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Seminar standard day & Time: Thursday 10:45am in T169, refreshments @ 10:30am Please contact Fatima Ebrahimi if you would like to give a seminar, suggest a speaker or would like to be notified of seminars by email. Please note: All visitor information is to be included on the Site Access Notification form and is the responsibility of the PPPL host to complete/ process. Future seminars are subject to changes due to speakers availability. Local, flexible speakers may be asked to reschedule their seminars to give guests an opportunity to deliver talks. |
| SEE BELOW FOR DATES & TIMES OF UPCOMMING AND DETAILS ON PAST SEMINARS |
| Special Theory Seminar Monday, October 22, 2012 10:45 AM, in Display Wall Room Title: Rapid Frequency Chirps of Toroidal Alfvén Eigenmode* Abstract: |
Special Theory Seminar Title: Gyrokinetic Particle Simulation of Kinetic-MHD Processes in Fusion Plasmas Speaker: Ihor Holod, University of California, Irvine Recent progress in the development of GTC code to include electromagnetic capabilities is reported. The fluid-kinetic hybrid electron model implemented in the GTC is discussed. Linear simulations of various Alfven modes is presented for verification and benchmark, emphasizing the significance of kinetic effects of thermal and energetic particles. Nonlinear simulations of BAE mode have revealed the role of energetic particles in the fast frequency chirping. Linear electromagnetic simulations of microturbulence are reported, recovering the finite-beta stabilization effect, ITG-CTEM transition and the onset of kinetic-ballooning mode. Future development plans are discussed.
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Special Theory Seminar Title: Beyond Linear Polarization in Gyrokinetic Theory |
| Special Theory Seminar Tuesday, July 17, 2012 10:45 AM, in T169 Title: Omnigenity as Generalized Quasisymmetry in Stellarators- to view persentation click here Speaker: Matt Landreman, MIT Any viable stellarator reactor will need to be nearly omnigenous, meaning the radial guiding-center drift velocity averages to zero over time for all particles. While omnigenity is easier to achieve than quasisymmetry, we show here that several properties of quasisymmetric plasmas also apply directly or with only minor modification to the larger class of omnigenous plasmas. For example, concise expressions exist for the flow and current, closely resembling those for a tokamak, and these expressions are explicit in that no magnetic differential equations remain. A helicity (M, N) can be defined for any omnigenous field, based on the topology by which B contours close on a flux surface, generalizing the helicity associated with quasisymmetric fields. For generalized quasi-poloidal symmetry (M=0), the bootstrap current vanishes, which may yield desirable equilibrium and stability properties. A concise expression is derived for the radial electric field in any omnigenous plasma that is not quasisymmetric. The fact that the tokamak-like analytical calculations are possible in omnigenous plasmas despite their fully 3D magnetic spectrum makes these configurations useful for gaining insight and benchmarking codes. A construction is given to produce omnigenous B(theta,zeta) patterns with stellarator symmetry. |
Thursday, July 12, 2012 10:45 AM, in T169 Title: delta-f Monte-Carlo calculation of NTV in perturbed tokamaks and demonstration of bounce harmonic resonance Speaker: Kimin Kim, Princeton Plasma Physics Laboratory Non-axisymmetric magnetic perturbations can fundamentally change neoclassical transport in tokamaks by distorting particle orbits on deformed or broken flux surfaces. This so-called nonambipolar transport is highly complex, and eventually a numerical simulation is required to achieve its precise description and understanding. A new df particle code (POCA) has been developed for this purpose using a modified pitch-angle collision operator preserving momentum conservation. Basic features of POCA are introduced with benchmarking tests in the axisymmetry. In the non-axisymmetry, NTV torques are calculated and compared with a combined and 1/n theory, which indicates an essential physics of NTV transport such as dB2 dependence and strong resonance. Benchmarking with the theories show good agreements on NTV torque profiles and amplitudes depending on collisionality. In addition, the first numerical demonstration of bounce harmonic resonance is presented. By including electric potential, POCA confirms the neoclassical offset rotation, and the bounce harmonic resonance between ExB and bounce frequency. Er scan on NTV shows strong peaks at the resonant ExB frequency consistently with the theory. A new type of bounce orbit modified by the bounce harmonic resonance is observed, which prevents phase mixing of bounce orbit thus enhances the NTV. Finally, a preparation for experimental application of POCA is discussed. |
| Special Theory Seminar Tuesday, July 10, 2012 10:45 AM, in T169 Symmetry of Momentum Conservation in Gyrokinetics Bruce Scott, Max-Planck-IPP, EURATOM Association Gyrokinetic field theory is set up in terms of a Lagrangian with canonical structure -- dependent field variables appear only in the time component. The gyrokinetic and associated field equations are the resulting Euler-Lagrange equations for the gyrocenter coordinate positions and field amplitudes. Canonical structure leaves the symplectic part time-independent and axisymmetric, from which energy and canonical momentum conservation follow. Conversion from canonical to plasma momentum uses the charge conservation equation which follows from continuity. The specific role of the time-dependent polarization current is emphasized. It is shown that the contributions in the zonal toroidal momentum transport equation due to higher-order field components in the Hamiltonian can be put into the same symmetric structure as the lowest-order piece. Hence arguments about symmetry and cancellation should be applied equally to all orders, not just the lowest. Some measurements of the PDF of these terms in computations are given. |
