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Past Seminars

Gyrokinetic delta-f Simulation of Collisionless Trapped Electron Mode Turbulence

Jianying Lang: CIPS - University of Colorado at Boulder

Friday October 26 2007, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

Turbulent transport driven by Collisionless Trapped Electron Modes (CTEM) is systematically investigated using three-dimensional gyrokinetic delta-f Particle-In-Cell (PIC) simulations. Scalings with local plasma parameters, including density gradient R/Ln, electron temperature gradient R/LTe, magnetic shear S hat, electron to ion temperature ratio Te/Ti and the inverse aspect ratio r/R, are studied. Simulation results are compared with previous simulations and theoretical predictions. Zonal flow suppression in nonlinear CTEM turbulence depends on electron temperature gradient and electron to ion temperature ratio. We introduce zonal density as another nonlinear saturation mechanism in the parameter regime where zonal flows are unimportant. The generation of zonal density is explained by a mode coupling model that agrees with the simulation. The mode coupling model also shows how the zonal density can stabilize the most unstable mode, which qualitatively agree with the simulation results.

Coarse-Graining the Distribution Function in Particle-In-Cell Simulation of Turbulent Transport in Tokamak Plasmas

Yang Chen: CIPS - University of Colorado at Boulder

Friday October 12 2007, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

In gyrokinetic simulation of turbulent transport in tokamak plasmas using the delta-f particle-in-cell method, it is frequently observed that the mean-square-value of the particle weights continue to grow after the turbulent fluxes reach a steady state. This is "the Growing Weight Problem" in PIC simulations. Since the particle weights are defined to be proportional to the distribution function, a situation where the particle distribution function continues to grow but the turbulent fluxes reach steady state is paradoxical. I will show how this "energy paradox" is resolved by examining the entropy evolution of the plasma. A numerical scheme for solving the growing weight problem will be described. Delta-f is periodically deposited on a five-dimensional phase-space grid, then re-evaluated at the particle position using interpolation. Using three-dimensional toroidal Ion-Temperature-Gradient Driven turbulence as an example, the numerical scheme is demonstrated to effectively suppress the long-term increase of the particle weights, while keeping the turbulent flux unchanged.

Is the Bohm Criterion satisfied in multiple ion species plasmas? Adventures with laser-induced fluorescence (LIF)

Greg Severn: University of San Diego Dept. of Physics

Monday October 01 2007, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

Xenon ion laser-induced fluorescence (LIF) measurements in low temperature Xe I I plasmas (Te ? 1eV , Ti ? 1/40eV , ni ? 10 9 cm?3 ) have been achieved. The transition studied involves the metastable state (3 P1 )5d [3]7/2 , at 108423.07 cm?1 . The LIF scheme involves transitions which have been misidentified in the literature. At any rate, these studies have permitted measurements of the ion velocity distribution functions for both ions in a two ion species (Ar+Xe) plasma, so as to make possible the first experimental tests of the Generalized Bohm Criterion. Is the Bohm Criterion satisfied in multiple ion species plasmas? Come to the seminar and find out!

Double Layers and Associated Nonlinear Structures in Earth's Auroral Zone: A Vlasov-Simulation Perspective

David Newman: CIPS - University of Colorado at Boulder

Friday September 14 2007, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

Observational evidence for localized electrostatic field structures in a variety of near earth space plasma environments is now well established. The most frequent such observations are of the typically bipolar parallel electric field signatures of electron and ion phase space holes. However, another class of nonlinear structures double layers with their less frequently observed unipolar parallel fields, may in fact play a more important role in regions such as Earth's auroral ionosphere. In particular, a double layers is able to support a large parallel potential drop over a short distance, thereby contributing directly as well as indirectly to the acceleration of both electrons and ions. Kinetic plasma simulations are important tools for the purpose of studying the dynamics of double layers and phase-space holes. Simulations based on direct integration of the Vlasov equations, while less commonly employed than particle simulations, have proven to be particularly well-suited to this effort. Because standard Vlasov methods become numerically demanding when modeling systems in more than one spatial dimension, various "reduced" Vlasov algorithms have been developed to facilitate these studies. Results of recent Vlasov-based simulations will be presented, addressing issues such as the stability and perpendicular structure of double layers, as well as their role in the generation of phase-space holes and the perpendicular heating of ions.

