetic field is introduced in averaged reduced MHD equations to study the coupling between interchange modes and drift waves in helical plasmas. Simulation results for cylindrical plasmas and a progress of the extension will be reported. [Preview Abstract] |
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GP6.00102: Simulation of DIII-D shot 132707 using Hybrid MHD-Gyrokinetic Code (HMGC) Andreas Bierwage, Sergio Briguglio, Giuliana Fogaccia, Gregorio Vlad, Fulvio Zonca, Ming-Sheng Chu, Michael Van Zeeland, Liu Chen, William Heidbrink, Zhihong Lin DIII-D shot 132707 was dedicated to the SciDAC GSEP project focusing on energetic particle turbulence and transport, and is intended to serve as a validation case for global codes used to study Alfv\'{e}n eigenmodes. We report and discuss first global nonlinear simulation results obtained with Hybrid MHD-Gyrokinetic Code (HMGC) [1] for this case. \\[3pt] [1] Briguglio S, Vlad G, Zonca F and Kar C, Phys. Plasmas 2, 3711 (1995) [Preview Abstract] |
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GP6.00103: Extension of the FMCFM framework to the neoclassical, paleoclassical, and gyrokinetic transport models Srinath Vadlamani, Alexei Pankin, Scott Kruger, Alex Pletzer, Johan Carlsson, John Cary, Stefan Muszala, Mark Fahey, Jeff Candy Recent improvements to the Framework for Modernization and Componentization of Fusion Modules (FMCFM) are described. The FMCFM framework provides a common interface to the transport modules and libraries such as those in the National Transport Code Collabation (NTCC) module library[1]. The FMCFM interface facilitates access to the transport models from integrated modeling codes and allows interlanguage interfaces using Babel[2]. The new interface to transport modules has been applied to the the GLF23 and MMM95 transport models. Current work of incorporating neoclassical NCLASS and Kapisn codes, a paleoclassical transport model[3] and the GYRO nonlinear tokamak microturbulence package will be presented. A new flux mapping tool that is being included to the FMCFM project in order to interface the equilibrium data generated with legacy equilibrium solvers such EFIT and TEQ is described in this report. The functionality is demonstrated in Framework Application for Core-Edge Transport Simulations (FACETS) project. [1] A. H. Kritz et al. Comp. Phys. Comm. 164,108(2004). [2] G. Kumfert et al. LLNL Tech Report UCRL-CONF-222279. [3]J. D. Callen, Nucl. Fusion 45,1120(2005). [Preview Abstract] |
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GP6.00104: Integrated simulations of tokamak physics using the FACETS framework Ammar Hakim, J. Cary, J. Carlsson, S. Kruger, Miah, S. Vadlamani, A. Pankin, J. Larson, L. McInnes, J. Candy, T. Ronglien, T. Epperly, R. Cohen, M. Fahey, J. Kuehn, P. Worley, A. Malony, S. Shinde, E. Feinbush, G.W. Indireshkumar, C. Indireshkumar, C. Ludescher, L. Rnaderson, D. McCune, A. Yu. Pigarov The FACETS project aims to provide a framework for whole-device simulations of tokamaks. FACETS not only provides infrastructure for developing new physics components but also provides mechanisms for integration and tight coupling of existing components to perform whole-device simulations. In this poster we review the software infrastructure of FACETS and also present results from coupled simulations with core, edge and wall components integrated into FACETS. The core component, FACETS::core, solves core transport equations in the tokamak core. FACETS::core uses the GLF23 model to compute turbulent fluxes at each flux surface and advances the transport equations in time using an nonlinear multigrid scheme. FACETS::core achieves significant speed ups, on the order of 100x to 200x, when compared to the ASTRA core solver. This speedup is achieved using a combination of implicit solvers and flux computations performed in parallel. For the edge computations the UEDGE code is used. [Preview Abstract] |
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GP6.00105: Benchmarking the parallel FACETS core solver Alexander Pletzer, Ammar Hakim, Mahmood Miah, John Cary, Scott Kruger, Srinath Vadlamani, Alexei Pankin The Framework Architecture for Core-Edge Transport Simulations (FACETS) is a SciDAC project targeting whole-device plasma simulations in tokamaks such as ITER. A key component in the multi-physics FACETS effort has been the development of a core transport solver (FACETS::core) that is both robust and runs in parallel. FACETS::core can interface to any of the flux calculators available through the Framework for Modernization and Componentization Fusion Modules (FMCFM), including GLF23 and MMM95. Electron and ion temperatures are advanced implicitly using the nonlinear fluxes from GLF23 (or other model). Here, we present results comparing the stability and accuracy of FACETS::core with the ASTRA transport code. Although FACET::core is slower than ASTRA on a per time step basis, the multigrid algorithm and PETSc/SNES solver applied by FACETS::core allow the latter to take orders of magnitude larger time steps, conferring to FACETS::core a 5-10x overall performance improvement over ASTRA. This, combined with the capability of FACETS::core to scale to tens of processors, contributes towards a wall clock time reduction of the core transport computation by a factor 200-500x. [Preview Abstract] |
