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Journal of Directed Energy
Volume 3, Number 1 Fall 2008

The papers listed below constitute Volume 3, 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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Effect of Natural Frequencies on Stresses in Impulsively Loaded Pressurized Thin-Walled Cylinders
George W. Sutton, SPARTA, Inc.; Robert Krech and William T. Laughlin, Physical Sciences, Inc.

Since the invention of lasers in 1960, there has been interest in their use for missile defense. Present interest is in their use for phase-boost intercepts that prevent the warhead from reaching its target while also preventing the deployment of penetration aids and/or submunitions. Whereas present programs are based on continuous-wave lasers, there is interest in powerful pulsed lasers that could burst a booster in a single pulse. Previous two-dimentional analyses have yielded large differences in results. To resolve this issue, vibration measurements were made on a pressurized steel tank. Experimentally measured vibration frequencies and pressure dependence agree with some of the previous researchers' results. Three-dimentional vibration modes were investigated theoretically and applied to a realistic solid-propellant-pressurized booster. Several megajoules of laser pulse energy are required to damage it, in agreement with those same previous researchers' results.
KEYWORDS: Booster, Composite, Damage, Laser, Pulsed
PAGES 1-14

Large-Spot Material Interactions with a High-Power Solid-State Laser Beam
C. D. Boley, S. N. Fochs, and A. M. Rubenchik, Lawrence Livermore National Laboratory

We study the material interactions produced by the beam of a 25-kW solid-state laser in experiments characterized by relatively large spot sizes (~3 cm) and the presence of airflow. The targets are iron or aluminum slabs of thickness 1 cm. In the experiments with iron, we show that combustion plays an important role in heating the material. In the experiments with aluminum, we observe a sharp transition from no melting to complete melt-through as the irradiance on target increases. A layer of paint greatly reduces the requirements for melt-through. We explain these effects and incorporate them into an overall computational model.
KEYWORDS: Airflow, Combustion, High average power, Large spot size, Solid-state laser
PAGES 15-24

Kalman Estimation of Anisoplanatic Zernike Tilt
Mindaugas Dagys, Zilvinas Kancleris, and Rimantas Simniskis, Semiconductor Physics Institute

A resistive sensor (RS) devoted to the measurement of a high-power microwave (HPM) pulse is presented. The performance of the RS is based on the electron-heating effect in semiconductors. It can measure HPM pulses directly; it is resistant to large power overloads and demonstrates very good long-term stability. The RS can produce an output signal of the order of a few tens of volts without any amplification circuit, and it can measure nanosecond-duration HPM pulses. Different types of the RS developed, manufactured, and tested in our laboratory are presented. They are the cross-waveguide RS for an intermediate pulse power level, the waveguide-type RS with diaphragm for HPM pulse measurements, the coaxial-type RS for the measurement of microwave pulses in a wide frequency range, and the RS for a millimeter-wave region.
KEYWORDS: Electron heating, High power microwave pulse measurement, n-Si, Sensors
PAGES 25-37

Studies of Vircator Operation at FOI: Electrode Material Erosion Studies
Sten E Nyholm, Mose Akyuz, Patrik Appelgren, Mattias Elfsberg, Tomas Hurtig, Anders Larsson, and Cecilia Möller, Swedish Defence Research Agency (FOI)

The high-power microwave (HPM) pulse generation process in a narrowband source is studied in single-pulse reflex triode experiments an repetitive axial vircator experiments. Electrode erosion is identified as an important limiting phenomenon for efficient durable HPM generation. The stability of the pulse generation mechanism in a burst of ten pulses at 10 Hz in an axial vircator is examined. Different electrode material combinations display markedly different behaviors with respect to pulse amplitude, dominating frequency, pulse shape, and electrode erosion. Graphite emitters have proven to generate comparatively stable pulses, in a single burst and over several bursts. The vircator impedance during microwave emission is also more stable with graphite as emitter material than with velvet. These observations can be utilized to improve the operating performance of vircators and similar HPM sources.
KEYWORDS: Electrode material erosion, High-power microwave, Vircator
PAGES 39-53

Development and Verification of a Narrowband High-Power Microwave Test Asset
D. Elkins and J. Fortinberry, Redstone Technical Test Center