Adventures in Plasma-Assisted Combustion

Louis Rosocha: LANL

Monday May 07 2007, 1:30 - 2:30pm in Gamow Tower, Room F931 Show Abstract

The application of electric fields to flames has been studied at least as far back as 1814, was applied to flame combustion in the 1920's and was further developed into several applications in the last half of the 20th Century. When the electric field strength is sufficient to cause electrical breakdown of a fuel or fuel/air mixture, plasma effects will dominate. Plasma effects can increase electron and ion temperatures and promote combustion through the formation of 'active' species (such as free radicals) or the dissociation of fuel molecules into smaller, more-easily combusted fragments. Plasma-assisted combustion (PAC) is now a timely topic worldwide, possibly having applications that can allow more efficient fossil-fuel usage, the conversion of low-grade fuels into higher-grade fuels, and the reduction of pollution through ultra-lean burn combustion. This chapter focuses on non-equilibrium ("cold" or "non-thermal") plasma applications to combustion, particularly for enhancing combustion stability, efficiency, and reducing undesirable emissions. This is in contrast to equilibrium ("hot" or "thermal") plasmas (e.g., spark plugs, plasma jets/torches). This talk will present a brief historical background on electric field and plasma effects on combustion and will then discuss non-equilibrium plasmas, as mainly applied to combustion stability, efficiency, and pollution reduction in more detail. Plasma-based ignition will be only briefly mentioned because it is considered a specialized, although important, topic within the PAC field. Selected examples from the literature will be presented, but the talk will primarily focus on work carried out at the author's institution that provides examples of nonequilibrium plasma applications to combustion enhancement. In non-equilibrium (or non-thermal) plasmas, energetic electrons are primarily responsible for generating the desired chemical species relevant to combustion, while the ions and background gas are 'cool'.

Simulations of beam-beam interaction for the PEP II collider

Kiran Sonnad: Stanford Linear Accelerator Center (SLAC)

Wednesday February 28 2007, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

While the PEP II luminosity recently reached a new peak of 10^34 cm^?2s^?1, simulations show that this peak can be doubled in the future by changing certain parameters. Luminosity is generally defined as a phase space overlap integral of two colliding beams and is a measure of the performance of the collider. The PEP II B-factory is an electron-positron collider located at SLAC. This talk will begin with a few details of the PEP II facility, an introduction to the physics of beam-beam interaction, followed by a brief explanation of the simulation method. Finally, results will be presented that led to determining the final set of parameters that yielded a simulated luminosity of 2x10^34 cm^?2 s^?1. Besides this, results will also be presented showing that the current peak luminosity can be immediately increased by about 10% by choosing an optimum set of accelerator parameters.

Charged Dust Dynamics Near Planetary Surfaces

Josh Colwell: LASP - University of Colorado at Boulder

Friday December 08 2006, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

Planetary surfaces exposed to the solar wind and high energy solar radiation develop a charge due to photoemission, collection of solar wind electrons and ions, and secondary electron emission. Dust particles on the surface can be lifted off the surface and transported by the plasma sheath electric field. Observations of a lunar "horizon glow" by several Surveyor spacecraft on the lunar surface in the 1960s and detections of dust particle impacts by the Apollo 17 Lunar Ejecta and Meteoroid Experiment (LEAM) have been explained as the result of micron-sized charged particles lifting off the surface. The NEAR/Shoemaker spacecraft observed unusual deposits of fine material in some craters on the asteroid Eros that may be the result of electrostatic transport of dust. I will give an overview of observations from the Moon and Eros and numerical simulations of the process of charged dust transport in a dayside photoelectron sheath.

The Earth's Tail Current Sheet: Observationally-Constrained Vlasov Models

Chris Cully: LASP - University of Colorado at Boulder

Friday November 10 2006, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

The interaction of the solar wind with the Earth's magnetic field creates several large sheets of electric current, including the ~10 MA tail current sheet which flows above geosynchronous orbit. A variety of nonlinear plasma processes occur within this sheet and its dynamics control much of the Earth's magnetosphere. Understanding its equilibrium configuration is thus an important step in resolving broader issues including current-driven instabilities, reconnection and the overall dynamics of the magnetosphere. Despite the large physical size, the equilibrium structure is frequently too thin to be describable within the framework of magnetohydrodynamics; consequently, realistic models must be built with Vlasov-Maxwell theory. Detailed observations of the tail current sheet are available from many satellites, most notably a set of 4 European Space Agency satellites known as Cluster. However, by comparing Cluster observations to available current sheet models, it becomes clear that the existing models are generally insufficient for describing the configuration. By using the observations as a guide, I will discuss the generalization of an existing class of models to create an exact semi-analytic Vlasov model that can better represent the real tail current sheet.