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GP6.00106: The effect of plasma triangularity on turbulent transport: modelling TCV experiments by linear and non linear gyrokinetic simulations Alessandro Marinoni, Stephan Brunner, Yann Camenen, Stefano Coda, Jonatha Graves, Xavier Lapillonne, Antoine Pochelon, Olivier Sauter, Laurent Villard Experimental evidence in the TCV tokamak revealed that the core electron heat transport is significantly reduced by a negative triangularity ($\delta$) configuration. This variation in heat transport cannot be explained by the flux surfaces compression; rather, it must be due to a (partial) stabilization of the microinstabilities at play in a positive d configuration. The present work is a theoretical investigation of the effect exerted by triangularity on plasma turbulence of actual TCV shots, using the gyrokinetic code GS2. A complex interplay of deeply and barely trapped particles, which react in opposite ways to triangularity, results in a stabilizing effect of negative $\delta$ on Trapped Electron Modes both in the linear and non-linear phases. The non-linear simulations are in qualitative agreement with the experimental results. [Preview Abstract] |
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GP6.00107: Intrinsic rotation and radial electric field in TCV plasmas Alessandro Bortolon, Basil Paul Duval, Alexander Karpushov, Yanis Andrebe We present intrinsic rotation results of TCV tokamak, based on upgraded CXRS diagnostic. Toroidal rotation $u_{\phi}$ has been measured by 2 horizontally viewing systems, each one recently equipped with 40 optic fibres for light collection and back illuminated CCD detector. Measurements now cover the mid-plane plasma diameter, up to the LCFS, with radial resolution of 6 mm and sample rate of 50 ms. A third high resolution vertical view simultaneously provides poloidal rotation profile $u_{\theta}$ on the LFS mid-plane. This setup has been used to characterize rotation and radial electric field $E_{r}$ (from the radial force balance) in L-mode plasmas with negligible external torque. $E_{r} \simeq -4$~kV/m is measured in gradients region, reversing to positive values in occurrence of $u_{\phi}$ inversions. For $\rho_{\psi} \simeq 1$, a sharp peak in $u_{\theta}$ produces a $E_{r}<0$ well with values down to $-5$~kV/m, reminiscent of H-mode pedestal physics. Assessing the $E_{r}$ profile dependence on plasma parameters and magnetic configuration, we will address $E_{r}$ impact on energy and momentum transport. [Preview Abstract] |
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GP6.00108: Steep-gradient tokamak edge turbulence and the nonlinear instability T. Stoltzfus-Dueck, J.A. Krommes, S.J. Zweben Turbulence just inside the last closed flux surface of a toroidal fusion device is strongly affected by the steepness of the equilibrium gradients, which enhances nonadiabatic electron response. Beginning with an isothermal three-field fluid model, a simple transformation of dependent variables decomposes the electrostatic potential into adiabatic and nonadiabatic portions. Under the assumption of ideal ballooning stability, the linear behavior of the three-field model over the entire parameter space is obtained, regions of drift-like and resistive-ballooning behavior are delineated, and criteria under which the single unstable branch may be expected to be only weakly nonadiabatic are identified. Using quadratic invariants of the model as well as arguments from statistical mechanics, it is demonstrated that turbulence in the steep-gradient region is nonlinearly sustained for typical tokamak parameters. Despite net forward transfer of energy, energy return from large-$k$ fluctuations to small-$k$ fluctuations can effectively excite nonadiabaticity at small $k$, which enhances gradient drive. [Preview Abstract] |
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GP6.00109: Universal properties of the edge turbulence in different fusion experiments and its driving mechanisms Matteo Agostini, P. Scarin, N. Vianello, R. Cavazzana, F. Sattin, G. Serianni, M. Spolaore, R. Maqueda, S. Zweben, B. LaBombard, J. Terry, Y. Yagi, H. Sakakita, H. Koguchi, S. Kiyama, Y. Hirano The edge turbulence of different plasma fusion devices (Tokamaks and Reversed Field Pinches) is studied by means of the Gas Puff Imaging (GPI) diagnostic and Langmuir probes. It is shown that the Probability Density Functions (PDF) of edge fluctuations depend on the time scales of the fluctuations themselves: they are clearly non-Gaussian for smaller time scale fluctuations, and become Gaussian for larger ones. This indicates that the turbulence has an intermittent behaviour in all the devices. This deviation from the Gaussian distribution is due to the presence in the GPI signals of strong emission bursts associated with coherent structures. The time scale where the PDF becomes Gaussian is linked with the turbulence injection scale, consistent with energy cascade that generates structures at smaller and smaller time scales. The different injection scales for the different experiments are linked with the characteristic length of the electron pressure profile, which should act as source of free energy for the turbulence. [Preview Abstract] |