| Special Theory Seminar Wednesday, June 13, 2012 10:45 AM, in T169 Title: New plasma techniques for mass separation Speaker: Renaud Gueroult, PPPL There is a growing recognition of the need for high-throughput mass separation devices for nuclear waste remediation. Plasma filters offer theoretically an advantage over other types of mass separation in that velocities can be large, elements are dissociated and the costs of ionization are small compared to the cost of chemical separation. Although most of the past studies have been devoted to isotope separation, rotating plasma configurations might also be promising techniques to discriminate elements with large mass difference too. About a decade ago, plasma mass filtering techniques were introduced in order to process nuclear waste [Ohkawa2002]. In the Ohkawa filter, as well as in other separation devices based on centrifugal forces, the unconfined heavy stream is collected at the radial outer surfaces, which makes more difficult the collection of the heavy elements. The new mass filter proposed here utilizes centrifugal and magnetic confinement of ions in a way similar to the asymmetric centrifugal trap. This magnetic centrifugal mass filter is shown to be more proliferation resistant than other separation technology and the separation is largely axial, rather than radial, which makes easier the collection of separated elements and reduces the overall contamination of the device compared to other plasma centrifuges |
Special Theory Seminar
Tuesday, June 12, 2012
10:45 AM, in T169 Speaker: Wilfred A. Cooper, Ecole Polytechnique Federale de Lausanne (EPFL), Centre de Recherches en Physique des Plasmas, Association Euratom-Confederation Suisse, Lausanne, Switzerland Title: 3D Magnetohydrodynamic Tokamak Equilibria; Helical Core, RMP and Ripple Three-dimensional (3D) effects on the magnetohydrodynamic (MHD) equilibrium problem in tokamaks has become a topic of great interest in recent years. The magnetic field ripple due to discrete toroidal coils disturbs the outer edge of the plasma and has been considered so far as a small perturbation. Long-lived continuous modes, saturated internal kinks and snake structures can now be routinely computed as novel equilibrium states with the VMEC and ANIMEC 3D codes with imposed nested magnetic flux surfaces. Control of Edge Localised Modes (ELMs) has been effected with Resonant Magnetic Perturbation (RMP) coils which can alter the plasma edge shape. We describe core helical equilibrium states computed with the ANIMEC code for the DIII-D, TCV and MAST tokamaks on which dedicated experiments are under way or planned. Bifurcated equilibria (helical and axisymmetric) exist for weakly core-shear-reversed plasmas with a range of minimum q-values in the neighbourhood of unity. Free boundary calculations for the MAST tokamak combine the effects of a helical core structure with RMP and finite ripple deformations. The helical core distortion can change the edge modulation induced by the RMP to cause the plasma to scrape off at the outer side of the vacuum vessel. |
Thursday, June 7, 2012
10:45 AM, in T169 Title: Electron energization and wave dispersion in a mirror kinetic Alfven wave Speaker: Peter Damiano, PPPL
Alfven waves in which mirror force effects dominate the parallel electric field generation can be termed mirror kinetic Alfven waves (Nakamura, JGR 2000). Examples are global scale Alfven waves (Field Line Resonances) that stand along the Earth's closed dipolar magnetic field lines and have been observationally linked to the formation of some auroral arcs. We use a hybrid MHD-kinetic model, that self-consistently couples the cold plasma MHD equations to a system of kinetic guiding center electrons, to explore the characteristics of these global modes. It is found that mirror force effects result in potential drops sufficient to accelerate electrons to the keV energies associated with auroral observations and the wave energy dissipated in this acceleration can damp the wave in a few Alfven cycles. In conjunction with the acceleration, there is also a structuring in the time evolution of the electron energization that scales with the electron bounce frequency and a cross-scale coupling from the global driver to kinetic scales that fragments the parallel current region. The perpendicular Poynting flux associated with the parallel electric field disperses wave energy perpendicular the the magnetic field leading to a broadening of the parallel current region as electrons are accelerated along adjacent field lines. This mirror force induced wave dispersion dominates over that associated with electron inertial effects.
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Special Theory Seminar
Tuesday, June 5, 2012
3:00 PM, in T169 Gyrokinetic finite-size effect investigations with GENE
Tobias Goerler, Max Planck Institute for Plasma Physics, GarchingThe extension of the Eulerian gyrokinetic turbulence code GENE from a flux tube up to a full torus domain code allows systematic study of some of the most important questions regarding turbulent transport scalings for ITER and beyond. This particularly involves the relevance of finite-size effects and the transition to Gyro-Bohm scaling in various plasma regimes. By means of both - linear and nonlinear - simulations it will be shown that the local limit can be reproduced. In this context, it also becomes clear that it is useful to define a modified rho* value, reflecting the width of the profiles (not of the device). Exemplifying these findings, two extreme cases - an ASDEX-Upgrade L-mode discharge and a TCV electron internal transport barrier - will be discussed involving as much physics (realistic geometry/profiles, electromagnetic effects, collisions) as available in the present code version. |
Thursday, May 31, 2012 10:45 AM, in T169 Title: Electrostatic Current Drive in Tokamaks Speaker: Richard Nebel, Tibbar Technologies
Previous work[1,2] has shown that it is possible to lock and amplify m=1 modes from the boundary in an RFP by applying electrostatic fields. Furthermore, it is possible to do this without any magnetic field lines penetrating the boundary (i.e. the normal component of the magnetic field vanishes at the boundary). These can result in single-helicity RFP states which have good flux surfaces everywhere.[1,2]