Ion Drive: Space Propulsion Workhorse for the New Millenium

Paul Wilbur: Colorado State University

Friday October 27 2006, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

This seminar will focus on the operating principles and capabilities of ion thrusters. Processes applied to generate energetic electrons and thence ions, to extract and accelerate the ions, and to neutralize the resulting ion beam will be discussed. The reasons why these thrusters are being used so successfully for stationkeeping and orbit-raising missions on Earth satellites and why they were so successful on the Deep Space One mission will be mentioned. Numerical modeling results that describe the ion extraction and acceleration process will be compared to corresponding experimental results obtained using small arrays of ion extraction aperture pairs (gridlet studies). The great potential of electric thrusters for future advanced satellite and exploration missions including NASA's DAWN mission will be mentioned.

Lowering the Fusion Threshold

Dan Barnes: Coronado Consulting

Friday October 20 2006, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

Beam-target fusion is not of economic interest, as the beam drag power exceeds the fusion yield. This picture changes if one imagines using the heat produced by beam drag as a low entropy source (because of the high plasma temperature) rather than exhausting it. This elementary thermodynamics is fine, but the challenge is in the (conceptual) engineering. How to arrange a confined plasma to form a high efficiency heat engine and how to use the mechanical energy to form a beam? Known electrostatic fusion concepts are extended to "conventional" magnetically confined quasi-neutral plasmas. Rapidly (supersonic) rotating plasmas are particularly useful for this. The rotation forms a mechanical energy reservoir and the cross field electrical potentials are useful for particle acceleration.

In this talk, two new physics ideas are developed and applied to this problem. First idea: electrostatic wells are replaced by centrifugal wells and non-neutral plasma replaced by quasi-neutral plasma. Previously known physics are applied, leading to many arrangements which form a high-efficiency (<90%) heat engine. Simplest of all is the Pastukov problem, in which a single well confines a low-collisionality nearly thermal plasma. It is shown that a proper arrangement of magnetic field (essentially the open field of a field-reversed configuration - FRC) can make this into a heat engine, so that plasma heat becomes rotation. The energy cycle is completed by converting rotation to beam energy. It is shown how to use the high electrical potentials induced by rotation to electrostatically accelerate a beam into the confined plasma.

Second idea: plasma rotation can produce plasma waves from a static magnetic perturbation, using nothing more than the Doppler effect. These waves can also be used to produce a desired beam by resonant absorption. Another use for such waves is to drive currents. As already demonstrated experimentally, such currents can form a FRC.

All of this leads to lowering the fusion threshold. In particular, required temperatures are greatly reduced, leading to very small, very high-power density systems. The non-thermal fusion also means that aneutronic fuel cycles can be used. Some examples are given. Finally, a small experiment to test these physics is being planned and some details of this design are given.

Left-Handed Structures for Accelerator Applications

Dr. Sergey Antipov: Argonne Wakefield Accelerator/Illinois Institute of Technology

Wednesday October 11 2006, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

Metamaterials are artificial periodic structures made of small elements and designed to obtain specific electromagnetic properties. As long as the periodicity and the size of the elements are much smaller than the wavelength of interest, an artificial structure can be described by a permittivity and permeability, just like natural materials. When the permittivity and permeability are simultaneously negative in some frequency range, the metamaterial is called double negative or left-handed and has some unusual properties. Left-handed metamaterials (LHM) have potential applications in active and passive devices at millimeter waves and at much higher frequencies. Waveguides loaded with metamaterials are of interest because the metamaterials can change the dispersion relation of the waveguide significantly. Slow backward waves can be produced in a LHM-loaded waveguide without corrugations. The dispersion relation of a LHM-loaded waveguide has several interesting frequency bands which are described. Left-handed structures can be employed at X-band accelerators to suppress wakefields.

Plasma Sputtering Effects in Planetary Dust Dynamics

James E. Howard: CIPS - University of Colorado at Boulder

Friday October 06 2006, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

Plasma sputtering can greatly reduce the size of charged dust grains orbiting within the magnetosphere of Saturn in only a few decades.With mass and charge varying in time, the resulting equations of motion are non-hamiltonian. Except for the small influence of solar radiation pressure, this systems is axisymmetric and the question arises as to the existence of global invariants in the absence of a hamiltonian. For larger grains, where gravity dominates we show that a formal hamiltonian may be constructed by treating the velocities as canonical momenta. For smaller grains, where the planetary magnetic field dominates, a hamiltonian description is apparently not possible. Nevertheless, an exact invariant is derivable from the axisymmetric equations of motion. Implications for the history and structure of Saturn's E ring and for observations by the Cassini orbiter CDA and UVIS experiments will be discussed.

