# Past Seminars

## Photoelectric Charging of Dust in Space

### Amanda Sickafoose: CIPS - University of Colorado at Boulder

Friday September 24 1999, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

We have examined the photoelectric charging of dust 90-106 microns in diameter dropped through UV illumination and dropped past a UV illuminated surface having a photoelectron sheath. Experiments are performed in vacuum with illumination from a 1 kW Hg-Xe arc lamp that has a spectrum extending to 200 nm (6.2 eV). We present and compare the photoelectric charging properties of particles composed of zinc, copper, graphite, lunar regolith simulant (JSC-1), and martian regolith simulant (JSC Mars-1). We find that the photoelectric charging properties of the elemental materials are consistent with charging models calculated from the theoretical capacitance and charge on an isolated spherical grain. Dust dropped through UV illumination loses electrons due to photoemission, while dust dropped past an illuminated surface gains electrons from the photoelectron sheath. The photoelectric charging properties of JSC-1 and JSC Mars-1 are more difficult to interpret due to residual charge on the dust. The results suggest that JSC Mars-1 is more susceptible to photoelectric charging than JSC-1. The relation of this work to similar phenomena in the solar system is discussed.

## Bifurcation of the Period-4 Orbits in the Standard Map

### Yoshi H. Ichikawa: Nagoya University

Friday August 27 1999, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

The period doubling bifurcation process in the two-dimensional area preserving mapping is investigated on the basis of simmetry structure analysis. In particular a case of the period-4 orbits in the standard map has been studied throughly to analyze boundary islands formation around the principal period-4 island, and the onset of the hyperbolic bifurcation without reflection. It is illustrated explicitly that the hyperbolic bifurcation without reflection gives rise to the birth of twin orbits with the periodicity of the mother orbit.

## Chaotic Particle Transport Across a Magnetic Island Due to Electrostatic Drift Waves

### Yasutaro Nishimura: CIPS - University of Colorado at Boulder

Friday May 07 1999, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

The effect of poloidally mode coupled, ballooning type electrostatic drift waves on a magnetic island has been studied both analytically and numerically. It has been shown quantitatively that particle orbits become stochastic and their behavior can be a possible candidate for the radial plasma transport across a magnetic island of a tokamak. The transport is significant in that it takes place even when the flux surface is not destroyed. The mechanism of the stochasticity generation is understood as an overlapping of secondary islands caused by resonance between periodic particle motions in the magnetic island and Fourier modes of E x B drift due to the electrostatic drift waves. The diffusion process perpendicular to the island magnetic surface has been shown to follow the Gaussian type and can be influential for the deterioration of the plasma confinement. In addition, local diffusion process in the vicinity of Kolmogorov, Arnold and Moser (KAM) surfaces is discussed.

## Calculation of Finite-length, Hollow-beam Equilibria

### Jinhyung Lee: CIPS - University of Colorado at Boulder

Friday April 30 1999, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

Finite-length equilibria occur in a number of intense-beam and plasma applications. Penning traps permit the study of intra-beam collective effects, as the additional freedom gained from having an internal conductor permits greater control over the plasma profile, so that monotonic, but not constant, plasma profiles can be obtained. On the basis that the thermal velocity of background neutrals and the drift velocity of the electrons are much lower than the thermal velocity of the electrons, and the rotation frequency is small compared to the gyrofrequency, the equilibrium equation can be reduced to a self-consistent Poisson equation where the source depends on the potential. We solve for these equilibria using a Gauss-Seidel relaxation method. Our results show the shape of the equilibria for various electrode configurations.

## Is the Dynamics of the Magnetosphere Due to Chaos of Noise?

### Brent Goode: CIPS - University of Colorado at Boulder

Friday April 23 1999, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

In 1985 it was shown that linear analysis techniques were inadequate to describe the behavior of the magnetosphere, and so nonlinear studies of the magnetosphere were begun. When a finite dimension for an attractor was found in magnetospheric data it was thought that conclusive proof was found for chaotic behavior. However, later studies found that random noise with a certain frequency spectrum, called colored random or pink noise, could also give a finite result for the correlation dimension. Since then a controversy has been raging as to whether the magnetospheric was chaotic or stochastic. The definition and method for computing the correlation dimension will be discussed as well as the properties of colored random noise. Also to be discussed is how the properties of colored random noise can lead to a test to determine the difference between a colored random noise time series and a time series from ordinary differential equations that are chaotic.

