# TREND Fair 2008

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**August 8, 2008**

### On this page... TREND 2008 Presentations

- Chaotic Ion Orbits in Alfvenic Magnetic Reconnection Outflows, Benjamin Brown
- Gyroklystron Design: A High-Power, High-Gain, Second-Harmonic Gyroklystron for K-Band Microwave Amplification, Melanie Veale
- Sync Map Description of Coupled Oscillator Systems, Gilad Barlev
- Spherical Heat Source in Superfluid Helium, Kaitlyn Tuley
- Large-Scale Deformation of Giant Unilamellar Vesicles, Alex Steinkamp
- Bright and Dark Field Imaging of Plasmon Resonance in Nanoparticles, Armand Rundquist
- Flow Birefringence of Aqueous Polyacrylamide Solutions, Auralee Morin
- Rejuvenation of a Depreciated Photocathode, Jessica Leung
- Comparison of Driven Surface Waves in Three Fluid Environments, Mariya Dryga
- Off-Axis Vortex Ring Collisions, Brian Vlastakis

### Chaotic Ion Orbits in Alfvénic Magnetic Reconnection Outflows

(Poster)

#### Benjamin Brown, University of Colorado Boulder

Advisors: Professor James F. Drake and Dr. Marc Swisdak

In our research, we simulate the unbound orbits of ions in the Alfvénic outflows of various magnetic reconnection events. Expanding on previous research, we include the influence of the out-of-plane Hall magnetic field inside the ion diffusion region. We focus on the behavior of the ions near the discontinuities of the Hall field, especially at the field reversal midplane, comparing results from numerical and analytical models. We present results of particular interest on the chaotic reflections of the ions at the reversal midplane and the resulting ion temperature distribution.

### Gyroklystron Design: A High-Power, High-Gain, Second-Harmonic Gyroklystron for K-Band Microwave Amplification

#### Melanie Veale, Washington University in St. Louis

Advisor: Professor. Wesley Lawson

The goal of this project is to design a high-gain (50-60dB) microwave amplifier circuit, operating at 22.848 GHz, for high electric field gradient testing. The design consists of a thermionic magnetron injection gun (MIG) connected to a multicavity gyroklystron circuit. First, an existing MIG design must be modified to work at 22.848 GHz and optimized for the best combination of the following beam properties: axial velocity spread, perpendicular velocity to axial velocity ratio, and average guiding center radius. This is accomplished using gun simulation codes (TRICOMP), so the numerical convergence of these simulations is also investigated. Second, a four-cavity gyroklystron circuit design is optimized for efficiency using simulation codes (MAGYKL). Promising designs must also be checked to ensure stability. Finally, in addition to optimizing a design for operation at 22.848 GHz, the potential feasibility of similar designs at other frequencies is also investigated. This is done by using the TRICOMP codes to examine the effect of operating frequency (magnetic field) on key beam properties.

### Sync Map Description of Coupled Oscillator Systems

(Poster)

#### Gilad Barlev, Kenyon College

Advisors: Professor Edward Ott and Assistant Professor Michelle Girvan

The Kuramoto model describes the tendency of coupled oscillators to synchronize when the coupling strength is above a critical value. Through Monte Carlo simulations, we study the behavior of a variation on the discrete-time Kuramoto model and verify certain properties of the model, namely the critical coupling value and the rate of relaxation towards synchronization. We then apply the model to nondirected networks with community structure to investigate synchronization within communities. Further, we propose a method for the discovery of community structure within networks based on observations of the time-averaged degree of synchronism between pairs of oscillators within our system.

### Spherical Heat Source in Superfluid Helium

#### Kaitlyn Tuley, University of Maryland College Park

Advisor: Professor Daniel Lathrop

At absolute zero temperature helium is superfluid, as described by the nonlinear Schrödinger equation. We examine the wave function for a spherical heat source suspended in the superfluid. Analytic solutions exist and are found for the fifth-order equation. A numerical solution for the cubic equation is found using Runge-Kutta integration. Just above absolute zero temperature, helium can be modeled as a mixture of two fluids: superfluid, as described by the nonlinear Schrödinger equation, and normal fluid, as described by classical fluid dynamics. We will examine the role of the nonlinear Schrödinger equation in this two-fluid model, and an experiment is proposed.

### Large-Scale Deformation of Giant Unilamellar Vesicles

#### Alex Steinkamp, Harvey Mudd College

Advisor: Associate Professor Wolfgang Losert

Vesicles are self-assembled spherical membranes formed from lipids. Not only are giant vesicles (10-100 micron diameter) good experimental tools for mimicking the dynamics of cell membranes, smaller vesicles also have many potential medical applications as drug-delivery devices. An understanding of the shape changes of vesicles is the first step towards these applications. Using holographic laser tweezers and phase contrast microscopy, giant unilamellar vesicles (GUVs) are stretched from spherical shapes into ellipsoids. Using previously untested theory involving fluid flow along with flicker spectroscopy, an analysis of the relaxation rate and thermal fluctuations allows for the extraction of the viscosity and bending rigidity of the lipid membrane.