The Source of Magnetic Shear that Drives Flares and Coronal Mass Ejections

Ward "Chip" Manchester: University of Michigan

Friday July 07 2006, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

Current mechanisms of coronal mass ejection (CME) initiation tend to rely on ad hoc assumptions to energize coronal magnetic fields to erupt. Most notably, artifi cial shearing of coronal magnetic arcades has been employed for nearly three decades to model fl ares and CMEs while no self-consistent explanation for the shearing motions was known. This talk will focus on the recent discovery that such shearing motions are driven by the Lorentz force that naturally arises when bipolar magnetic fields emerge from the photosphere into the corona. These spontaneous shearing motions will be shown to produce eruptions in a fully self-consistent manner in both magnetic arcades and flux ropes. The shearing motions transport axial flux and energy from the submerged portion of the field to the expanding portion, strongly coupling the solar interior to the corona. This physical process is very robust for explaining the highly sheared state of the magnetic field associated with prominences, and why these magnetic fields erupt in flares and CMEs.

Magnetohydrodynamic Activity Inside a Sphere

David Montgomery: Dartmouth College

Friday May 12 2006, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

Spontaneously arising magnetic fields occur widely throughout the universe. Attempts to explain them go back at least to Gauss in 1838 and define the "magnetic dynamo problem." In the context of magnetohydrodynamics (MHD), it is easy to see that some of the possible motions of an electrically-conducting fluid are capable of amplifying arbitrarily small magnetic fields. If the amplification continues, the magnetic fields B and their associated electric currents J become large enough that the Lorentz force JxB ceases to be negligible and begins to participate nonlinearly in the mechanical motion of the fluid, which sometimes may be turbulent. The difficulty lies in making the results quantitative and in accountng for the kinds of magnetic fields (planetary, solar, laboratory, or remotely astrophysical) that are created and observed. We have been doing numerical MHD computations motivated jointly by recent laboratory experiments on liquid sodium (e.g., [1,2]) and the need to account for magnetic fields generated inside spheres [3] in planetary models. An important number is the ratio of fluid viscosity to resistivity (in dimensionless units, the "magnetic Prantdl number"); it has much to say about the ease or difficulty of exciting dynamo processes. The subject will be reviewed at an elementary level, and then samples of our recent computations discussed.

[1] A. Gailitis et al, Phys. Rev. Lett. 86, 003024 (2001).

[2] P.D. Mininni and D.C. Montgomery, Phys. Rev. E72, 056320 (2005).

[3] P.D. Mininni and D.C. Montgomery, "Magnetohydrodynamic activity inside a sphere," arXiv:physics/0602147 (submitted to Phys. Fluids, 2006).

Global Invariants for Variable-Mass Systems

James E. Howard: CIPS - University of Colorado at Boulder

Friday April 07 2006, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

We investigate the effect of mass loss on the invariants of systems having translational or rotational symmetry. These systems are nonhamiltonian in the physical momenta but, in the case of non-velocity-dependent potentials can be made formally hamiltonian by treating the velocities as canonical momenta. Applying Noether's theorem to the formal Hamiltonian then yields global invariants corresponding to its symmetries. For velocity-dependent potentials such as occur for motion in magnetic fields, an exact invariant is constructed from the (nonhamiltonian) vector field. For adiabatic orbits the motion is thereby reduced from 3D to a 2D manifold with time-dependent effective potential parametrized by the conserved momentum. The results are applied to single particle motion in an axisymmetric gravitational field, the time-dependent Kepler problem, and charged particle motion in linear and axisymmetric magnetic fields. Finally, we indicate how the results might be combined to describe the motion of charged dust grains about Saturn.

Double Layers, Ion Holes, and Other Non-Linear Structures in Auroral Plasmas Implosions

Daniel Main: CIPS - University of Colorado at Boulder

Friday February 24 2006, 2:00 - 3:00pm in Gamow Tower, Room F931 Show Abstract

Observations from auroral satellite missions have shown for decades that electron and ion distributions in the auroral region are consistent with being accelerated through magnetic field aligned electric fields (so called parallel electric fields). More recently, FAST (an auroral spacecraft), has directly measured parallel electric fields. However, In a collisionless plasma, there is no generally accepted theoretical description of how parallel electric fields are self-consistently supported.