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GP6.00110: Convective Blobs in the Presence of Sheared Flows in a Magnetized Laboratory Plasma Lincan Yan, Mark Gilmore, Christopher Watts Intermittent convective plasma transport across magnetic field lines, so called ``blobs'' has been one of the most important issues in fusion-related edge plasma physics, and is thought to play a key role for cross-field plasma transport in the tokamak scrape-off layer. Experiments have shown evidence of ``blobby'' plasma transport, in which intermittent bursts of fluctuations in ion saturation currents measured by Langmuir probes were analyzed. To investigate the details of how this transport interacts with plasma flows, experiments are being conducted in both the LArge Plasma Device (LAPD) and HELicon-CAThode (HELCAT). HELCAT is a linear device at UNM measuring 4 m in length, 50 cm in diameter, with B $<$ 0.22 T. It can operate two sources simultaneously, both RF helicon and cathode plasmas, with peak helicon-produced densities, n $\sim $ 10$^{13}$ cc and cathode-produced densities, n $\sim $ 10$^{12}$ cc. Sheared ExB flows, generated via biased concentric rings, are utilized to modify the flow profile. Fluctuations and flux are monitored with probe arrays are measured by a Mach probe. Blobs are observed at both LAPD and HELCAT cathode plasma. No blobs have been found in HELCAT helicon plasmas so far. It is also found that strong shear flow exists at the plasmas edge in LAPD. Experimental and analysis results will be presented. [Preview Abstract] |
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GP6.00111: Characterization of Drift-wave Turbulence in the Sheared, Cylindrical Slab Kevin Lee, Kenneth Gentle We present an experimental characterization of drift-wave turbulence in the Helimak, a finite realization of the sheared, cylindrical slab used in turbulence calculations. Measurements of electrostatic turbulence are made both using an large fixed array of langmuir probes and a moveable array on a motorized probe drive. We examine such non-spatially oriented quantities as turbulence levels, fluctuation frequencies, and phases between density and electrostatic potential fluctuations. Measurements on dispersion relations and coherence lengths in both the radial and vertical directions are used to characterize the turbulence in the plane perpendicular to the magnetic field. Radial turbulent transport is also investigated. In addition to this information, we present a study of fluctuations parallel to the field lines, including measurements of parallel coherence lengths and parallel wavenumbers. Furthermore, we characterize fluctuations of both radial and vertical magnetic fields. We explore the relationships between density, potential, and magnetic turbulence. Finally, a description of nonlinear aspects of the turbulence in this configuration such as mode coupling and intermittency is offered. To complete our characterization, comparisons to theory are given where possible. Supported by DOE-OFES grant DE-FG02-04ER54766. [Preview Abstract] |
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GP6.00112: Photon-assisted Beam Probes for Low Temperature Plasmas and Installation of Neutral Beam Probe in Helimak Alvaro Garcia de Gorordo, Gary A. Hallock, Nirmala Kandadai The Heavy Ion Beam Probe (HIBP) diagnostic has successfully measured the electric potential in a number of major plasma devices in the fusion community. In contrast to a Langmuir probe, the HIBP measures the exact electric potential rather than the floating potential. It is also has the advantage of being a very nonperturbing diagnostic. We propose a new photon-assisted beam probe technique that would extend the HIBP type of diagnostics into the low temperature plasma regime. We expect this method to probe plasmas colder than 10 eV. The novelty of the proposed diagnostic is a VUV laser that ionizes the probing particle. Excimer lasers produce the pulsed VUV radiation needed. The lasers on the market don't have a short enough wavelength too ionize any ion directly and so we calculate the population density of excited states in a NLTE plasma. These new photo-ionization techniques can take an instantaneous one-dimensional potential measurement of a plasma and are ideal for nonmagnitized plasmas where continuous time resolution is not required. Also the status of the Neutral Beam Probe installation on the Helimak experiment will be presented. [Preview Abstract] |
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GP6.00113: Experimental Relation of Velocity Shear to Turbulence Suppression Kenneth Gentle, William Rowan, Ken Liao, Kevin Lee The Helimak is an approximation to the infinite cylindrical slab, but with open field lines of finite length. Radially segmented isolated end plates allow application of radial electric fields that drive radial currents. Above a sharp threshold in applied voltage (driven current), the fractional turbulent amplitude is greatly reduced, as is the radial turbulent particle transport. Stabilization is observed for both positive and negative bias. Concurrent measurements of the ion flow velocity are made by Doppler spectroscopy. The ions are cold and give no diamagnetic contribution to the velocity. The turbulence reduction cannot be explained by the standard model of flow shear reducing the radial correlation length and thus turbulent amplitude. For positive bias, neither flow shear increase nor radial correlation reduction accompany the turbulence reduction. For negative bias, the turbulence level and radial correlation decrease before flow shear increases at high bias. [Preview Abstract] |