More recent work using these same techniques has shown that it is possible to drive current in tokamaks with the same electrostatic techniques. These electrostatic studies have uncovered a new nonlinear MHD relaxation principle. This new principle states that if helical electrostatic fields are applied to a plasma, it tries to relax to a state where the magnetic field aligns with the electrodes. Thus, if an m=1, n=1 driving electrostatic field is applied at the boundary, the plasma tries to relax to a state where q ~ 1 everywhere even if no loop voltage is applied to the plasma. Thus, it is possible to operate a tokamak steady-state without out applying a loop voltage via ohmic heating coils (i.e. the OH coils are not required). Furthermore, this principle not only works for the m=1, n=1 mode it also works for the m=1, n=2 mode. This results in a q ~ ½ plasma and suggests that ohmic ignition of tokamak reactors is possible. It is also possible to drive current with the m=2, n=1 mode which results in a reversed-shear discharge with q ~ 2. All of these are discharges evolve to steady-states suggesting that these solutions are universal attractors. All of the simulations are 3-D which indicates that they should also eliminate disruptions. The strong flow in the plasma (2%-10% of the Alfven speed) tends to keep the plasma in single helicity with good flux surfaces everywhere. At Tibbar Technologies we are primarily interested in using this new MHD relaxation principle to build DC-DC electrical transformers. This technology is important for both photovoltaic power and wind power. We are presently building an experiment at Tibbar Technologies to test this new physics. 1. R. A. Nebel, J. M. Finn, 2011 Sherwood Theory Meeting, Austin, TX (2011). 2. R. A. Nebel, J. M. Finn, Paper BP9.00075, 2011 APS-DPP meeting, Salt Lake City, UT 2011). |
Thursday, May 24, 2012
10:45 AM, in T169 Reduced-order-model based feedback control of the Hasagawa-Wakatani Equations Imène Rym Goumiri, Department of Mechanical and Aerospace Engineering, Princeton University
In this work, we demonstrate the development of model-based feedback control for stabilization of an unstable equilibrium obtained in the Hasegawa-Wakatani (H-W) model, a classic model in plasma turbulence. First, snapshot-based approximate balanced truncation, a model reduction technique that has been proved successful in flow control design problems, is applied here to obtain a low dimensional model of the linearized H-W equation. A model-based feedback controller is then designed for the reduced order model using linear quadratic regulators (LQR). The controller is then applied for the original linearized and nonlinear H-W equation to stabilize the unstable equilibrium and suppress transition to drift-wave induced turbulence. |
Date/Time: May 16 (Wednesday) 2012 2:15AM *SPECIAL DAY & TIME* Title:TBA |
Date/Time: April 25 (Wednesday) 2012 10:45AM Title: Gyrosymmetry: global considerations |
Date/Time: April 19 (Thursday) 2012 10:45AM Title: TBA |
Date/Time: April 16 (Monday) 2012 10:45AM * Special Day* Title: A closer guiding-center look at Gyrokinetic theory* Two representations of Hamiltonian guiding-center theory are possible depending on whether the guiding-center Poisson bracket (i.e., the symplectic structure) or the guiding-center Hamiltonian contains higher-order corrections due to the nonuniformity of the magnetic field. The guiding-center Banos hierarchy is introduced to help guide the derivation of higher-order effects in the symplectic representation. The resulting guiding-center polarization and magnetization are thus shown to arise naturally from higher-order guiding-center theory within a two-step derivation of nonlinear gyrokinetictheory.
* In collaboration with Alain J. Brizard, Saint Michael’s College |
Date/Time: April 6 (Friday) 2012 10:45AM Title: Gyrokinetic Particle Simulations of Kinetic-MHD processes Abstract: Confinement and stability properties of fusion plasmas depend on nonlinear interaction of multiple physical processes such as microturbulence, energetic particle instabilities, magnetohydrodynamic (MHD) modes, heating and current drive. I will summarize the status of first-principles simulation of these kinetic-MHD processes using the gyrokinetic particle code as an integrated simulation model for developing the predictive capability for burning plasmas. I will highlight recent progress in the studies of nonlinear wave-particle interactions underlying the transport processes including: (1) convective flux driven by the constraint of the longitudinal invariant in the trapped electron mode turbulence; (2) nonlinear frequency oscillation of Alfven eigenmodes induced by phase space coherent structures; and (3) scaling of energetic particle transport due to wave-particle decorrelation and orbit-averaging. |
Date/Time: April 5 (Thursday) 2012 10:45AM Speaker: Maxime Lesur Title: Phase-space turbulence, and nonlinear instabilities driven by self-organized structures Abstract: Coherent phase-space structures are an important feature of plasma turbulence. They can drive nonlinear instabilities, intermittency in drift-wave turbulence, interact with zonal-flow, and cause transport that departs from quasilinear predictions. Thus it is crucial to revive efforts toward a comprehensive understanding of turbulence, not merely as an ensemble of waves, but as a mixture of coupled waves and localized structures. My strategy is to develop the theory step-by-step, starting from the simplest model. The Berk-Breizman (BB) model is a tractable paradigm for wave-particles interactions, in the presence of extrinsic dissipation. Despite its apparent simplicity, this model exhibits a wealth of complex nonlinear behavior, including spontaneous creation and evolution of phase-space structures. In this seminar, I will review nonlinear wave-particle interactions, the BB model and its experimental applications to laboratory and space plasmas, notably energetic particle-driven Alfvén wave experiments. Then I will focus on two novel points. The first point is a new theory which describes the growth of coherent phase-space structures called as holes and clumps, which can in turn drive the wave by direct momentum exchange due to the dissipation. This mechanism explains the existence of nonlinear instabilities in both barely unstable and linearly stable (subcritical) regimes. The second point is numerical evidence of the breakdown of quasi-linear theory in the presence of structures. Extending the BB model to multiple resonances, simulations show that coalescing holes survive much longer than the classical quasilinear diffusion time and dominate the nonlinear evolution
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Date/Time: March 27 (Tuesday) 2012 10:45AM ** SPECIAL DAY** Title: TBA |
Date/Time: March 22 (Thursday) 2012 10:45AM Title: Finite-beta simulation of microinstabilities Abstract: |
Date/Time: March 21 (Wednesday) 2012 10:45AM *SpecialDay* Title: Neoclassical theory in a tokamak pedestal |
Date/Time: March 15 (Thursday) 2012 10:45AM Feng Wang (~ 30 min.)
