Theory Department Weekly Seminar
Date/Time: MONDAY, June 8, 2009, 10:45AM
Location: Theory Conference Room T169

Title: "Gyrokinetic Turbulence in Space and Astrophysical Plasmas"

Speaker: Gregory G. Howes, University of Iowa

Abstract:

Plasma is a ubiquitous form of matter in the universe, nearly always found to be both magnetized and turbulent. One must understand this behavior to interpret the observations of many astronomical environments, including the galactic interstellar medium, accretion flows around stars and black holes, and the solar wind streaming outward from our Sun. Although the turbulence at large scales is
well-described by fluid theory, for scales smaller than the collisional mean free path, the dynamics must be described instead by kinetic theory. Because the dissipation of turbulence leading to plasma heating occurs at these small scales, the heating mechanisms are, by nature, kinetic. This transfer of turbulent energy through an inertial range from the driving scale to dissipative scales in a kinetic plasma followed by the conversion of this energy into heat is a fundamental plasma physics process. The ability to make in situ measurements of turbulence in the solar wind, for which the entire inertial range is weakly collisional, presents a great opportunity to improve our understanding of turbulence in kinetic plasmas. In this talk, I will present a theoretical model of this kinetic turbulent cascade. The first nonlinear simulations of the kinetic turbulent cascade at the scale of the ion Larmor radius show good qualitative agreement with observations of turbulence in the solar wind. Progress in our understanding of kinetic turbulence necessarily requires a combined effort of theoretical modeling, nonlinear numerical simulations, and observational constraints from the turbulent solar wind.

Theory Department Weekly Seminar
Date/Time: June 3, 2009, 10:30AM
Location: MGB Auditorium

Title: "Mathematical theory of Landau damping in the nonlinear regime"

Speaker: Prof. Cedric Villani, Institute for Advanced Study and
Ecole Normale Superieure de Lyon

Abstract:

Sixty years ago, Landau discovered a paradoxical collisionless relaxation effect in plasmas. The Landau damping is now one of the cornerstones of classical plasma physics. From the mathematical point of view, it has remained elusive so far, since the best available results prove the existence of some damped solutions, without saying anything about their genericity. I shall report on new advances, and a whole new mathematical theory, for this problem. I will discuss the physical implications of these results. This is joint work with Clement Mouhot.

Theory Department Weekly Seminar
Date/Time: May 21 2009 10:45AM
Location: Theory Seminar Room (T169)

Title: A noble analysis of radial electric field formation, turbulencetransport, and H-mode transition based on the gyrocenter shift

Speaker: Dr. Kwan Chul Lee, University of California at Davis

Abstract:
A new concept of turbulence transport and diffusion coefficient are derived from the microscopic ExB drift at tokamak boundary by characterization of the gyrocenter shift induced by ion-neutral collisions. It is found that when the viscosity force of the ion-neutral collision is counted, Reynolds number of the poloidal ion flow in the vicinity of separatrix vary over the critical value between turbulent and laminar flows depending on the plasma parameters such as temperature, neutral density and density fluctuation level, which explains the mechanism of H-mode transition [1]. In this talk, the concept of gyrocenter shift and formation mechanism of radial electric field [2][3] will be also presented.

[1] K.C. Lee, Plasma Physics and Controlled Fusion, Vol-9 number-6, 065023 (2009)

[2] K.C. Lee, Physics of Plasmas, Vol-13, 062505 (2006)

[3] K.C. Lee, Physical Review Letters, Vol-99, 065003 (2007)

Theory Department Weekly Seminar
Date/Time: May 14 2009 10:45AM
Location: Theory Seminar Room (T169)

Speaker: Dr. Igor Kaganovich, PPPL

Title: Collective Effects of Intense Ion and Electron Flows Propagating
Through Background Plasma.

Abstract:
After a short historical introduction on the subject, several collective effects observed during propagation of intense ion or electron beams through a background plasma will be described: 1) neutralization of ion beam space charge by a background plasma; 2) collisionless ion heating by an intense electron beam due to development of the Weibel instability; 3) operation of the Hall thruster with intense secondary electron emission.

Theory Department Weekly Seminar
Date/Time: March 26 2009 1:30 PM ** SPECIAL TIME**
Location: Theory Seminar Room (T169)

Speaker: Dr. Alexander Lazarian
Department of Astronomy,University of Wisconsin-Madison

Title: Turbulence in Magnetized Plasmas: Astrophysically important cases

Abstract:
Astrophysical plasmas usually are turbulent with magnetic fields playing a dominant role. This presents major impediments for detailed numerical modeling of astrophysical processes, as Reynolds and Lundquist numbers of astrophysical fluids cannot be possibly attained employing brute force approach. This calls for better understanding of scalings related to MHD turbulence. I shall start my discussion with the Goldreich-Sridhar 95 model of MHD turbulence and show that the attempts to improve the model are still problematic. Then I discuss the spacecraft data indicative of the dispersive range of turbulence below the ion gyroscale and propose a model of turbulence at those small scales. I shall show that in the presence of neutrals the turbulence of partially ionized plasma gets radically different while a resurrection of the normal turbulent cascade including only ions is expected at small scales. In addition, I shall discuss how MHD turbulence is modified in the presence of energy flux imbalance and collisionless effects.