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GP6.00114: Transport and Turbulence Studies on HL-2A Tokamak X.R. Duan In this paper the results of turbulence and transport studies on HL-2A tokamak are summarized. Using the density profile and perturbation analysis, a spontaneous and quasi-steady state particle transport barrier has been evidenced in the ohmic plasmas on HL-2A tokamak without external momentum or particle input. A density threshold for the occurrence of the barrier was identified. The particle diffusivity profile is well-like, and the convection is found to be inward outside the well, and outward inside the well. The formation of the barrier coincides with the TEM/ITG transition. The non-local transport with new features induced by SMBI fuelling has been investigated. The results show that the duration of core T$_{e}$ rising depends on the SMBI modulation parameters and may be prolonged. The distinct characteristics of low frequency quasi-mode (QM) fluctuations of several ten kilohertz and the higher frequency ambient turbulence (HFAT) of 100 kHz or higher are measured with high spatiotemporal resolution Langmuir probe arrays. Three dimensional wave number spectra and dispersion relations are investigated. Besides, \textbf{t}he toroidal symmetry of the low frequency zonal flow of f$<$4 kHz is identified with toroidally distributed the probes in the edge plasma for the first time. In addition, the mode structure of the density fluctuation at GAM frequency is observed. [Preview Abstract] |
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GP6.00115: MAGNETIC FUSION TECHNOLOGY |
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GP6.00116: Robust Matching System for the ITER Ion Cyclotron System D. Swain, R. Goulding, D. Rasmussen, M. Vervier, A. Messiaen, P. Dumortier The ITER ion cyclotron system is required to deliver 20 MW to the ITER plasma under a number of different operating scenarios. The EU will fabricate the antenna, the US will supply the matching system and transmission lines, and India will deliver the rf sources and high-voltage power supplies. A brief description of the complete ion cyclotron system will be presented, and different design options for the matching system will be discussed. Emphasis will be on analyzing the ability of the system to operate effectively during sudden changes caused by plasma perturbations (e. g., ELMs), and on the robustness of matching algorithms. Particular challenges are: the possibility of relatively low loading of the antenna by the plasma because of a large plasma-antenna distance; the resulting high voltages in the matching system (which must be minimized by good system design); the need to install a number of large matching components in the tight space available near the tokamak; and the requirement for operation and maintenance in a radiation environment. [Preview Abstract] |
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GP6.00117: The Influence of Plasma Density on RF Breakdown in Antenna Systems J.B.O. Caughman, F.W. Baity, D.A. Rasmussen One of the main power-limiting factors in antenna systems is the maximum voltage that the antenna can sustain before breaking down. In practice, antenna structures can typically sustain much higher stand off voltages during vacuum conditions compared to operating conditions in the presence of the edge plasma. In an effort to quantify this affect, the influence of plasma density on RF breakdown is being studied in a resonant 1/4-wavelength section of vacuum transmission line terminated with an open circuit electrode structure. A small inductively coupled plasma source is used to inject plasma into the high-voltage electrode gap. The maximum electric field that can be sustained without breakdown is on the order of 30-40 kV/mm for vacuum conditions, but this value is substantially reduced in the presence of plasma and magnetic field. Details of the influence of plasma density and magnetic field strength, including the effect of electrode materials (Ni and Cu), will be presented. [Preview Abstract] |
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GP6.00118: Development of neutral beam injection system by use of washer gun plasma source Heizo Imanaka, Hirotaka Kajiya, Yuichi Nemoto, Akiyoshi Azuma, Tomoaki Asai, Takuma Yamada, Michiaki Inomoto, Yasushi Ono For the past ten years, we have been investigating high-beta Spherical Tokamaks (ST) formation using reconnection heating of their axial merging in the TS-4 experiment, University of Tokyo. The produced ST was observed to have the maximum beta of 50-60{\%} right after the merging of two STs. A key issue after the formation is to maintain the produced high-beta ST over 100 Alfven times for its stability check. A new low-cost pulsed neutral beam injection (NBI) system has been arranged for its sustainment experiment. Its advantages are 1) low voltage (15kV for low-field side of ST) and high current (20A), 2) maintenance-free, 3) low-cost. The conventional filament plasma source was replaced by the washer gun to realize air-cooled and maintenance free NBI system. In its startup experiment, we already extracted the maximum beam current of 3.7A for then acceleration voltage of 10kV successfully. This result suggests that the increase in the acceleration voltage and several conditioning work will realize its designed beam parameters of 15kV, 20A. [Preview Abstract] |
