TREND Presentations

Synchronization of Chaotically Oscillating TWTs (Presentation)

Michelle Adan, Stetson University, DeLand, FL
(Advisor: Dr. J. Rodgers)

Increased dependence upon TWTs for communications systems has also stimulated a demand for higher spectral efficiency. This efficiency is possible with chaotic signaling, which is also more robust with respect to noise than linear modulation. Existing systems utilize TWT operation in the linear regime. However, close to saturation levels the signal response shifts from the linear regime into nonlinearity, and then chaos. Given a system of two coupled TWTs, the receiving TWT can synchronize with the chaotic transmitter to amplify the signal with little distortion. We will reproduce such a system with two TWTs of identical central frequency and maximum power to determine how strong the coupling between the two tubes is necessary for synchronization.

Characterization of Cesiated Tungsten Dispenser Photocathodes (Presentation

Anne Balter, Oglethorpe University, Atlanta, GA
(Advisors: Prof. P. O'Shea and N. Moody)

Photocathodes are an emerging source of high quality electron beams for accelerator-based applications such as short wavelength free-electron lasers. Photocathodes produce high quality electron beams by using a drive laser to switch the beam on and off at time scales as short as several picoseconds. Currently, the problem with photocathodes is that they are either robust and efficienct, but not both. This research explores critical aspects of photocathodes to enable the fabrication of an efficient and robust photocathode. It is well known that depositing a near-monolayer of cesium on a metal photocathode surface reduces the work function and therefore increases the quantum efficiency (QER). For this to occur, the photocathode surface must be atomically clean; this can be optimally achieved with ion beam cleaning. A beam of energetic argon ions was shown to double the maximum attainable QE for cesiated tungsten, but little is known about the effects of the cleaning on surface structure. The ion beam cleaning procedure used in this research has been optimized and its effects on the surface of a sintered tungsten cathode were explored. Robustness in cesiated-metal photocathodes is limited because the submonolayer coating of cesium is quickly degraded, due to natural desorption, ion back-bombardment, and contamination. A novel solution has been proposed and demonstrated at the University of Maryland: a low-temperature dispenser photocathode which allows rejuvenation of the cesium coating on a sintered tunsten surface. Tests were done on this prototype cathode to determine its operating parameters and overall utility. Each part of this research has contributed to a deeper understanding of photocathode theory and the long-term goal of fabricating an efficient and robust photocathode.

Gate Network Dynamics (Presentation)

Amanda Fournier, University of Southern California, Los Angeles, CA
(Advisor: Prof. D. Lathrop)

Aside from their pervasive use in computing, networked logic gates have been used as simple models for a number of more complicated systems, including cellular chemical reaction networks and genetic expression. Theoretical work on logic gates has proven that for ideal logic gates, which have no time delays and update synchronously, these networks' behavior will always be periodic. In-lab observations, however, indicated that simple networks of logic gates exhibit complex and sometimes chaotic behavior that varied by, among other things, supply voltage and by initial conditions. This project experimentally examined the behavior of these networks, the bifurcations between different behaviors, and the factors that cause and influence those behaviors. The knowledge gained may afterwards be useful to electronic engineers who use logic gates and to researchers who consider them models of biological systems.

Doppler Detection in Chaotic Microwave Signals for Improved Radar Resolution (Presentation)

Kristina Gaff, Dickenson College, Carlisle, PA
(Advisor: Dr. J. Rodgers)

This project seeks to experimentally extract Doppler information from a chaotic signal for radar applications. As opposed to conventional radar systems, the wideband nature of a chaotic signal allows for unambiguous resolution of the range and velocity of objects while retaining a strong immunity to interference and detection. Generated using a traveling wave tube, a chaotic signal is transmitted, phase modulated and returned, simulating the reflection off a moving object. Analysis of the transmitted and phase-shifted signals using the wideband ambiguity function provides time and frequency correlation for various states of the chaotic oscillator. These plots reveal the distinct Doppler signature and result in a best-case Doppler signal-to-noise ratio of 35 dB.

3D Imaging of Wet Granular Matter (Presentation)

Leonard Goff, University of Maryland, College Park, MD
(Advisor: Prof. W. Losert)

Knowledge of the internal particle arrangement of a granular material is important for understanding its internal dynamics. By using a laser sheet scanning method, the coordinates of individual grains can be determined in 3D. An improved method of extracting this information from a set of 3D data has been developed by adapting a 2D feature extraction algorithm to work in 3D. This method is used to study the evolution of a granular sample's internal structure as a rod is forced into it. The average radial particle displacement exhibits an exponential decay when plotted against radial distance from the rod, and the average displacement is maximum just under the tip of the rod.