Wei Shen (~ 30 min.)
Institute of Fusion Theory and Simulation, Zhejiang University, Hangzhou, China Nonlinear simulations of Sawteeth and energetic particle transport are carried out using the kinetic/MHD hybrid code M3D-K. MHD simulations show repeated sawtooth cycles due to a resistive (1,1) internal kink mode for a model tokamak equilibrium with a broader pressure profile. However, sawteeth do not occur for a peaked pressure profile. In this case, a steady state saturated (1,1) mode follows the initial sawtooth crash. Furthermore, test particle simulations are carried out to study the energetic particle transport due to a sawtooth crash. The results show that energetic particle distribution is flattened depending on pitch angle and energy. For trapped particles, the redistribution occurs for particle energy below a critical value in agreement with previous theory.
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Date/Time: March 1 (Thursday) 2012 10:45AM Title: TBA
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** Special Theory Seminar** Date: Wednesday, February 22, 2012 Speaker: Prof. J. F. Drake, Univ. of Maryland
Title: Particle acceleration during magnetic reconnection Abstract: Key topic in the physics of magnetic reconnection are the the mechanisms for plasma heating and the production of energetic particles. I will discuss our efforts to model these processes in two very distinct environments: impulsive flares and the sectored region of the heliosheath. We suggest that in both cases a multi-island reconnection model is required to explain the observations. Recentobservations of flares reveal that essentially all electrons in the flaring region undergo acceleration and that the pressure of the energetic component can approach that of the magnetic field. Abundanceenhancements of high mass-to-charge ions suggest that the acceleration of minority species is more efficient than protons. Sawtooth events in RFPs reveal similar behavior. In the outer heliosphere the Voyager spacecraft have revealed that the termination shock is not the source of anomalous cosmic rays. We suggest that the sectored structure of the heliospheric magnetic is compressed across the termination shock so that collisionless reconnection onsets in the heliosheath and is the source of anomalous cosmic rays. The high beta environment of the heliosheath leads to magnetic island dynamics that differ greatly from the low beta environment of flares but in both cases Fermi reflection in contracting islands drives the plasma towards the marginal firehose condition, which ultimately controls the spectra of energetic particles. |
Date/Time: January 27 (Friday) 2012 10:45AM Title: "Numerical Solution of a Dilute and Disperse Gas-Particle Flow" Abstract: Both Eulerian and Lagrangian formulations are commonly used when numerically simulating a dilute and disperse particle-phase coupled with a fluid. Eulerian methods can provide advantages over Lagrangian tracking schemes in terms of computational expense (large number of particles required to achieve a realistic concentration) and parallel scalability of the numerical algorithm. However, the mathematical characteristics of the particle-phase transport equations have implications that must be considered when developing the numerical scheme. In this seminar, the equations governing the motion of a dilute and disperse gas-particle flow, the mathematical characteristics of these equations, and methods for their solution will be presented. Numerical results will be described that demonstrate the capabilities of the approach for the solution of coupled gas-particle flows.
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Date/Time: January 26 (Thursday) 2012 10:45AM
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Date/Time: January 18 (Wednesday) 2012 10:45AM
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Date/Time: January 17 (Thursday) 2012 10:00AM 10:00 am to 10:50 am 1:00am to 11:50 am
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Date/Time: January 16 (Monday) 2012 1-2:30pm Title: Spontaneous Generation of Turbulence in Magnetic Reconnection In space and astrophysical plasmas, magnetic reconnection occurs most commonly in parameter regimes where collisions are weak and where the macroscopic scales are much larger than the underlying kinetic scales. While progress has been made in understanding certain features of collisionless reconnection within 2D models, very little is know regarding the influence of realistic 3D dynamics. In this talk, a combination of linear Vlasov theory and petascale kinetic simulations are employed to examine the 3D evolution of reconnection layers with a finite guide field. This configuration is unstable to tearing modes at resonant surfaces across the initial layer corresponding to oblique angles relative to the 2D models, which permit only a single resonant surface. In real 3D systems, magnetic islands correspond to extended flux ropes, which can evolve and interact in a variety of complex ways not possible in 2D models. To examine this physics, we employ the 3D particle-in-cell code VPIC running on the Jaguar machine. These simulations feature the formation and interaction of flux ropes within the initial current layer, followed by the generation of secondary flux ropes within new current sheets that develop nonlinearly. For sufficiently large systems, this gives rise to a turbulent evolution in which flux ropes and current sheets are the basic building blocks. Examples are given for parameter regimes relevant to the Earth’s magnetosphere, as well as electron-positron plasmas of relevance to astrophysical applications.