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GP6.00119: Control of beam characteristics on a washer gun based pulse neutral beam injector Akiyoshi Azuma, Hirotaka Kajiya, Yuichi Nemoto, Tomohiko Asai, Heizou Imanaka, Yasusi Ono, Yuichi Takase An economical neutral beam system has been developed by using a single washer gun, pulsed operation, and a simple electrode system. Our initial experiments revealed successful beam extraction up to 10 kV and 5 - 10 A. In this work control of beam profile and parameter have been studied by changing the operation parameters of washer-gun (i.e. potential, working gas pressure and discharge voltage). To investigate beam parameter in detail, transparent vacuum tube have been installed onto neutralizing part. Plasma parameters on a ion source of line integration electron density and electron temperature are measured by triple probe and CO2 laser interferometer. Newly installed radiation and Faraday cup measurement in a focusing area observe characteristics of extracted beam. From these results, we will discuss correlation between the parameters of ion source and extracted beam. [Preview Abstract] |
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GP6.00120: Dynamic measurements of the structure of a vapor cloud formed during high powered fusion relevant disruptions Travis Gray, Michael Jaworski, Vijay Surla, David Ruzic The Divertor Erosion and Vapor shielding eXperiment (DEVeX) at the University of Illinois at Urbana-Champaign is designed to produce plasmas with densities on the order of 10$^{21}$ m$^{-3}$ with a total plasma temperature (T$_i$ + T$_e$) of several hundred eV. This is accomplished with the rapid discharge of a 64 kJ capacitor bank through a conical shaped $\theta$-pinch coil. The general purpose of the facility is to generate energetic plasma flows to study plasma-material interaction relevant to disruption conditions in TOKAMAKs. Here, the first measurements of the plasma flow and the resultant vapor cloud produced during the plasma strike are presented. Lithium is used as the plasma facing material due to its low melting temperature and high vapor pressure as well as for its resurgent use in the fusion community. Measurements of the vapor cloud dynamics are accomplished with an array of fiber optics extending perpendicularly away from the lithium target. Vapor cloud density is measured by resonance absorption of the 670 nm lithium line. This work is important to plasma facing component (PFC) lifetime and viability as the presence of a vapor cloud can absorb the incident energy of a disruption. [Preview Abstract] |
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GP6.00121: Surface analysis studies on the wall conditioning of TCSU Aydin Tankut, James Grossnickle, George Vlases, Fumio Ohuchi To overcome the temperature barrier set by impurity radiation losses in the earlier TCS experiments, TCS-upgrade (TSCU) has been built with particular focus on obtaining UHV compatible wall surfaces. To further minimize impurity ingestion into plasma, wall-conditioning techniques including glow discharge cleaning (GDC), siliconization and Ti-gettering are being carried out in TCSU. The chemical and morphological characterization of the wall surfaces is done using x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). This study presents an overview of the surface analytical studies on the wall surfaces throughout the first year of TCSU operation. The effect of GDC, siliconization and Ti-gettering on TCSU surfaces, along with subsequent glow discharges and plasma shots, will be discussed. Additional data from experiments carried out in a separate system for detailed study of these techniques will be presented. [Preview Abstract] |
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GP6.00122: Structural studies of carbon dust samples exposed to NSTX plasma Fuming Jiang, Yevgeny Raitses, Charles Skinner, Thomas Duffy Raman spectra were measured for different dust samples exposed to the NSTX plasmas with and without Lithium coating on the walls, unexposed dust samples, carbon deposits produced in an atmospheric pressure helium arc discharge, and heat-treated carbon samples. For the unexposed particles, the high energy G-mode peak ($\sim $1580 cm$^{-1}$) is much stronger than the defect-induced D-mode peak ($\sim $ 1350 cm$^{-1}$). For dust particles exposed to the plasma, the ratio of G-mode to D-mode peaks is lower and becomes even less than 1. This behavior indicates on a strong increase of structural disordering in plasma exposed samples. Because we found similar plasma-induced structural modifications in dust particles exposed under the different background gases (hydrogen and helium) and the different plasma conditions of the arc and NSTX experiments, our results suggest that the observed structural modifications can be unrelated to hydrocarbon compositions on the dust surface or implantation. [Preview Abstract] |
