Listed below are the Theory Seminars Scheduled for -2010
Standard
Time: Thursday
Refreshments
are at 10:30am
Seminar
is at 10:45am
| SEE BELOW FOR DATES & TIMES OF UPCOMMING SEMINARS AND PAST SEMINARS |
Date/Time: March 10 (Thursday) 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 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) |
Date/Time: March 3 (Thursday) 2011 10:45AM TITLE: Ponderomotive forces, wave dispersion, and action conservation Abstract: |
| Date/Time: February 24 (Thursday) 2011 10:45AM Location: Theory Seminar Room (T169) ------------------------------------------- Speaker : Walter Guttenfelder, PPPL TITLE: Electromagnetic transport from microtearing mode turbulence in NSTX |
Date/Time: February 23 ( Wednesday) 2011 10:45AM Abstract: The SWIM center has the scientific objectives of improving our understanding of interactions that both RF wave and particle sources have on extended-MHD phenomena, and improving our capability for predicting and optimizing the performance of burning plasmas. The center has built an end-to-end computational system that allows physics codes to be able to function together in a parallel environment and connects them to utility software components and data management systems. We have used this framework to couple together state-of-the-art fusion energy codes to produce a unique multi-physics simulation capability. A physicist's overview of the Integrated Plasma Simulator (IPS) will be given and applications described. For example the IPS is being employed to support ITER with operational scenario studies. A computational approach to coupling MHD with RF has been developed, and initial numerical studies of RF effects on tearing modes with reduced models have been completed. And the IPS is being used to investigate the possibility of parallelization in the time domain of plasma turbulence calculations. The talk will emphasize the wide range of simulation work-flows that can be composed using the IPS. |
Date/Time: February 17 (Thursday) 2011 10:45AM Title: Turbulent momentum transport in magnetized plasmas: results from linear devices and thoughts for studies on toroidal confinement devices Abstract: Turbulent momentum transport is thought to play an important role in the formation of sheared ExB flows in confined plasmas and thus may be a key piece of important macroscopic transport phenomena such as critical gradient behavior and transport barrier formation. In addition, recent experiment and theory suggest it may play a crucial role in the formation of so-called "intrinsic rotation" in confined plasmas, where the plasma acquires a net rotation in the absence of external momentum input. Thus turbulent momentum transport studies are of fundamental importance for magnetic fusion. In this talk we provide an overview of studies of turbulent momentum transport in a linear plasma device. Using a mixture of multi-point probe studies, digital signal processing techniques and fast imaging diagnostics, we show how coherent drift waves transition into a turbulent spectrum, and how coherent sheared zonal flows arise from the turbulence during this transition. The results show the key role that turbulent symmetry breaking plays in the formation of large scale ordered flows out of turbulence, and suggest several laboratory and confinement device experiments. |
| Date/Time: February 11 (Friday) 2011 10:45AM *** Special Day*** Location: Theory Seminar Room (T169) ------------------------------------------- Speaker : Prof B. Coppi, MIT |
Date/Time: February 4 (Friday) 2011 10:45AM *** Special Day*** Title: Observation Features and Physical Mechanisms of Substorms We present key features of substorm observations and their physical mechanisms. In particular, we emphasize the fine structure in the onset arc and the associated magnetic fluctuations in Pi1 and Pi2 frequency ranges and their exponential growing behaviors before the onset of substorm expansion phase. We will discuss the possible physical mechanism of substorm onset in the magnetosphere and the formation of auroral breakup arc in the ionosphere. We will also present the nonlinear evolution of the onset arc breakup and magnetic fluctuations into turbulent states, the current disruption and magnetic field dipolarization processes and the dispersionless particle injection during the expansion phase. |
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 |
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. |
| SEE BELOW FOR DATES & TIMES OF UPCOMMING SEMINARS AND PAST SEMINARS |
***** SPECIAL DAY & TIME ***** Date/Time: December 15 (Wednesday) 2010 10:30AM In this talk I will present an overview of hyperbolic balance laws with applications to solution of moment systems of the Boltzmann equation. Key properties of hyperbolic balance laws, important for developing accurate and robust schemes, will be presented. Schemes from high-order finite-volume and discontinuous Galerkin families will be described. These schemes are not only high-order but also capture shocks by careful use of limiter functions around discontinuities. |
| ***** SPECIAL DAY & TIME *****