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Date/Time: January 10 (Tuesday) 2012 10:45AM Title: Gyro-center shift as explanation for the various anomalies in magnetized plasmas Abstract: The theory of gyro-center shift has been developed and solved numerous anomalies in the plasma physics. The gyro-center shift theory consists of three formulae, first the gyro-center shift current arisen from the momentum loss of ions by collision with neutrals, second the turbulence induced diffusion coefficient, and third one is the Reynolds number for ion-neutral frictions [1][2]. This talk will present the theoretical development of the gyro-center shift and its applications including : radial electric field formation in Tokamak, H-mode transition, turbulence diffusion as anomalous transport, reversed motion of arc discharge cathode spot [3], equatorial electro-jet (EEJ) in ionosphere , Bohm diffusions [4] , and Enhanced Pedestal H-mode in NSTX [5]. [1] K. C. Lee, Phys. Plasmas 13, 062505 (2006) [2] K. C. Lee, Plasma Phys. Control. Fusion 51, 065023 (2009) [3] K. C. Lee, Phys. Rev. Lett. 99, 065003 (2007) [4] K. C. Lee, Phys. Plasmas, submitted (2011) [5] K. C. Lee, R. Maingi, C. W. Domier, R. Kaita, et al. presented at 13th H-mode Workshop (2011)
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Date/Time: January 5 (Thursday) 2012 10:45AM Title: "Large scale electron acceleration by parallel electric fields during magnetic reconnection" During reconnection in magnetized plasma stress in the magnetic field is reduced through changes in the field line topology. The process is often accompanied by an explosive release of magnetic energy and is implicated in a range of astrophysical phenomena. In the Earth's magnetotail, reconnection energizes electrons up to hundreds of keV and solar flares events can channel up to 50% of the magnetic energy into the electrons resulting in superthermal populations in the MeV range. Electron energization is also fundamentally important to astrophysical applications yielding a window into the extreme environments. Here we show that during reconnection powerful energization of electrons by magnetic-field-aligned electric field (E||) can occur over spatial scales which hugely exceed previous theories and simulations. We find that E|| is supported by non-thermal and strongly anisotropic features in the electron distributions not permitted in standard fluid formulations, but routinely observed by spacecraft in the Earth’s magnetosphere. This allows for electron energization in spatial regions that exceed the regular de scale electron diffusion region by at least three orders of magnitude. |
Date/Time: December 6 (Tuesday) 2011 2:00pm ** SPECIAL DAY & TIME** Speaker: Hank Strauss, HRS Fusion Title/abstract Disruption Simulations with the M3D code
Disruptions are a serious concern in ITER. Electromechanical stress from a disruption might damage the wall, blanket, and other structures. Simulations of disruptions [1] are being carried out using the M3D extended MHD code [2]. Primarily the disruptions studied are vertical displacement events (VDEs) interacting with kink modes, the scenario thought to be the worst case for ITER. Toroidal peaking factor, halo current fraction, and wall forces are calculated, in particular the ”sideways” asymmetric wall force. The relation of halo and hiro [3] current is clarified, as well as the relation between hiro current and toroidal variation of the toroidal current, observed in JET. Issues of velocity boundary conditions and halo modeling are discussed. Recent simulations of NSTX disruptions [4] will be shown.
[1] H. R. Strauss, R. Paccagnella, J. Breslau, Phys. Plasmas 17, 082505 (2010).
[2] W. Park, E.V. Belova, G.Y. Fu, X. Tang, H.R. Strauss, L.E. Sugiyama, Phys. Plas- mas 6, (1999) 1796.
[3] L. Zakharov, Phys. Plasmas (2008) 15 062507. [4] J. Breslau, H. Strauss, R. Paccagnella, and S. Jardin, B.A.P.S. 56, PP9.00051 (2011) |
Date/Time: December 1 (Thursday) 2011 10:45am Title: Transport-driven toroidal rotation in the tokamak edge |
Date/Time: November 22 (Tuesday) 2011 2:00pm
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Date/Time: November 10 (Thursday) 2011 10:45AM Title: "Understanding disruptions in tokamaks" |
Date/Time: November 3 (Thursday) 2011 10:45AM Title: "Scalable Flowing Liquid Lithium System (FLiLi) for tokamaks" Abstract: The talk presents a practical system for implementation of a flowing liquid lithium layer in tokamaks. The suggested scheme satisfies all requirements for FLiLi for development of the LiWall Fusion regime on exisiting devices and for its utilization in future tokamaks with a burning plasma. The thin LiLi layer is insensitive to MHD effects and yet unknown j × B forces, transparent to the heat flux, sufficient for plasma pumping. The FLiLi system is scalable in both poloidal and toroidal directions, compact, and has minimal necessary LiLi inventory in the machine.
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Date/Time: October 27 (Thursday) 2011 Time: 10:45am Speaker: Prof. Auishan Cai, University of Science and Technology Title: Influence of energetic ions on tearing modes Abstract: In contrast with the stability effects of trapped energetic ions on tearing modes, the effects of circulatingenergetic ions (CEI) on tearing modes depend on the toroidal circulating direction, and are closely relatedto the momentum of energetic ions. CEI provide an additional source or sink of momentum to affecttearing modes. For co-CEI, tearing modes can be stabilized if the momentum of energetic ions is largeenough. On the other hand, the growth of tearing modes can be enhanced by counter-CEI. Further, apossibility to suppress the island growth of neoclassical tearing modes by co-CEI is pointed out." |
Date/Time: October 13 (Thursday) 2011 Time: 10:45am Abstract: |
Date/Time: September 22 (Thursday) 2011 10:45AM Title: Ideal MHD stability and performance of ITER steady state scenarios with Internal Transport Barriers |
Date/Time: September 14 (Wednesday) 2011 10:45AM ** Special Day & Time** Title: " Gyrokinetic simulation study of ITG turbulence and zonal flow in LHD with high ion temperature" |
Date/Time: September 13 (Tuesday) 2011 3pm ** Special Day** Title: " Effects of profile shear and kinetic electron response on momentum transport" |
Date/Time: September 12 (Monday) 2011 10:45AM ** Special Day** Title: Symmetry Constraints on the Dynamics of Magnetically Confined Plasma Abstract In respect of their symmetry properties, toroidal magnetically confined plasmas have much in common with the Taylor-Couette flow. A symmetry-based analysis (equivariant bifurcation theory) has proved very powerful in the analysis of the latter problem. This talk discusses the applicability of the method to nuclear fusion experiments such as tokamaks and pinches. The likely behavior of the simplest models of rotationally symmetric tokamaks is described, and found to be potentially consistent with observation. |
Date/Time:September 6 (Tuesday) 2011 ** Special Day** Special TIME: 1:30pm
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Date/Time: September 6 (Tuesday) 2011 ** Special Day** Special TIME: 11:00 am
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Date/Time: August 8 (Monday) 2011 ** Special Day** TIME: TBA Abstract- Click here to see abstract |
Date/Time: July 21 (Thursday) 2011 10:45AM Tite: Plasmoid-dominated magnetic reconnection
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Date/Time: July 7 (Thursday) 2011 10:45AM Title: Reference Magnetic Coordinates (RMC) for toroidal confinement systems Abstract The issue of an appropriate coordinate system for ergodic magnetic fields is as old as the world of 3-D toroidal configurations and remains unresolved. The destruction of nested magnetic surfaces even by small 3-D perturbations leads to a sudden change of topology of magnetic field. As a result, the coordinate systems cannot be longer based on tracing the magnetic field lines. At the same the high plasma anisotropy requires use of field aligned coordinates. The additional demand comes now from disruption simulations which need adaptive grids for resolving the plasma edge. The talk presents the RMC (introduced in 1997, 1 year B.L.) as the best simply nested coordinates for ergodic confinement fields together with a practical algorithm (not involving the field line tracing) for advancing RMC.