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GP6.00123: ITER First Wall Component Design and Development Michael Ulrickson, Joseph Kotulski, Steve Goods, Rebecca Coats, Michael Pasik, Tina Tanaka-Martin, Dennis Youchison The ITER First Wall (FW) consists of 468 panels with Be tiles and copper heat sink on a stainless steel structure. The design has evolved from poloidal fingers to toroidal fingers because of excess stress caused by revised estimates of the halo current loads on the FW. This change was made to permit changing the FW without having to remove the entire FW/SM set. We have completed simulation of plasma current disruption forces for six disruption cases for all 18 different styles of FW/SM in ITER. The toroidal, radial and poloidal force and torque have been used to calculate the loads on the mounting points. Using the flux surfaces from ITER plasma equilibria, we have calculated the heat loads on the FW surface for a variety of surface shapes. We have completed fabrication and testing of a FW Qualification Mockup. We have completed cyclic thermal testing of both an EU and a US FWQM. [Preview Abstract] |
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GP6.00124: An Experiment to Tame the Plasma Material Interface R.J. Goldston, J. Menard, J.P. Allain, J.N. Brooks, J.M. Canik, R. Doerner, A. Hassanein, M. Kotschenreuther, G.J. Kramer, H.W. Kugel, R. Maingi, S.M. Mahajan, R. Majeski, C.L. Neumeyer, R.E. Nygren, L.W. Owen, T.D. Rognlien, D.N. Ruzic, D.D. Ryutov, S.A. Sabbagh, C.H. Skinner, V.A. Soukhanovskii, M.A. Ulrickson, P.M. Valanju, R.D. Woolley Approaches to heat flux handling and tritium retention that may work for ITER do not generally extrapolate to Demo, and certain ITER parameters, such as first-wall temperature and loss power / major radius do not approach those of Demo. Thus research will be required in parallel with ITER to bridge the gap to Demo. A series of key questions sets the requirements for the research capabilities of a device fill this gap, such as heating power / major radius, flexibility in poloidal field configuration, plasma facing component flexibility in materials (solid and liquid) and in temperature, plasma pulse length, and access for surface and plasma diagnostics. [Preview Abstract] |
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GP6.00125: Vertical Control for Burning Plasmas in Ignitor F. Villone, G. Rubinacci, F. Bombarda, G. Ramogida The vertical position and shape controller for Ignitor has been designed on the basis of the CREATE\_L linearized plasma response model,\footnote{R. Albanese, F. Villone, \textit{Nucl. Fusion} \textbf{38}, 723 (1998)} which assumes an axisymmetric system and describes the electromagnetic interaction of the plasma with the surrounding structures by a small number of global parameters (i.e., $\beta_{pol}, l_i, I_p$). In particular, the vertical stabilization system has been designed assuming that the vertical plasma centroid position can be estimated by a suitable linear combination of the available magnetic measurements. A possible partial failure of these magnetic diagnostics has already been taken into account, showing a good resilience to such events. However, in case of severe failures, it will be necessary to resort to a completely different (i.e. non-magnetic) measurement of the vertical position.\footnote{F. Bombarda, et al., \textit{35th EPS Plasma Phys. Conf.} P4.073 (2008)} As an example, we apply this method to the simulated signal of a double, soft X-ray spectrometer looking at the top and bottom of the plasma edge. The spatial and spectral features of these segnals seem, in many cases, sufficient to discriminate beween actual movements of the plasma column and changes in the plasma paramters. [Preview Abstract] |
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GP6.00126: Construction and Initial Tests of MAIZE: 1 MA LTD-Driven Z-Pinch * R.M. Gilgenbach, M.R. Gomez, J.C. Zier, W. Tang, D.M. French, Y.Y. Lau, M.G. Mazarakis, M.E. Cuneo, M.D. Johnston, B.V. Oliver, T.A. Mehlhorn, A.A. Kim, V.A. Sinebryukhov We report construction and initial testing of a 1-MA Linear Transformer Driver (LTD), The Michigan Accelerator for Inductive Z-pinch Experiments, (MAIZE). This machine, the first of its type to reach the USA, is based on the joint HCEI, Sandia Laboratories, and UM development effort. The compact LTD uses 80 capacitors and 40 spark gap switches, in 40 ``bricks'', to deliver 1 MA, 100 kV pulses with 70 ns risetime into a matched resistive load. Test results will be presented for a single brick and the full LTD. Design and construction will be presented of a low-inductance MITL. Experimental research programs under design and construction at UM include: a) Studies of Magneto-Raleigh-Taylor Instability of planar foils, and b) Vacuum convolute studies including cathode and anode plasma. Theory and simulation results will be presented for these planned experiments. Initial experimental designs and moderate-current feasibility experiments will be discussed. *Research supported by U. S. DoE through Sandia National Laboratories award document numbers 240985, 768225, 790791 and 805234 to the UM. MRG supported by NNSA Fellowship and JCZ supported by NPSC Fellowship / Sandia National Labs. [Preview Abstract] |
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GP6.00127: Lead Activation Neutron Yield Measurement System Used in ICF Experiment Yang Cunbang, Feng Jie, Liang Zhiyuan Lead activation measurement system is used to measure very low yield neutron of DT or DD reaction, activation sample is Pb sheath that surrounds a NaI(Tl) detector or Pb disks in front of it. Using this detector, adding amplifier, single-channel and multi-channel scaling system in computer, we can measure the $\gamma $ rays from the activated sample(207mPb) with 0.8s half-life decay and calculate the yield. Because half-life decay is too short to move activated sample to measure room manually or mechanic, we place the detector as close as 32cm from the laser target. To minimize this effect of the X-rays burst from the laser irradiated target, we use a gate circuit to remove the detector high voltage about 200ms to 1s adjustable, and restores it less than 100ms after the shot. The drift of the peak address of the pulse-height spectrum is unavoidable because of the NaI(Tl) detectors and the electronic system. We used an embedded multi-channel pulse height analyzer in this system to monitor and adjust the peak address of the spectrum in time. This system is used successfully in ICF experiment on SG-II facility. [Preview Abstract] |