## Global Dynamics of Charged Dust Particles in Planetary Magnetospheres

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

Friday April 16 1999, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

We study the global stability of charged dust grains orbiting an axisymmetric planet with co-rotating magnetic field. The magnetic field and induced electric field are described in an inertial frame using the magnetic stream function $\Psi$. The combined gravitational, magnetic, and electric forces are modelled by a two dimensional effective potential $U^e(\rho,z)$, parametrized by the conserved angular momentum $\pp$. The critical points of $U^e$ then locate the equilibrium circular orbits, nonequatorial as well as equatorial. The stable equilibria form the nuclei of potential wells, which can contain large populations of dust grains. These potential wells have their own topological structure, so that a particle which loses local stability can still be trapped globally. Explicit Lyapunov stability boundaries are derived for both positive and negative charges in both prograde and retrograde orbits. Thus, radial stability is lost when a critical point of $U^e$ undergoes a ta! ngent bifurcation, while transverse stability is lost via a pitchfork bifurcation. For a given position near a given planet stability depends only on the charge-to-mass ratio $q/m$, which for a spherical dust grain is proportion to $\Phi/a^2$, where $\Phi$ is the ambient plasma potential and $a$ is the grain radius. The results are applied to Saturn and Jupiter.

## Resistive X-point Modes in Tokamak Boundary Plasmas

### J.R. Myra: LRC

Friday April 09 1999, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

The edge and scrape-off-layer (SOL) of a tokamak plasma, while comprising only a few percent of the minor radius, is of great importance in understanding global tokamak confinement and heat deposition on the divertor plates. In this talk, a tutorial introduction to some of the relevant physics governing boundary plasma instabilities and turbulence will be presented, including the use of the collisional fluid Braginskii equations, the role of X-point geometry on wave physics, and the types of modes and instability drives thought to be important. Then, the results of recent work will be presented in which we have identified the curvature-driven resistive X-point mode as the dominant instability of a characteristic L-phase discharge in the DIII-D tokamak. This mode, expected to be a generic L-mode edge/SOL instability, is electromagnetic in the "bad curvature" region, but transitions to an electrostatic mode near the X-point due to the combined effects of resistivity and X-point magnetic shear. Motivated by observations of elevated electron temperature near the X-point, we investigate heating and parallel energy flow induced by the resistive X-point mode. We speculate that energy in the unstable waves flows to and dissipates in the X-point region, heating the electrons. Progress in understanding the nonlinear saturation levels and implications of this mode for perpendicular wave-induced transport will be discussed.

## The Plasma Dispersion Relation for Arbitrary Smoothed Distribution Functions: Landau Contours in the Lland of the Non-analytic

### David L. Newman: CIPS - University of Colorado at Boulder

Friday April 02 1999, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

Landau's prescription for determining the dispersion relation of waves in a collisionless plasma with a given velocity distribution for each species lies at the heart of kinetic plasma theory. However the treatment of damped modes requires the analytic continuation of the distribution functions for complex velocities.

In practical application, velocity distributions are often determined experimentally, or are numerically generated by computer simulations. These general distribution functions cannot be assumed analytic, nor can they necessarily be well-modeled by a superposition of analytic functions (e.g., Maxwellians).

An alternative perspective on the problem of determining the dispersive properties of damped plasma waves will be presented. This method draws a connection between the plasma susceptibility and the convolution (or deconvolution) of the distribution function with simple Lorentzians. This approach lends itself to the numerical determination of the plasma susceptibility for piecewise-analytic distributions that have been subjected to Gaussian smoothing.