Many commercial and military systems are complex networks of electronic monitoring and control subsystems, including vehicles, aircraft, and even the modern power grid. Coupled with the explosion of wireless devices and the potential for front- and backdoor radio frequency (RF) coupling into these modern systems, susceptibility to high-intensity RF environments generated by high-power microwave (HPM) sources and weapons is becoming a focused concern for Department of Defense Electromagnetic Environmental Effects testers and evaluators. To better characterize the HPM threat and its effects on modern Army electronic systems and to begin development of appropriate HPM test assets to emulate this threat, Redstone Technical Test Center has retrofitted its RADAR Environment Emulation System to generate high-intensity fields that would be common to narrowband and mesoband HPM sources. This HPM test source has been utilized to evaluate the effects of RF on various types of electronic components to include computers, cell phones, and handheld radios. Results that detail variations in the test parameters and their related effects on the electronic subsystems to determine upset and failure levels are presented.
KEYWORDS: Directed energy, High-power microwave, Narrowband, Test and evaluation
PAGES 145-150

Advances in Cesium Dispenser Photocathodes: Modeling and Experiment
E. J. Montgomery, D. W. Feldman, P. G. O'Shea, Z. Pan, and N. Sennett, Institute for Research in Electronics and Applied Physics, University of Maryland; K. L. Jensen, Vacuum Electronics Branch, Naval Research Laboratory; N. A. Moody, Los Alamos National Laboratory

Photocathodes are critical to the design of electron sources in high-power free-electron lasers but must maintain operational readiness and reliability with a long lifetime despite requisite high current density (hence high quantum efficiency), possible drive laser heating , and vacuum contamination. We at the University of Maryland have already demonstrated extended lifetime of cesiated metal photocathodes via the application of the dispenser cathode concept, a mature thermionic cathode technology, as part of our ongoing effort to develop the controlled porosity dispenser photocathode (CPD). This effort is now being extended to high-quantum-efficiency semiconductor coatings. The most efficient semiconductor coatings, notably those responsive to visible wavelenths (e.g., alkali antimonides), are prone to cesium loss in harsh operating environments; the dispenser concept promises in situ rejuvenation of cesiated surface layers by gently heating the cathode and allowing cesium to diffuse controllably to the surface through a porous substrate from a subsurface reservoir. Photocathode lifetime and robustness can be significantly enhanced. Essential to the advancement of the high-quantum-efficiency semiconductor, CPD is a comprehensive understanding of cesium's behavior. We here discuss the use of cesium in dispenser photocathodes in three photoemission topics: lower temperature operation of a modified cesium dispenser, development of a model for the diffusion of cesium on the surface of such a dispenser, and fabrication of cesium-based semiconductor coatings on the dispenser surface (cesium antimonide; Cs3Sb) for increased quantum efficiency.
KEYWORDS: Alkali antimonides, Cesium, Diffusion, Dispenser photocathode, Free-electron laser
PAGES 66-79

Hybrid Electric Oxygen-Iodine Laser Performance Enhancements and Measurements
G. F. Benavides, A. D. Palla, D. L. Carroll, J. T. Verdeyen, D. M. King, J. K. Laystrom-Woodard, and T. H. Field, CU Aerospace; J. W. Zimmerman, B. S. Woodard, and W. C. Solomon, University of Illinois at Urbana-Champaign

Recent experiments have led to improvements in the hybrid electric oxygen-iodine laser (ElectricOIL) system that significantly increased the discharge performance, supersonic cavity gain, and laser power output. The continuous-wave laser operating on the I(2P1/2) ? I(2P3/2) transition of atomic iodine 1,315 nm was pumped by the production of O2(a1?) in a radio frequency (RF) discharge in an O2/He/NO gas mixture. Results with both molecular iodine injection and partially predissociated iodine are presented. Flow temperatures were maintained at reasonable levels even with RF powers of around 2 kW. More than 30% of the discharge power is now being coupled into the active oxygen gas flow. A gain of 0.10% cm-1 and a laser power of 6.2 W are reported. Modeling with the BLAZE-IV model is in good agreement with data.
KEYWORDS: DOIL, ElectricOIL, Electric oxygen-iodine laser, EOIL, Singlet delta oxygen
PAGES 80-96

Volume 3, Number 1, Journal of Directed Energy

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