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GP6.00128: Recent Progress in Two-Dimensional Thomson Scattering Measurement System by Multiple Reflections and the Time-of-Flight of Laser Light Shingo Ito, Hiroyuki Nonaka, Takashi Sumikawa, Mitiaki Inomoto, Yasushi Ono, Kazumichi Narihara Two-Dimensional Thomson Scattering measurement (2-D TS) has been developed using multiple reflections and the time-of-flight (TOF) of laser light. It enables us to measure the r(radial)-z(axial) profiles of electron temperature and density. The multiple reflections of YAG laser light are used to cover the whole r-z plane of the compact torus plasma, and the time delay of the scattered light along the laser beam is arbitrarily arranged by adjusting the multiply reflected laser light path in order to reduce the number of detectors. We detected the Thomson scattering lights from three measurement points successfully, proving validity of this method and then extended it to 3 x 3 measurement system, realizing the 2-D measurement for the first time. We already measured the 2-D Raman scattering lights for absolute calibration of this system and will report details of the 2-D measurement of Thomson scattering lights. [Preview Abstract] |
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GP6.00129: Diagnostics Development for the Ignition Experiment Ignitor G. Pizzicaroli, F. Bombarda, A. Licciulli, M. Fersini, D. Diso, H. Kroegler The Ignitor experiment is designed to reach ignition conditions. The short, but intense neutron flux will pose challenging conditions for diagnostics, such as magnetic sensors or bolometers, in direct proximity, or in direct view, of the plasma. An R\&D program is in progress to manufacture mineral insulated magnetic coils with a reduced sensitivity to radiation effects. A double layer, MgO insulated Ni coil has been produced and tested. The wire is wound on an alumina core and the coil is housed in an alumina box for high refractoriness and minimum vacuum degassing. A lanthanide glass ceramic has been used as sealant for the box. At the same time, alternative methods to provide critical plasma position information during the high performance discharges in Ignitor are being explored. For example, the radiation emitted at the plasma edge by Mo$^{+14}$ can be monitored by means of a soft X- ray spectrometer equipped with a GEM detector, which allows high counting rates ($>$ 1 MHz) and provides good energy resolution and flexibility of design. A 10$\times$10 cm$^2$ multichannel prototype with its associated fast read-out system is being assembled. A layout of the complete spectrometer compatible with the Ignitor port design has been carried out, and the bolometer system design has been updated. [Preview Abstract] |
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GP6.00130: Effects of Radial Profiles in the H-Regime for Ignitor G. Cenacchi, A. Airoldi, P. Detragiache, B. Coppi The radial profiles of the main plasma parameters in the central region of the plasma column and their connection to those at the edge have an important influence on the levels of fusion power that Ignitor can achieve by accessing the H- regime. It is well established by now that the fusion power can be strongly degraded ($\propto B_T^{3.5}$) by decreasing the magnetic field\footnote{R.V. Budny, et al., Report PPPL- 4300 (March 2008)} and consequently the plasma current and the density limit. Ignitor does not have the last constraint but maintaining a reasonable magnetic safety factor is important. Therefore the maximum design field (13 T on axis) is considered. Then an analysis of the operating parameter space (in the H-regime) using a zero-dimensional model shows that a considerable ample space exists when $Q=10$ is attained for a plasma pressure profile moderately peaked ($p_0/\left\langle p\right\rangle = 2.9$) and various scaling expressions for $\tau_E$ and $P_{thr}$. A considerably improved performance (with $Q$ up to 100) can be achieved by a modest increase in the assumed density profile peaking, leading to an attractive regime of operation with moderate power flux to the wall (below about 20 MW) when a newly-proposed scaling expression\footnote {D.C. McDonald, et al., \textit{Plasma Phys. Control. Fusion} \textbf{48}, A349 (2006)} for $P_{thr}$ is used. [Preview Abstract] |