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Date/Time: June 23 (Thursday) 2011 10:45AM Title/abstract TBA |
Date/Time: June 21 (Tuesday) 2011 10:45AM **Special Day** Title: Energy Consistency and Momentum Conservation in Gyrokinetics Abstract The modern theory of gyrokinetics is briefly reviewed. Dynamics is described in terms of a Lagrangian with canonical structure -- dependent field variables appear only in the time component. The gyrokinetic and associated field equations are derived from the same Lagrangian by varying gyrocenter coordinate positions and field amplitudes. Energetic consistency follows from the general symmetry implied by the support of the equations by the Lagrangian. The Noether theorem tells what is conserved though conservation proofs are done independently. Both local and global conservation forms are given. Conversion from canonical to plasma momentum uses the charge conservation equation which follows from continuity. The specific role of the time-dependent polarisation current is emphasised. In the plasma momentum form the original motivation for considering higher-order drifts is removed exactly. The correspondence to the MHD momentum conservation law is given. The results are automatically valid for any ordering since all ordering is done in the construction of the Lagrangian in the beginning and none is done thereafter. A simple 1-page cheat sheet is given on the blackboard at the end of the presentation.
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Date/Time: June 16 (Thursday) 2011 10:45AM Title: "The generalized Fjortoft constraint for two dimensional gyrokinetics" to see presentation click here Abstract: The 1953 paper by Fjortoft established a simple argument for understanding the upscale transfer of energy that occurs in incompressible two-dimensional Navier-Stokes turbulence. This argument starts from the simple observation that the existence of a second quadratic invariant, "enstrophy," places a constraint on how energy can be redistributed spectrally under nonlinear interaction. This sets the stage for the famous "inverse cascade" of fluid turbulence. The gyrokinetic equation also conserves two quadratic quantities under nonlinear interactions. To isolate the effects of this property, we focus on a simple limit: the electrostatic two-dimensional gyrokinetic equation, with a homogeneous equilibrium. We show that the relationship between the two invariants establishes a simple constraint on spectral transfer, in analogy to that of 2D Navier-Stokes turbulence. However, in this case the inverse transfer of energy (appropriately defined) is only one possible outcome. If suitably driven, a constrained dual forward cascade can also occur, revealing the possibility that large scales can be "forcibly" damped by fluctuations at much smaller scales. We discuss the consequences for zonal flows and other features of realistic saturated turbulence.
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Date/Time: June 2 (Thursday) 2011 10:45AM Title : Electron - Magnetohydrodynamic (EMHD) studies in an inhomogeneous plasma Abstract: The Advent of high intensity femto-second laser pulses and the availability of fast diagnostic techniques have now made it possible that plasma
can be triggered at very fast electron time scales and its response at these fast time scales can now also be watched in considerable detail.
