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Journal of Directed Energy
Volume 5, Number 1 Spring 2013

The papers listed below constitute Volume 5, Number 1 of the Journal of Directed Energy. Print copies of issues of the Journal of Directed Energy are available through the online store..
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Elemental Theory of a Relativistic Magnetron Operation: Dispersion Diagram
Andrey D. Andreev, Kyle J. Hendricks, Shawn Soh, Mikhail Fuks, and Edl Schamiloglu; Raytheon Missile Systems and other affiliations

The cold dispersion diagram (resonant frequencies) of the S-band and L-band sixcavity (A6) relativistic magnetrons are calculated assuming that (1) the A6 resonant system has either a solid or transparent cylindrical cathode inside and no output irises, and (2) the A6 resonant system with the solid cylindrical cathode is coupled (opened) via either one (the classical MIT-A61 magnetron) or three (the AFRL-A63 magnetron) output capacitive iris(es), situated at the back of the anode resonant cavity(ies), with the output waveguide(s). Results of the calculations show that when the solid cylindrical cathode is replaced by the transparent cylindrical cathode, the dispersion diagram of the closed (without output irises and waveguides) A6 resonant system is not significantly changed and approximates that of the A6 resonant system without a cathode, when the width of the interaction space (the distance between the cathode and the anode) is large enough. Otherwise, when the width of the interaction space is small enough, the dispersion diagram is significantly modified. Calculations of resonant frequencies of the open (with output irises and waveguides) AFRL-A63 resonant system with the solid cylindrical cathode show that its dispersion diagram corresponds to that of a rising-sun magnetron, where the frequency of the p mode is significantly lower than the frequency of the p mode of the closed symmetrical A6 resonant system.
KEYWORDS: Resonant cavity, Induced RF field, Mode pattern, Resonant frequencies, Dispersion diagram
PAGES 1-41

Progress in First Principles Modeling of HPM Effects
Larry D. Bacon, Jeffery T. Williams, Michael J. Walker, and Alan Mar; Sandia National Laboratories

The past few decades of research into radio-frequency directed energy (RFDE) have witnessed the development of analytical and computational tools for modeling highpower microwave (HPM) effects that are capable of nearly complete electrical and mechanical characterizations of entire systems. These developments include advances in numerical electromagnetics and semiconductor physics, multiphysics modeling, meshing and gridding tools, and sheer computing power. We have progressed from analyzing nearly canonical coupling problems to fully coupled, electromagnetic-electrothermaldevice physics models of small but realistic systems. At this juncture, we ask, How deterministic can we make our assessment of RFDE effects? We must be able to estimate, within acceptable bounds, the variability and repeatability of RFDE effects. Even with precise knowledge of the physical geometry of the target, significant variability and strong orientation dependence of RFDE effects remain due to the multiple ports of entry feeding the terminal pairs of interest. This paper explores our progress in predicting end-to-end effects from first principles, defined as follows: modeling as much of the physics as necessary to capture the significant effects in a specific problem. We focus on the derivation, implementation, and validation of an active Thevenin equivalent network approach (ATHENA) to solving the linear coupling and nonlinear circuit response selfconsistently and efficiently.
KEYWORDS: HPM effects prediction, Multiport equivalent circuit, Modeling
PAGES 42-50

Nonlinear Transmission Line Performance under Various Magnetic Bias Environments
J.-W. Braxton Bragg, James C. Dickens, and Andreas A. Neuber; Texas Tech University,