## Transport Experiments with Non-neutral Plasma in a Modified Penning Trap

### Scott Robertson: CIPS - University of Colorado at Boulder

Friday March 12 1999, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

The Penning trap confines plasma of electrons (or ions) by using an electrostatic potential to prevent the escape of particles along magnetic field lines. A modified Penning trap has been constructed which has an azimuthal magnetic field made by wires along the axis. The resulting field is helical and particles execute bounce orbits along helical field lines. Drifts cause a particle path in the +z direction to be at a slightly larger radius than in the -z direction so that the orbit is oval, like a rubber band. Collisions cause a step-wise change in position with a characteristic length determined by the width of the oval rather than the the Larmor radius. Experiments in the new trap allow aspects of transport theory for toroidal devices to be isolated and tested. The first experiment shows that the transport due to electric mobility scales with the width of the oval drift orbit (the "banana width") and not with the Larmor radius.

## Plasma Space Propulsion: Plume Transport in Hall-Effect Thrusters

### Lyon King: NIST

Friday March 05 1999, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

Recent advances in available on-board electrical power in satellites has moved plasma propulsion devices from the laboratory to space-based application. In the past year alone more than 80 spacecraft employing some type of plasma propulsion system were flown on commercial vehicles. This talk will present a brief introduction to plasma thrusters including Arcjets, Resisojets, Electrostatic Ion Thrusters, and Magnetoplasmadynamic Thrusters before entering a more detailed analysis of the plasma transport properties in the plume of closed-drift Hall-effect thrusters. The Hall thruster utilizes static crossed electric and magnetic fields to accelerate ionized xenon producing a reactive thrust. The experimental investigation reported utilized in-situ probes to quantify the transport of charged and neutral species in the flowing exhaust plume as well as the construction of a molecular beam mass spectrometer to provide species-dependent measurements of the heavy-particle energy distributions. Among the phenomena discovered was the existence of an anomalous population of ions having apparent acceleration voltages of up to three times that applied to the discharge electrodes. Discussion of such phenomena in the context of charge- and momentum-transfer collisions will be presented.

## Efficient Class Library for Differential Algebra

### John Cary: CIPS - University of Colorado at Boulder

Friday February 26 1999, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

Numerical, as opposed to symbolic, differential algebras have a growing number of uses in computational science. These differential algebras can be used for simultaneous calculation of some quantity plus a certain number of its derivatives up to some order in some number of variables. Solving a differential equation for a differential algebra element gives the Taylor series expansion for the transfer map - the solution for initial conditions near some particular initial condition. In recent years, it has been recognized that numerical differential algebras are most easily treated by Object Oriented Programming methods in C++, a language that allows operator overloading. In this talk we will review numerical differential algebras and object oriented programming methods. We then discuss the implementation of numerical differential algebras within C++ and show that by judicious choice of data layout, the effiency of the numerical calculations can be increased by an order of magnitude.

## Particle Transport in Three Dimensional Stellarator Equilibria

### Yasutaro Nishimura: CIPS - University of Colorado at Boulder

Friday February 19 1999, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

The effect of poloidally mode coupled, ballooning type electrostatic drift waves on a magnetic island has been studied both analytically and numerically. It has been shown quantitatively that particle orbits become stochastic and their behavior can be a possible candidate for the radial plasma transport across a magnetic island of a tokamak. The transport is significant in that it takes place even when the flux surface is not destroyed. The mechanism of the stochasticity generation is understood as an overlapping of secondary islands caused by resonance between periodic particle motions in the magnetic island and Fourier modes of E x B drift due to the electrostatic drift waves. The diffusion process perpendicular to the island magnetic surface has been shown to follow the Gaussian type and can be influential for the deterioration of the plasma confinement. In addition, local diffusion process in the vicinity of Kolmogorov, Arnold and Moser (KAM) surfaces is discussed.

## A Simple Explanation of the Ballooning Transformation and TAE Modes

### Scott E. Parker: CIPS - University of Colorado at Boulder

Friday February 12 1999, 1:00 - 2:00pm in Gamow Tower, Room F931

## Spatiotemporal Evolution of Stimulated Brillouin Scattering in Homogeneous Plasmas

### Rodolfo E. Giacone: CIPS - University of Colorado at Boulder

Friday February 05 1999, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

The spatiotemporal evolution of stimulated Brillouin scattering (SBS) in homogeneous plasmas and some aspects of the influence that nonlinear and kinetic effects have on the evolution of SBS were studied.