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GP6.00131: Adoption of MgB$_{2}$ Superconducting Magnets for the Ignitor Machine G. Grasso, B. Coppi, G. Giunchi The progress made in the fabrication of MgB$_{2}$ long cables, and related superconducting magnets of relatively large dimensions has led to the decision of adopting this material for the vertical magnetic field coils of the Ignitor machine. These will be the largest magnets (about 5 m in diameter) of the machine and will be cryocooled at the operating temperature of 15 K: a temperature compatible with the He-gas cryogenic cooling system of Ignitor of the actual machine design as well as with the projected superconducting current density of the MgB$_{2}$ material, at the magnetic field values $(\simeq 4-5$ T) in which these coils are designed to operate. The MgB$_{2}$ coils solution will avoid the adoption of a separate liquid-He cryogenic system that otherwise should be used for conventional superconducting NbTi wires. MgB$_{2}$ superconductors hold the promise of becoming suitable for high field magnets by appropriate doping of the material and of replacing gradually the normal conducting coils adopted, by necessity, in high field experiments. Therefore, an appropriate R\&D program on the develpment of improved MgB$_{2}$ material and related superconducting cabling options has been undertaken, involving different institutions. [Preview Abstract] |
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GP6.00132: The ICRH System for The Ignitor Experiment M. Sassi, A. Coletti, R. Maggiora, B. Coppi The ICRH system (80-120 MHz) is an important component of the Ignitor experiment as it provides the flexibility to reach ignition or nearly ignited regimes following different paths in parameter space and to shorten the time needed for this. The system is designed with a modular configuration and launches the power into the plasma through RF strap-antennas based on 4 straps per port. Each module consists of 4 high power generators whose power is split over two ports (8 straps). A 30 $\Omega$ vacuum transmission line transfers 0.4 MW of power per strap for a total power of 1.6 MW per port in order to keep the maximum electric field below 5 kV/cm in the vacuum region of the straps and trasmission line. The RF configuration of the modules allows a full phase controls (toroidal and poloidal) of the straps though a PLL phase control. Two modules, involving 4 ports, produce 6 MW at 115 MHz for the envisioned RF ``accelerated ignition'' scenario. A detailed design of the ICRH antenna has been carried out, including the Faraday shield, the current straps, the vacuum transmission lines and the vacuum feed- through. Its integration of the antenna with the plasma chamber is under way. The mechanical assembly of the relevant components is fully detailed and ready for a prototype manufacturing. [Preview Abstract] |
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GP6.00133: Installation of the Ignitor Machine at the Caorso Site S. Migliori, S. Pierattini, F. Bombarda, G. Faelli, M. Zucchetti, B. Coppi The actual cost of building a new experiment can be considerably contained if infrastructures are already available on its envisioned site. The facilities of the Caorso site (near Piacenza, Italy) that, at present, houses a spent nuclear power station, have been analyzed in view of their utilization for the operation of the Ignitor machine. The main feature of the site is its robust connection to the electrical national power grid that can take the disturbance caused by Ignitor discharges with the highest magnetic fields and plasma currents, avoiding the need for rotating flywheels generators. Other assets include a vast building that can be modified to house the machine core and the associated diagnostic systems. A layout of the Ignitor plant, including the tritium laboratory and other service areas, the distribution of the components of the electrical power supply system and of the He gas cooling sytem are presented. Relevant safety issues have been analyzed, based on the in depth activation analysis of the machine components carried out by means of the FISPAC code. Waste management and environment impact issues, including risk to the population assessments, have also been addressed. [Preview Abstract] |
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GP6.00134: Analysis of Ignitor Discharges with Double X-point Magnetic Configurations A. Airoldi, G. Cenacchi, B. Coppi The Ignitor experiment\footnote{B.Coppi, A.Airoldi, F.Bombarda, et al.,\textit{Nucl. Fusion} \textbf{41}, 1253 (2001)} was proposed and designed to achieve ignited and sub-ignited conditions in well confined deuterium-tritium plasmas. Thanks to its unique features (high magnetic field up to 13 T, high plasma current up to 11 MA, and high plasma density up to $5 \times 10^{20} \rm m^{-3}$), Ignitor is the only device capable of exploring plasma regimes that are relevant to a net power producing D-T reactor and are not accessible to other existing or planned machines. Double X-point scenarios with magnetic field up to 13 T and plasma current up to 9 MA are analyzed. In these configurations, the access to a high confinement state is assumed when the available plasma heating power, supported by the injected auxiliary power, is larger than the L-H threshold value, according to recent suggested scalings\footnote{D.C. McDonald, A.J. Meakins, et al., \textit {PPCF} \textbf{48}, A439 (2006)}. The H-regime is modeled by a global reduction of the thermal transport coefficients used for the L-regime. Situations in the presence and in the absence of sawtooth oscillations have been investigated. Quasi-stationary conditions can be attained when a process producing re- distribution of pressure and current profiles is active. [Preview Abstract] |
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