Date/Time: December 7 (Tuesday) 2010 11:00AM |
| ------------------------------------------- Date/Time: December 2 (Thursday) 2010 10:45AM Location: Theory Seminar Room (T169) ------------------------------------------- Title: Non-axisymmetric Response and Stability Calculations Using a Non-Ideal Two-Fluid Model Speaker: Dr. Stuart R. Hudson, PPPL Please see abstract in this link, 2010_Toki_Hudson_abstract.pdf |
Date/Time: November 29 (Monday) 2010 2:00pm Speaker: Dr. Nathaniel Ferraro,General Atomics Abstract: The ideal-magnetohydrodynamics (MHD) model has been generally successful in describing many aspects of the macroscopic stability and response characteristics of tokamak plasmas. However, more sophisticated modeling is needed, both to describe fundamentally non- ideal phenomena such as tearing modes, and to validate the more efficient ideal MHD calculations in regimes to which ideal-MHD is not strictly applicable. Here we apply a comprehensive extended-MHD model to two important and related tokamak phenomena: the calculation of edge-localized mode (ELM) stability, and the plasma response to non- axisymmetric external fields. The effect on these phenomena of including resistivity, rotation, and two-fluid effects is explored. These calculations are carried out using the initial-value finite- element code M3D-C1. |
Theory Department Weekly Seminar Title: Gyrokinetic computation of tokamak edge turbulence Speaker: Dr. Bruce Scott Edge turbulence is computed using a generalised fluxtube delta-f gyrokinetic formulation. Energetic consistency in the model is reviewed. A local fluxtube model conserves a free energy, not the total thermodynamic energy, following Hasegawa-Wakatani in a general sense. Gradient terms provide drive, and collisions and numerical dissipation (Landau damping, ultimately) provide saturation. Within the local model, self consistent profile effects can be studied via appropriate boundary conditions. Full flux-surface edge turbulence results are obtained with realistic scale separation for the first time. Instabilities occur at the scale of circa ten ion gyroradii, while nonlinear redistribution fills the spectrum. A key feature of edge turbulence is the strong nonlinearity: all available degrees of freedom are maintained at finite amplitude, most especially a long-wave shear-Alfven component. Transport scaling of the turbulence is determined more by saturation through this component than the drive. In the turbulence, the dominant drive is by a long-wave MHD which is self maintained but is very weak in the linear regime. The resulting scaling qualitatively diverges from the linear growth rates. The turbulence is robust enough to avoid the L-H transition seen in experiments --- discussion of the role of stratification is given. Progress in several efforts to build stratification into total-f computations of the tokamak edge is reviewed.
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Theory Department Weekly Seminar Title: Momentum conservation in total-f gyrokinetics Speaker: Dr. Bruce Scott |
Theory Department Weekly Seminar Title: Hybrid MHD-kinetic electron simulations of a standing shear Alfven wave |
Theory Department Weekly Seminar Title: Plasma Instabilities in the Lower Earth’s Ionosphere and Their Macroscopic Effects The Earth’s ionosphere represents a natural laboratory where many exciting plasma physics phenomena occur. The lower-E/upper-D region ionosphere, roughly between 80 and 130 km of altitude is of a special interest to observers and modelers. In this range, electrons are magnetized, whereas ions are largely demagnetized due to frequent collisions with the neutral atmosphere. A sufficiently strong DC electric field perpendicular to the geomagnetic field gives rise there to large-scale ionospheric currents named electrojets. This DC field also generates low-frequency plasma instabilities which produce plasma turbulence observed by radars and rockets. Similar instabilities are also generated in elongated plasma trails left behind fast-moving meteoroids and in long-lived plasma clouds named sporadic-E layers. In my talk, I will review the E/D-region instabilities and related macroscopic processes, such as the formation of nonlinear currents and strong anomalous electron heating which has been observed at high latitudes during magnetospheric storms and substorms. These anomalous processes may affect significantly the height-integrated ionospheric conductances and explain why existing global MHD codes employed for predictive modeling of space weather regularly overestimate cross-polar cap potentials. I will show results of recent theoretical and modeling efforts involving particle-in-cell supercomputer simulations.