In this talk, I will show that parallel electric fields can be supported by double layers (DL). I will then show how we solve for such a double layer using methods similar to Bernstein, Green and Kruskal [1957] (the so called BGK method). The distribution functions that we use to construct the DL are modeled from FAST data. Finally, to test whether such a DL is stable, I have initialized a Vlasov simulation with a typical auroral cavity plasma, and have included a double layer to see how it evolves. I will briefly discuss the Vlasov algorithm for evolving distribution functions. One of the new features of the simulation is that we have included two ion species (H+ and O+) in addition to electrons. As a result of having two ion species, I will show how ion phase space holes and other non-linear structures, which are often seen with FAST, form in the simulation.

nterplay of Flows and Magnetic Fields in Solar and Space Environments

Nong Xiang: CIPS - University of Colorado at Boulder

Monday December 12 2005, 3:00 - 4:00pm in Gamow Tower, Room F931 Show Abstract

The delta-f algorithm has been implemented in the PIC code VORPAL. With this approach, the mode conversion from the extraordinary (X) mode to the electron Bernstein wave (EBW) below and above the second electron cyclotron harmonic frequency is simulated. It is found that a full X-B mode conversion below the second cyclotron harmonic frequency can be established for the parameters optimizing the maximum mode conversion efficiency coefficient giving by the linear theory. When the driving frequency is in the vicinity of the third and fourth cyclotron harmonics, however, the mode conversion becomes less efficient even for the optimized parameters. A new X-B mode conversion scenario due to the complicated dispersive behavior of the EBW is revealed in which the present linear theory of X-B mode conversion may fail. It is also shown that the mode conversion and propagation of EBWs are affected by the existence of the electron cyclotron harmonic resonance. If the amplitude of the incident X wave is sufficiently large, resonant mode-mode coupling is observed in the X-B mode conversion.

Collaborators: John R. Cary, Daniel C. Barnes and Johan Carlsson

Fusion Research Using Dynamic Hohlraum Radiation-driven Capsule Implosions

Jim Bailey: Pulsed Power Sciences Center

Friday November 18 2005, 1:30 - 2:30pm in Gamow Tower, Room F931 Show Abstract

The goal of inertial confinement fusion research is to create miniature thermonuclear energy bursts. This requires heating and compressing a deuterium-tritium mixture to stellar interior conditions in a terrestrial laboratory. The dynamic hohlraum approach converts electrical energy from the Z pulsed power machine into x-rays that drive spherical capsule implosions. The hot dense implosion core plasma emits thermonuclear neutrons and x-rays that are used to diagnose and optimize the implosion. In particular, Ar tracer atom emission is measured with time- and space-resolved spectrometers that provide data suitable for a tomographic reconstruction of the implosion core temperature and density profiles. The challenges and opportunities provided by dynamic hohlraum fusion research will be described.

Thermal Fluctuations in a Pure Electron Plasma: A Touch-free Thermometer for Plasmas

Nobuyasu Shiga: NIST Time and Frequency Division

Friday November 04 2005, 1:30 - 2:30pm in Gamow Tower, Room F931 Show Abstract

We have detected thermally excited charge fluctuations in a pure electron plasma over a temperature range of 0.05 < kT < 10eV. These fluctuation spectra have both a global mode component and a random particle fluctuation component.

At low temperatures, the m? = 0, kz = 1, 2, 3, . . . Trivelpiece-Gould modes (standing waves of density fluctuation along the z-axis; i.e., center of mass motion, breathing mode, and higher modes) are weakly damped and dominate, since the random particle component is suppressed by Debye-shielding. As the temperature increases, the broad random particle component increases in between the modes. The thermally excited mode is physically interesting because it exhibits both the individual particle behavior and the collective mode (wave) behavior of equilibrium plasmas. Also, the thermally excited mode leads to an important application, which is a passive temperature diagnostic of electron plasmas.

Interplay of Flows and Magnetic Fields in Solar and Space Environments

Giovanni Lapenta: Los Alamos National Laboratory

Wednesday October 12 2005, 4:00 - 5:00pm in Duane Physics, Room G1B20 Show Abstract

Flows and magnetic fields are primary actors in many processes in space and solar environments. Understanding the interplay of flows and magnetic fields is key for developing a predictive model of processes involving energy conversion between magnetic and kinetic energy. Processes where these issues are paramount include coronal mass ejections, solar wind formation and magnetic disturbances in the space weather around the Earth.

In the present seminar, I describe my recent work in the field of flow-magnetic field interaction applied to processes typical of the solar and Earth environment. I will describe the specific examples of the genesis of the slow solar wind in the solar corona and of reconnection in the Earth's magnetosphere. I will describe fundamental processes related to the interplay of flows and magnetic fields and I will address how microscopic and macroscopic processes interact to determine the overall evolution. A unique tool to handle such multiple scale problems at within a fully kinetic approach, CELESTE3D, will also be described.