Nonlinear transmission lines (NLTLs) loaded with ferrimagnetic materials act as solidstate sources capable of generating subnanosecond rise-time pulses and megawatt-level microwave oscillations depending on the specific geometries, materials, and external bias fields. NLTLs stand apart from other high-power microwave (HPM) sources such as vacuum HPM tubes in the sense that they are able to operate at internally ambient or higher pressures, are cost effective, and occupy relatively small volumes. Microwave oscillations are formed through constructive interaction between an incident pulse and an external magnetic bias field causing damped gyromagnetic precession of the ferrites magnetic moments. The Landau-Lifshitz-Gilbert equation describes the magnetic moment dynamics, and evaluation of this equation reveals that the bias field greatly affects precessional magnitude and frequency. Pulse sharpening partly occurs due to the nonlinear permeability sharply approaching unity (initially of order 1000) with the application of a saturating pulse front. Further pulse sharpening is observed when an external magnetic field is applied, which indicates that the magnetization dynamics and flux reversal involved with precession also plays an important role. The shape and magnitude of the external bias field highly affects the performance of NLTLs, and several configurations are examined. The rise times and microwave generation are studied for a single line having a length of 1 m with ferrites having dimensions 3 mm X 6 mm (ID X OD) under varying bias magnitudes and lengths.
KEYWORDS: Damped gyromagnetic precession, Ferrimagnetic, Nonlinear transmission line
PAGES 51-57

Far-Field Laser Intensity Drop-Outs Caused by Turbulent Boundary Layers
Stanislav Gordeyev, Jacob Cress, and Eric Jumper; University of Notre Dame

Usually aero-optical effects are quantified in a time-averaged manner, such as timeaveraged spatial root-mean-square of optical path difference or time-averaged Strehl ratio (SR) on a target. However, for airborne free-space, laser-based communication systems, instantaneous SR should be studied as well. An attached transonic boundary layer, for example, provides a relatively high time-average SR; however, experimentally it was discovered that it has many sharp intensity drop-outs, which typically last for a millisecond or so. Left untreated, these drop-outs might lead to significant data loss, potentially slowing down or even disrupting airborne laser-based communications. This paper presents experimentally measured instantaneous near-field wavefront statistics due to laser transmission through subsonic boundary layers. The resulting far-field SR for various flow conditions and aperture sizes are also presented. Using scaling laws for boundary layers, a simple relation between flight conditions and the relative amount of time when the SR drops below a prescribed threshold is developed. The model leads to development of a method for predicting system performance for a free-space communication system. The method is discussed along with possible approaches to using it for designing and optimizing current and future laser-based communication systems. In addition, statistics of the instantaneous drop-outs and analysis of the relative intensity variations caused by boundary layers are presented and discussed.
Erratum: Footnote 7 should read: Wang, M., Mani A., and Gordeyev, S., Annu. Rev. Fluid Mech. 44, 299 (2012).
KEYWORDS: Boundary layer, Communication, Laser
PAGES 58-75

Simulation of Aero-Optics over Conformal and Flat Window Turrets
Michael D. White, Philip E. Morgan and Miguel R. Visbal; Ohio Aerospace Institute and other affiliations

This work is focused on conformal and flat window turrets in a Mach number range of 0.35 to 0.4 and Reynolds number based on turret diameter of approximately 2 million. The flow is solved using a hybrid Reynolds averaged Navier-Stokes/high-fidelity implicit large eddy simulation (hybrid RANS/LES) solution methodology. The optics are computedwith both integration of the optical path difference and high-order integration of the parabolic beam equations. Results of the simulation show that the conformal turret has much better optical qualities over a far greater range of look angles than the flat window turret. At backward-look angles, the flat window turret may have advantages due to smaller and more regular turbulent structures than the conformal turret. Qualitative agreement is seen in the optics between the simulations and experiment, despite differences between the actual tunnel environment and the simulation.
KEYWORDS: Aero-optics, High-order compact differences, Hybrid turbulence methods, Turrets
PAGES 76-92

Lasers Based on Highly Doped Lu2O3 Ceramics
Woohong Kim and others, Naval Research Laboratory and other affiliations

The performance of highly doped sesquioxide-based ceramics for high-energy, solid-state lasers is first discussed. We also report on powder synthesis and the effect of the powder quality and fabrication process on optical quality and lasing performance of lutetia (Lu2O3 ) ceramic. Various powder synthesis methods such as coprecipitation and flame spray pyrolysis, and postprocess techniques such as jet milling and ultrasonication are also discussed to provide an efficient route to obtain fine-grained ceramics that are desirable for scaling to high-power lasers.
KEYWORDS: Ceramic lasers, Sesquioxides, Power scaling, Fine-grain ceramics
PAGES 93-104

Volume 5, Number 1, Journal of Directed Energy

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