### Bright and Dark Field Imaging of Plasmon Resonance in Nanoparticles

#### Armand Rundquist, Utah State University

Advisor: Associate Professor Edo Waks

Despite the fact that many types of nanoparticles are too small to be effectively imaged by an optical microscope, they still exhibit important interactions with light. In particular, charge carriers in the conduction band can cause intense scattering of incident light at different wavelengths, a phenomenon known as surface plasmon resonance. In this experiment, we explore methods of examining the plasmon resonance of single nanoparticles optically and spectroscopically. Ultimately, such a technique can be employed in detecting the plasmon resonance of quantum dots (semiconductor nanocrystals small enough to have quantized energy states), providing useful information about their charge carrier density in an excited state.

### Rejuvenation of a Depreciated Photocathode

(Poster)

#### Jessica Leung, Rensselaer Polytechnic Institute

Advisor: Professor Patrick O'Shea

Photocathodes are the preferred source of electron bunches for high-power free electron lasers. A photocathode emits electrons in response to incident light through the photoelectric effect. Photocathodes are characterized in part by the number of electrons emitted per incident photon: the quantum efficiency (QE). Optimal QE is maintained by using a clean and low-pressure vacuum chamber. Experiments were performed to find the minimum cleaning time for a cesiated metal cathode to characterize contamination of the cathode with O2, CO2, and N2O (decreasing QE), and to reapply a fresh layer of cesium, showing successful rejuvenation of QE.

### Flow Birefringence of Aqueous Polyacrylamide Solutions

#### Auralee Morin, Rensselaer Polytechnic Institute

Advisor: Professor Daniel Lathrop

Many polymeric liquids exhibit changes in their optical properties when exposed to shear stress due to the anisotropy introduced as the polymer chains become aligned with the direction of flow. One such property is the birefringence of the sample, in which the index of refraction along the axis of anisotropy varies from that of the axis orthogonal to it. Flow birefringence data have been used in a wide variety of applications, most importantly stress field distribution analysis, microstructural analysis, and model testing for polymer behavior as well as testing the limits of tools in rheology such as the stress-optical rule. These kinds of analyses have been conducted on a variety of types of fluids, including polymer melts, polymer solutions, and colloidal suspensions. Determining how these fluids behave under shear stress is vitally important in industry, where, for example, a small change in viscosity could potentially disrupt the optimal performance of the system. Consequently, studies of this sort are valuable to both pure rheology and applied science and engineering, and the multifaceted nature of much of the previous work reflects this. In my study, I will be subjecting polyacrylamide solutions to oscillatory shear stress and tracking the resulting changes in birefringence (if any). A high-speed camera will be used to collect these data, as this will allow analysis to be conducted via overall intensity changes and also possibly via fringe pattern changes (isoclinics/isochromatics) if the sample is strongly birefringent in the region where the stress-optical rule still applies. At present, the optical setup is in the process of being constructed.

### Comparison of Driven Surface Waves in Three Environments

#### Mariya Dryga, Emory University

Advisor: Professor Daniel Lathrop

Free surface waves in glycerin-water can be produced by a sinusoidal vertical acceleration of the container. Such a setup can produce periodic surface waves, quasiperiodic waves, or jets. Less is known about the properties of these states when the solution is below a fluid other than air. At the resonance frequency of type m = 0 surface waves, we compare the states exhibited when the fluid above is air, oil, or vacuum.

### Off-Axis Vortex Ring Collisions

(Poster)

#### Brian Vlastakis, North Carolina State University

Advisor: Professor Daniel Lathrop

Vortex dynamics are an important part of fluid mechanics. Understanding vortices are crucial towards advancements in aerodynamics and chaotic motion. Axisymmetric propagating vortex rings are of special interest because they are self-maintaining and allow for the transfer of mass a large distance before diffusing. This experiment allows for the observation of near-miss vortex ring interactions. The setup includes two vortex ring generators which will produce vortex rings with parallel or antiparallel trajectories. These trajectories will be close enough to allow vortex ring interaction while still avoiding a direct collision of vortex ring cores. Tests will include varying ring velocities as well as ring separation. I hope to observe and analyze scattering of these produced vortices as well as changes in the rings’ geometry. I also predict that oscillations will form in the vortex rings due to these interactions. The vortices may also exhibit turbulent behavior from these tests. Data from this experiment will provide additional knowledge on the complex nature of vortex rings and their interactions.