These studies, therefore, open up an entirely new regime of studies of plasma phenomena. Some of these phenomena can be suitably depicted by the
Electron Magnetohydrodynamics (EMHD) model, which is a simplified description for magnetized electron fluid in the presence of neutralizing stationary background ions. |
Date/Time: May 26 (Thursday) 2011 10:45AM Title: Reconnection in Naturally Arising Magnetic Fields Abstract: The fundamental questions of reconnection theory are why is the observed dissipation of the magnetic field sometimes enhanced by ten or twenty orders of magnitude from its characteristic value and what is the trigger for the enhanced dissipation. Electrodynamics provides an answer—the exponential increase in the separation between neighboring magnetic field lines. In toroidal magnetic configurations with small islands this exponentiation occurs only near the X-points of the islands, though it occurs almost everywhere in regions of stochastic field lines. In naturally occurring magnetic structures X-points and islands are ill defined, but neighboring magnetic field lines generically change their separation by a factor of exp(S) within a segment of the magnetic system of length L. The exponentiation S is easily calculated field-line-by-field-line in any bounded region in which a magnetic field is known—for example in a numerical simulation—and S evolves even when the magnetic evolution is dissipationless. The exponential increase in the separation implies an exponential increase in derivatives across the magnetic field of quantities that vary slowly along the magnetic field lines. Some dissipative mechanisms are proportional to these derivatives squared, so the dissipation becomes exponentially large, proportional to exp(2S). Reconnection is naturally triggered in regions in which S ~ 20. |
Date/Time: May 19 (Thursday) 2011 10:45AM Abstract: The variational formulation of the nonlinear gyrokinetic theory is based on a Lagrangian density that depends on the gyrocenter Vlasov distribution and the perturbed electric and magnetic fields. According to the Noether Theorem, the energy-momentum conservation laws are associated with symmetries of the gyrokinetic Lagrangian density with respect to time and space translations. This talk presents a tutorial on the Noether method and its applications in plasma physics. |
Date/Time: May 13 (Friday) 2011 10:45AM ** Special Day** TITLE: Rapid Evaluation of the Fokker-Planck Collision Operator* Abstract: The Fokker-Planck equation, which describes the evolution of the plasma over time, has a nonlinear term representing the collisions of various species of the plasma. Current plasma edge simulations do not take this collision effect into account, because of the difficulties in the accurate evaluation of this term. Using the Rosenbluth potential formalism, the collision operator can be written in terms of solutions of a Poisson and a biharmonic free space PDE. Due to the inherent axisymmetry of the input data, cylindrical coordinate solvers are preferred for efficient computation. Standard numerical techniques (based typically on finite differences and finite element approximations) encounter difficulties in achieving high order accuracy, especially in the computation of derivatives of the solution (required in the collision operator formulation), and in imposing radiation conditions at infinity. Our new solver achieves arbitrary order accuracy in cylindrical coordinates based on a combination of separation of variables, Fourier analysis and the careful solution of the resulting radial ODE. A weak singularity arises in the the continuous Fourier transform of the solution that can be handled effectively with special purpose quadrature rules and spectral accuracy can be achieved in derivatives without loss of precision. |
Date/Time: May 6 (Friday) 2011 10:45AM ***Special Day**** TITLE: A Particle-in-cell Method with Adaptive Phase-space Remapping for Kinetic Plasmas Abstract: The numerical solution of the Vlasov equation is usually performed by the particle-in-cell (PIC) method. However, it is well-known that, in some cases, the PIC method has difficulty in having an accurate description for the distribution function in phase space due to numerical noise, the inherent drawback of particle-based methods. In this talk, I will present an accurate and efficient PIC method for computing the dynamics of kinetic plasmas. The method overcomes the numerical noise by periodically remapping the distribution function on a hierarchy of locally refined grids in phase space. The positivity of the distribution function is enforced by redistributing excess phase space density in a local neighborhood. Remapping on phase space grid also provides an opportunity to integrate a collisional model and an associated grid-based solver. At the end of this talk, I will show our numerical results on a number of standard plasma physics problems, e.g., Landau damping and the two stream instability in both 1D and 2D cases. It is shown that remapping largely reduces the numerical noise and results in a more consistent second order method than the standard PIC method.
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Date/Time: April 28 (Thursday) 2011 10:45AM TITLE: Resistive wall mode kinetic stability theory advancements for refined Abstract: For future tokamaks to operate in a continuous, disruption-free manner, an understanding
of how the resistive wall mode (RWM) instability can be reliably stabilized must be gained.
The theory of RWM stabilization presented invokes kinetic effects as energy dissipation mechanisms [1]. In particular, rotational resonances between the mode, the plasma rotation, and
thermal particle motions have been shown to explain RWM instability at intermediate rota
tion in NSTX [2,3], significantly changing our understanding of the dependence of stability
on rotation. Further improvement of the theoretical model, as implemented in the MISK code,
is presently investigated to refine the qualitative agreement between computed RWM marginal
stability points and experimental results. One such refinement is the inclusion of energetic par
ticle effects, which are known to play an important role in kinetic RWM stabilization [3]. It is
shown that it is important to take into account the anisotropy of neutral beam injected energetic
ions to correctly account for their stabilizing effects. The RWM dispersion relation is cubic, in
general, leading to three distinct roots. It is shown that one root has a slow mode rotation (less
than the inverse wall time) while the other two rotate more quickly, one leading and one lag
ging the plasma rotation frequency. Elevated electron and ion collisionalities can each stabilize
one of these rotating roots. The role of collisions in both dissipating the mode energy and also
in damping the resonant kinetic effects is also outlined [4]. In future devices with lower col
lisionality, plasmas in rotational resonance will gain stability, but the plasma stability gradient
with rotation will increase, making it especially important to avoid unfavorable plasma rotation
profiles through rotation control or active mode control. |
Date/Time: March 24 (Thursday) 2011 10:45AM TITLE: Auroral Arc Theories - The Missing Link Abstract: In-situ and optical observations of visible auroral arcs - the most common pre-midnight auroral form - place tight restrictions on candidate theories. For example, electron energies within inverted-V arcs can reach 20-30 keV; these energies vary characteristically and often symmetrically with latitude; they have typical widths of order 20 km; they are highly elongated in longitude; they can occur as far south as the inner edge of the plasma sheet and as far north as the polar cap (though at lower energies); they often appear in multiple parallel sets of two to five or more; and although they sometimes oscillate, more often they exist with no basic change in form for several to tens of minutes. These basic properties have been well-known for many decades, yet most auroral theories address only a small subset - often only one - and no theory accounts for them all. As a result, there remains no consensus as to the essential mechanism(s) responsible for auroral arcs. One barrier to progress is a surprising lack of statistical information on basic arc behaviors. This talk describes initial work intended to remedy this situation, and discusses implications for theories of auroral arcs, focusing in particular on zero-frequency electromagnetic structures known as "stationary inertial Alfven waves" [Knudsen, JGR, 1996].