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Theory Department Weekly Seminar Title: Analyses of substantially different plasma current densities and safety factors reconstructed from magnetic diagnostics data
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Theory Department Weekly Seminar Speaker: Dr. Ernesto Mazzucato,PPPL Abstract: Our present understanding of confinement in tokamaks originated from the exchange of information on results from experiments in the U.S., Europe, Japan and the former Soviet Union. This is why we should redirect our present effort on the development of fusion reactors towards this type of synergistic international collaboration – without relying on a single experiment for addressing the physics of burning plasmas. It is for this reason that we should begin immediately the design and construction of a midsize tokamak capable of reaching large values of energy gain. In this presentation, I will discuss the possibility of achieving this goal using a tokamak where plasma recycling is minimized with a more efficient divertor than those currently used in tokamaks. |
Theory Department Weekly Seminar Title: Gyrokinetic absolute equilibria and turbulence Speaker: Dr. Jian-Zhou Zhu, University of Maryland/PPPL Abstract: A paradigm based on the absolute equilibrium of Galerkin-truncated inviscid systems to aid in understanding turbulence [T.-D. Lee, ``On some statistical properties of hydrodynamical and magnetohydrodynamical fields,'' Q. Appl. Math. 10, 69 (1952)] is taken to study gyrokinetic plasma turbulence: We keep a finite set of Fourier modes of the collisionless gyrokinetic equations and calculate the equilibrium statistics; possible implications for plasma turbulence in various situations are discussed. The new feature is that the integrations over the distributions are functional integrals because of the extra dependence on velocity of the gyrokinetic variable. For the case of two spatial and one velocity dimension, in the calculation with discretization also of velocity $v$ with $N$ grid points (our regularization exactly conserves $N+1$ quantities corresponding to an energy invariant and $N$ entropy-related invariants,) we show the existence of negative temperature states, corresponding to the condensation of much of the generalized energy into the lowest modes (depending on parameters), while there is an approximate equipartition of the generalized entropy. This indicates a generic feature of inverse energy cascade. Some classical results, such as those of Charney-Hasegawa-Mima are reproduced in the cold-ion limit. There is a universal shape for statistical equilibrium of gyrokinetics in three spatial and two velocity dimensions with just one conserved quantity. |
Theory Department Weekly Seminar Title: Radiation diagnostic of sub-Larmor-scale magnetic fields in lab and astrophysical plasmas Spontaneous rapid growth of strong magnetic fields is rather ubiquitous in high-energy density environments ranging from astrophysical sources (e.g., gamma-ray bursts and relativistic shocks), to reconnection, to laser-plasma interaction laboratory experiments, where they are produced by kinetic streaming instabilities of the Weibel type. In the talk, we will discuss spectral and temporal properties of radiation emitted by relativistic electrons in the course of the Weibel instability development and saturation. In our study we consider (i) anisotropic magnetic fields and electron velocity distributions, (ii) the effects of trapped electrons and (iii) extends the description to large deflection angles of radiating particles thus establishing a cross-over between the classical jitter and synchrotron regimes. The analytical and numerical results obtained from particle-in-cell simulations of the classical Weibel instability will be presented. Radiation emitted has a markedly non-synchrotron spectral energy distribution, which can be use as a benchmark of the sub-Larmor-scale magnetic fields in the system. |
Theory Department Weekly Seminar
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Theory Department Weekly Seminar Title: ECRH power deposition from a quasi-optical point of view Abstract: A quasi-optical description of the propagation and damping of the slowly varying wave amplitude across an arbitrary electron cyclotron wave beam is presented. This model goes well beyond those implemented in existing beam tracing codes, which typically require the spatial inhomogeneity across the wave beam to be small. The present model allows an accurate description of the wave beam evolution in the region of electron cyclotron power deposition, where the latter condition is quite generally broken. The additional physical effects from spatial inhomogeneity and dispersion included in the quasi-optical model are discussed in relation to their consequences for the power deposition profile. Quite generally, a broader power deposition profile is obtained in the quasi-optical calculations. The importance of these effects is analyzed in a number of scans varying the injection geometry for typical conditions in both the ITER and the TEXTOR tokamak. Optimization of the power deposition profile towards a minimal width is found to require a focused wave beam with a waist of typically 2 cm width localized near the electron cyclotron resonance region. Calculations are also presented for beams injected from the ITER Upper Port electron cyclotron resonance heating (ECRH) launcher as it is currently being designed. These show that the additional power deposition profile broadening from quasi-optical effects may result in a drop in the predicted efficiency for neoclassical tearing mode or sawtooth control by up to a factor of 2.
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Theory Department Weekly Seminar
Abstract: The talk outlines the basic aspects of plasma regimes of Fusion-Fission |
Theory Department Weekly Seminar Title: Theory of the Dense Plasma Focus: Basic Model and Recent Developments Speaker: Dr. Eric J. Lerner, Lawrenceville Plasma Physics, Inc. Abstract: There has been a revival in interest in the dense plasma focus, with
three large new machines coming on line in the past 18 months, and at
least two groups investigating the device’s possible use for pB11 |
Theory Department Weekly Seminar |
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