A one-dimensional analytical linear model based on a fluid description of the plasma was initially developed. It was found that the threshold intensity of the absolute instability and the steady-state spatial growth rate of the convective instability are both independent of the scattering angle. However, the saturation time of the convective instability exhibits a strong inverse dependence on the scattering angle.

The basic model was improved by extending the one-dimensional analysis to include two spatial dimensions and time. In order to assess the effects that the finite size of the laser beam has on SBS, wide and narrow laser-beam geometries were considered. Detailed comparison were made between the predictions of a reduced 1D and 2d models, which can be solved and analytically, and the results of 2D numerical simulations.

The influence that nonlinear and kinetic effects have on SBS was investigated by performing particle-in-cell (PIC) simulations. The results of these PIC simulations were compared against fluid simulations, and good agreement was obtained for sufficiently weak laser intensities. When the laser intensity is sufficiently strong for ion trapping to be significant, PIC and fluid simulations differ substantially. The SBS reflectivity is shown to depend sensitively on the frequency mismatch between the light wave used to seed the instability and the incident laser.

## Crystalline Order and Modes in Laser-cooled Ion Plasmas

### Travis Mitchell: NIST

Friday January 15 1999, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

Plasmas, the ionized states of matter, are usually hot and gaseous. However, a sufficiently cold or dense plasma can be liquid or solid. We trap beryllium ions in a Penning trap, and utilize laser cooling to reduce the ion temperature to less than 5 mK. By applying an asymmetric electric field that rotates in the same sense as the ions, we are able to phase-lock the rotation of these plasmas, therefore enabling precise control of the plasma density and shape. The ions freeze into Coulomb crystals, which we have studied through both Bragg diffraction and spatial imaging. The crystals have a rich phase structure, whose features are shared with such diverse systems as neutron star crusts, hard spheres, colloidal suspensions and semiconductor electron bilayers. Plasma modes can be excited with potentials applied to the trap electrodes, and directly imaged by changes in the ion resonance fluorescence produced by Doppler shifts from the coherent velocities of the mode. Enhanced radial transport is observed where modes are resonant with static external perturbations; similarly, the plasma angular momentum can be usefully changed through the deliberate excitation of azimuthally asymmetric modes. Precise control of the plasma's angular momentum and structure is important for possible applications such as frequency standards, quantum computing and antihydogen production.

## Effect of Edge Convection on Tokamak Confinement

### Daniel D’Ippolito: Lodestar Corporation

Friday December 04 1998, 1:00 - 2:00pm in Gamow Tower, Room F931

## Large-scale Gyrokinetic Plasma Turbulence Simulations

### Scott E. Parker: CIPS - University of Colorado at Boulder

Friday November 13 1998, 1:00 - 2:00pm in Gamow Tower, Room F931

## Nonlinear 2-stream Instabilities as an Explanation for Auroral Bipolar Wave Structures

### Martin Goldman: CIPS - University of Colorado at Boulder

Friday November 06 1998, 1:00 - 2:00pm in Gamow Tower, Room F931

## Gyro-averaged Plasma Kinetic Equations for Low-frequency Phenomena

### Yang Chen: CIPS - University of Colorado at Boulder

Friday October 30 1998, 1:00 - 2:00pm in Gamow Tower, Room F931 Show Abstract

I will briefly review the physics of the saturation of kinetic instabilities due to wave trapping, and the role of collisions in wave-particle resonant interaction. I will provide a general introduction to the delta-f method, then discuss some of the difficulties in extending delta-f method to cases with collisions, and show how the introduction of a convenient tool (the marker distribution in extended phase space) overcomes such difficulties. I will then try to apply the new method to the simulation of a recent TFTR experiment and discuss the result.

## Ionospheric Modification

### Alexander Gurevich: Lebedev Physical Institute

Monday October 19 1998, 1:00 - 2:00pm in Gamow Tower, Room F931