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Date/Time: March 15 (Tuesday) 2011 10:45AM TITLE: Modification of particle distributions by MHD instabilities Abstract: The modification of particle distributions by magnetohydrodynamic modes is an important topic for magnetically confined plasmas. Low amplitude modes are known to be capable of producing significant modification of injected neutral beam profiles, and the same can be expected in burning plasmas for the alpha particle distributions. Flattening of a distribution due to phase mixing in an island or due to portions of phase space becoming stochastic is a process extremely rapid on the time scale of an experiment but still very long compared to the time scale of guiding center simulations. In this work we introduce a new method of determining domains of phase space in which good KAM surfaces do not exist, giving exact resonance locations and island widths[1]. We use this method for quickly finding the evolution of the particle distribution without carrying out a full guiding center simulation. As examples, profile flattening due to particle avalanche caused by island overlap and large scale flattening of a beam distribution in DIII-D[2, 3] due to a large spectrum of low amplitude TAE modes are considered.This work was partially supported by the U.S. Department of Energy Grant DE-AC02-09CH11466. [1] R. B. White, Comm. in Nonlinear Science and Numerical Simulations, accepted (2011) [2] R. B. White, N. N. Gorelenkov, W. W. Heidbrink, M. A. Van Zeeland, Phys. of Plasmas, 17, 056107 (2010) [3] R. B. White, N. N. Gorelenkov, W. W. Heidbrink, M. A. Van Zeeland, Plasmas Physics Controlled Fusion, 52, 045012 (2010)
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| Date/Time: March 10 (Thursday) 2011 TIME: 11am Location: Theory Seminar Room (T169) ------------------------------------------- Speaker : Dr. Bedros Afeyan, Polymath Research, Inc. Title: Optimal Control of Laser-Plasma Instabilities in Plasmas Using STUD Pulses Abstract:If Spike Trains of Uneven Duration and Delay, or STUD pulses are used instead of continuous laser illumination of the plasma, and the laser pulses become ``on'' for a few inverse growth rates of the fastest instabilities, with comparable delays or "off" times between the spikes, then we can show that due to a variety of favorable mechanisms, coherent wave-wave interaction caused instabilities can be kept under control. Their growth can be kept linear and unable to turn into runaway processes as they often are in current plasmas, such as Raman scattering on the NIF. In addition, STUD pulses allow the actual true control of crossed beam energy transfer whenever it is desired and can be disallowed when it is not desirable. This is achieved by temporal interleaving the pulse trains between cones of beams in indirect drive and in a spatially random subset of a larger number of overlapping beams in direct drive. Theoretical and Computational results will be shown together with the innovative design elements of future experiments meant to validate our results. |
Date/Time: March 4 (Thursday) 2011 10:45AM TITLE:"Ponderomotive forces, wave dispersion, and action conservation" The ponderomotive force has long been a handy concept for describing how intense waves affect the dynamics of individual charged particles and plasmas. As introduced originally, it is the effective average force produced on particles by an inhomogeneous wave field. On the other hand, the ponderomotive energy is exactly what determines the wave dispersion, including the linear dispersion at arbitrarily small amplitudes. Thus, wave propagation and interaction with particles can also be approached as mode coupling problems. An abstract oscillation-center formalism, non-perturbative in the wave intensity, can then be developed that describes the average dynamics uniformly in any environment, from rf-driven magnetized plasmas to ultrarelativistic laser-plasma interactions or even interaction of laser light with cold atoms. In particular, the talk is focused on adiabatic effects of two types. First, fundamental properties of nonlinear forces on particles due to waves are contemplated, and examples are presented illustrating how unusual ponderomotive dynamics is predicted. Second, the particle influence on waves is addressed. A general nonlinear dispersion relation is derived for arbitrary stationary waves in plasma. Kinetic effects are included without solving the Vlasov equation, and the frequency shifts due to trapped particles are revised. Finally, the same formalism yields adiabatic conservation laws, which are then applied to plasmas undergoing densification in various contexts (compression, ionization, cosmological metric expansion). Existing results pertaining to slow transformation of linear waves in such plasmas are generalized and corrected.
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Date/Time: February 24 (Thursday) 2011 10:45AM |
| Date/Time: February 17 (Thursday) 2011 10:45AM Location: Theory Seminar Room (T169) ------------------------------------------- Speaker : Prof. George Tynan of UCSD |
| Date/Time: February 11 (Friday) 2011 10:45AM Location: Theory Seminar Room (T169) ------------------------------------------- Speaker : Prof B. Coppi, MIT |
Date/Time: January 20 (Thursday) 201110:45AM Title:"Gyrokinetic Particle Simulation of the Drift Compressional Modes in the Abstract: Global gyrokinetic particle-in-cell simulation code has been developed in the magnetic dipole geometry, and successfully verified against the shear Alfv\'{e}n wave, ion acoustic wave, and the drift compressional mode. A numerical scheme has been developed for gyrokinetic simulations of low frequency compressional modes to study the linear and nonlinear properties of the drift compressional mode. Linear gyrokinetic simulations were performed to investigate the effects of the kinetic ions and finite Larmor radius on the frequency and growth rate. Global simulations were also conducted to resolve the structure of the perturbation along the equilibriummagnetic field. The radial mode structures will also be studied. |
Date/Time: January 13 (Thursday ) 2011 10:45AM
ABSTRACT: Graphic processing units (GPU’s)---the graphic cards in most PC’s---are among the most powerful computing devices now available. How to adapt scientific codes to harness this computing power in GPU’s is an active research area in high performance computing. Recently using CUDA, we have developed an electromagnetic Particle-in-Cell code with charge-conserving current deposition that can run 30-100 times faster on a GPU than on a CPU. On a GeForce GTX-280 graphic card, the GPU PIC code can achieve a one-particle-step process time of 1.9 – 5.1 ns in 2D and 5.7 – 21 ns in 3D, depending on plasma temperatures. In this talk, we will discuss issues that we have encountered in our adaptation, such as thread assignment, reduction of algorithm branching and writing conflicts, shared memory usage, and parallel particle sorting. |
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