Journal of Directed Energy
Volume 1, Number 3 Fall 2005

The papers listed below constitute Volume 1, Number 3 of the Journal of Directed Energy. Print copies of issues of the Journal of Directed Energy are available through the online store.

Atmospheric Considerations in Engagement-Level Simulations of Tactical High-Energy Laser Systems (1,400 KB)
George Jumper, John Roadcap, Sara Adair, Guy Seeley, and Gerard Fairley; Air Force Research Laboratory and other affiliations

The atmosphere can have a tremendous impact on tactical high-energy laser (HEL) systems. The variable nature of the atmosphere can produce highly variable performance. System designers need accurate environmental models to optimize the design of these systems. This paper reviews the atmospheric impact of various atmospheric phenomena on laser performance and the measurement and modeling efforts of the Air Force Research Laboratory. Two of these phenomena, optical turbulence and cloud obscuration, have been included in an engagement-level model, DEEST (Directed Energy Environmental Simulation Tool), to determine the optical performance of a tactical laser system. DEEST is described, and various aspects of the model are discussed for the benefit of those who will be modeling tactical HEL systems.
KEYWORDS: Atmospheric effects simulation, Atmospheric transmission, Optical turbulence
PAGES 183-201

Optical Diagnostics for Pulsed Laser Ablation (600 KB)
Carl Druffner and others, Air Force Institute of Technology and other affiliations

The ablation of target material by pulsed laser deposition (PLD) techniques at high fluence produces highly energetic, ionized plumes with average kinetic energies of more than 50 eV. Optical diagnostics have been developed to monitor the spatial and temporal evolution of the highly nonequilibrium distributions of electronic, vibrational, and translational degrees of freedom. For example, emission time-of-flight profiles have been obtained by using gated imagery to further the development of process control during the pulsed laser deposition of the high-temperature superconductor YBa2Cu3O7-x. An intensified charge-coupled device (CCD) array was used to obtain a sequence of plume images at 10-ns temporal resolution and 0.2-mm spatial resolution. The resulting streaming speeds of (6-9) x 105 cm/s and temperatures of 20,000-600,000 K are characterized at selected positions across the full plume. Significant deviations from the forward-directed Maxwellian speed distributions are observed. Ablation plumes are often optically thick, and our recent emission spectroscopy experiments indicate sufficient radiation trapping to observe highly Planckian behavior from neutral and ionized atomic emission lines. Finally, the angular distribution of ejected material is highly forward peaked and fully characterized by high-speed imagery.
KEYWORDS: High-temperature superconductors, PLD, Pulsed laser deposition, YBCO
PAGES 203-218

Electric Discharge Pumped Oxygen-Iodine Laser Kinetics (400 KB)
Matthew Lange, Greg Pitz, Brian Smith, and Glen Perram; Air Force Institute of Technology

Chemical lasers offer the highest powers necessary for many weapons applications but require significant logistical support in the delivery of specialized fuels to the battlefield. The goal of a current multiuniversity research program is to demonstrate an oxygen-iodine laser with electrical discharge production of singlet oxygen. Typically, oxygen discharges are limited to about 15% yield for singlet oxygen. The electron excitation cross sections as a function of the ratio of the electric field vs. the number density of particles (E/N) are well established. However, the kinetics for electron and singlet oxygen interactions is considerably more difficult to study. Optical diagnostics for O2(a, b) and O have been applied to a double-microwave discharge flow tube. By examining the difference in singlet oxygen kinetics between the two discharges in series, considerable information regarding the excited-state, excited-state interactions is obtained. Under certain discharge conditions, the O2(a) concentration significantly increases outside of the discharge, even after thermal effects are accounted for. Unfortunately, the sensitivity of singlet oxygen yields from a microwave discharge to the initial excited oxygen inlet conditions is low, suggesting a kinetic limitation to generator performance.
KEYWORDS: Oxygen discharge kinetics, Oxygen iodine laser, Rotational temperature, Singlet oxygen, Spectroscopy
PAGES 219-232

Progress Toward an NCl3-Fueled Iodine Laser (200 KB)
William McDermott, Robert Coombe, Julanna Gilbert, Zane Lambert, and Mikaila Heldt; University of Denver Research Institute

The NCl-I laser has been demonstrated by using HN3 as a fuel. We discuss the possibility of constructing a NCl(a1?)-I transfer laser by using NCl3 as a fuel. Not only is NCl3 more stable but it also may eliminate the requirement for a combustor. We present kinetic modeling and the results of experiments designed to understand the basic physical processes in this system. Experiments have included determination of the time profile history of NCl(a), NCl (b), I, and chlorine recombination. The efficiency of production of chlorine atoms in a microwave discharge has been measured. We have also shown that the autodecomposition of NCl3 can be initiated by modest heating of the NCl3 flow, which supports our belief that a combustor will not be required. The NCl-I laser operates on gas species, eliminating the need for heterogeneous gas-liquid reactions such as used for the production of COIL chemical lasers. The lasing species is the same as in COIL, simplifying the scaling process since many optical, tracking, and propagation problems have been demonstrated in the ABL program. This work is supported by the JTO through the AFOSR-administered MRI program.
KEYWORDS: Chemical laser, NCl-I transfer laser, NCl3 reactions
PAGES 233-240

Measurement of Gain in a Supersonic, Combustion-Driven Generator for NCl(a1 Delta) (600 KB)
Gerald Manke II, Timothy Madden, Chris Cooper and Gordon Hager; Air Force Research Laboratory and other affiliations

The measurement of positive small signal gain on the 1.315-Ám spin orbit transition of atomic iodine following energy transfer from chemically generated NCl(a1?) is reported. Previous instances of gain produced by energy transfer from NCl(a1?) used dc discharges to generate F and Cl atoms; this report describes recent progress toward a true chemical laser device that uses a high-temperature chemical combustor and a supersonic reactor to generate NCl(a1?). These improvements represent a significant step toward the development and demonstration of a scalable All Gas-Phase Iodine Laser (AGIL) device.
KEYWORDS: AGIL, All gas iodine laser, Chemical lasers, COIL
PAGES 241-260

Investigation of Ultrafast Laser Ablation Using a Semiclassical Two-Temperature Model (400 KB)
J.K.Chen, J.E.Beraun, and D.Y. Tzou; Air Force Research Laboratory and other affiliations

A semiclassical two-temperature model is formulated, based on the Boltzmann transport equation, to solve thermal response of metals irradiated by an ultrashort-pulsed laser. In the simulation of laser material ablation, two competing mechanisms are considered: shock wave and phase explosion. Numerical analysis is performed for copper and gold films, respectively. It is shown that the present approach correlates well with experimental data over a wide range of laser fluences and pulse durations. It is also found that ultrashort-pulsed laser ablation could be caused by the generated shock wave for pulses of a few picoseconds or shorter and by the thermally induced phase explosion for the longer pulses.
KEYWORDS: Boltzmann transport equation, Hot-electron pressure, Phase explostion, Semiclassical two-temperature model, Shock wave, Ultra-short pulse laser ablation
PAGES 261-274

Journal of Directed Energy, Volume 1, Number 3

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