Theory Department Weekly Seminar
Date/Time: March 05 2009 AM 10:45
Location: Theory Seminar Room (T169)

Speaker: Prof. James Wanliss, Presbyterian College

Title: Complex behavior in Space Plasmas

Abstract:
We provide a new example of a natural plasma system exhibiting statistical signatures of complex scaling behavior involving scale-independent burst of activity. We examine statistical properties of multiscale energy dissipation in the inner magnetosphere of Earth based on the dynamics of the SYM-H index, a global marker of low-latitude geomagnetic fluctuations and storms in space plasma. 0n average, and for time scales shorter than 2 hours, temporal development of SYM-H bursts follows an algebraic form which is consistent with the predictions from the theory of nonequilibrium phase transitions in disordered media. Probability distributions of sizes and lifetimes of the activity bursts reveal no characteristic scales other than the scales imposed by technical limitations of the analysis. This behavior is observed for the range of SYM-H burst durations starting from about 5 minutes up to 10-15 days. The power-law exponents describing the probability distributions suggest that the main energy dissipation in the plasma of earth's inner magnetosphere takes place due to major space storms as opposed to smaller activations whose contribution is less significant despite their much higher relative occurrence. The results obtained provide new evidence for dynamical and statistical self-similarity in the inner magnetosphere.

Theory Department Seminar
February 26 2009 1:30 PM ** SPECIAL TIME**
Theory Seminar Room (T169)

Dr. Jonathan Arons
University of California, Berkeley

High Energy Emission from Pulsar Winds and a Possible Origin of Low
Wind Magnetization

Several lines of evidence suggest that relativistic winds from pulsars have flow energy dominated by kinetic energy at their termination, even though they emerge from the light cylinder as Poynting flux dominated flows. The wind sources are oblique rotators, thus the winds are ``striped'' - composed of interleaved sectors of oppositely directed ${\boldsymbol B} $ in a wide sector of latitude around the rotational equator. The electric current in the sheets separating the oppositely directed magnetic fields of the stripes, which provide the star's electric return current, may be composed of a high energy particle beam, propagating across the magnetic field in an almost unmagnetized channel of thickness comparable to the particles' formal Larmor radius. The beams in neighboring sheets have opposite propagation directions, and interact across the stripes through the long range electromagnetic field. Thus the beams are subject to an electromagnetic shear instability which has strong kinship to Weibel beam filamentation instabilities in unmagnetized plasmas. I outline the physics of this instability, apply it to the pair dominated winds from pulsars, both in the case when the return current is composed of ions or high energy positrons (angle between the angular velocity and the magnetic moment less than 90 degrees, an "acute" pulsar) and also in the electron beam return current case (angle between the angular velocity and the magnetic moment greater than 90 degrees, an "obtuse" pulsar). I argue that the instability saturates through magnetic trapping, which leads to the appearance of an anomalous resistance in the pulsar circuit, and show that this resistance can account for the reduction of the striped component of the winds' magnetic fields, through broadening of the current layers until they merge and the stripes disappear. I discuss some possible observational consequences of this magnetic dissipation in the apparently dark region between the light cylinder and the winds' termination shocks, and speculate on possible relations to dark flow regions of AGN jets.

Theory Department Seminar
February 24 (Tuesday) 2009 AM 10:45 ** SPECIAL DAY**
Theory Seminar Room (T169)

Dr. John R. Cary
Tech-X Corporation and the University of Colorado

The FACETS project: towards whole device modeling with concurrent,
tighly coupled components components

The multi-institutional FACETS project was started two years ago to develop new
computational capability for multiphysics, whole device modeling that can take
advantage of the current and future large parallel hardware. The physics goals include developing an understanding of how a consistent, coupled core-edge-wall plasma evolves, including energy flow, particle recycling, and the variation of power density on divertor plates with plasma under different conditions. FACETS is just now entering the research phase, as it has only recently developed coupled core-edge simulation capability. Hence this talk will concentrate on how we got tho this point. This includes a description of the software architecture with issues involved in developing concurrent component parallelism. It also includes what was involved in developing a new core solver (which provides parallelism, with a speedup of more than an order of magnitude). We also discuss some of the issues of verification and validation, as well as some software engineering issues dealing with testing and cross platform build issues. The conclusion will stress lessons learned about how to make a multi-institutional project work (or not), comments on code reuse versus rebuild, and methods for introduction of engineering into
computational physics for improved results.