Recovering Universal Properties of Quantum Chaotic Cavities in the Presence of Significant Direct Processes (Presentation)

Michael Johnson, University of Southern California, Los Angeles, CA
(Advisors: Profs. E. Ott and T. Antonsen)

It is conjectured that many statistical properties of chaotic systems are given by particular matrix ensembles. The Random Coupling Model is an extension of this conjecture to a description of the scattering and impedance matrices for quantum chaotic cavities connected to transmission lines. The model has been numerically and experimentally verified in the case where the ports are far enough apart that the direct coupling between them is not dominant. We numerically demonstrate success of the model when direct processes between the ports are significant and examine non-universal character introduced by ignoring such cross-talk.

Noise-Enhanced Stability of Attractor Ruins in Semiconductor Laser System with Optical Feedback (Presentation)

Nathaniel Karst, Olin College, Needham, MA
(Advisors: W. Ray and Prof. R. Roy)

We experimentally and numerically investigate the effect of noise on the intermittency between low-frequency fluctuations (LFFs) and coherence collapse (CC) in a semiconductor laser with time-delayed optical feedback. We observe a clear algebraic scaling of average residence time in the quiescent LFF state for increasing injection current. Increasing the noise strength included in our numerical simulation, we observe a translation in critical pump current at which bursting from LFF to CC begins. We conclude that noise acts to stabilize the LFF regime and briefly discuss the scaling of the critical pump current as a function of noise strength.

Avalanches in Foam Collapse (Presentation)

Andrew Rhines, Reed College, Portland, OR
(Advisor: Prof. Daniel Lathrop)

In a quasi-stable foam, long periods of static behavior are punctuated by brief (but violent) cascades of collapsing bubbles. By recording the audio produced in a steady-state foam, a statistical interpretation of avalanching collapse events is undertaken. The event spacing distribution comes in two separate power-law regimes for low and high burst frequencies. Power-law scaling also observed in avalanche size is substantiated by previous studies in non-destructive rearrangement processes. High speed video analysis of shockwave propagation within the foam further confirms this, in that direct rupture by shockwaves or ejected droplets is uncommon. The primary mechanism for initiation of avalanches, then, is the formation of additional film stresses due to topological rearrangement.

Droplet Pinch-Off in Liquid Sodium (Presentation)

Christine Vadovszki, Montclair State University, Upper Montclair, NJ
(Advisor: Prof. Daniel Lathrop)

Droplet separation in liquids results in very fine connecting capillaries moments before pinch-off. This thin filament of fluid contains a connecting neck that shrinks to zero diameter at a finite time singularity. Instead of using an optical technique, our goal is to study this portion of the pinch-off by running an electric current vertically through the droplet neck, yielding a resistance proportional to the minimum cross-section area of the pinch-off. To create this electrically connected pinch-off, we slowly draw apart two sodium-wetted electrodes until the sodium bridge between the two electrodes pinches off. A similar study carried out by a research group at UC Irvine used mercury as the liquid medium. They observed minimum filament diameters of 2.7 nanometers. By using less resistive sodium as our liquid medium, we hope to reduce the RC time constant of the neck diameter measurement circuit and thus improve resolution at the point of pinch-off. This experiment could thus yield data to smaller length scales than the previous study. A previous attempt at this experiment using sodium was not able to reach these reduced scales due to inductive effects. By this time working in an inert environment and reworking the electrical and mechanical design of our apparatus, we have attained the reduced RC time constant and improved measurements.

Bidirectional Chaos Communication in Ring Laser Systems (Presentation)

Brian Zhou, California Institute of Technology, Pasadena, CA
(Advisor: Prof. R. Roy)

We show that two mutually delay-coupled Ikeda ring oscillators (IR) synchronize isochronally when both are symmetrically driven by a third IR. This isochronal synchrony, unstable without the common drive signal, makes possible simultaneous, bidirectional communication between the two mutually coupled IRs. Our scheme represents the generalized synchrony of a spatially distributed system to a single drive signal and thus may have potential application to multiple-user communication networks based on optical chaos.