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DIRECTED ENERGY
PROFESSIONAL SOCIETY
Journal of Directed Energy |
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Volume 1, Number 4 |
Winter 2006 | |
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The papers listed below constitute Volume 1, Number 4 of the Journal of Directed Energy.
Print copies of this, and other issues of the Journal of Directed Energy are available through the
DEPS online store.
Enjoy access to the complete technical paper(s) through links in the paper titles.
Eight-Watt Coherently Phased Four-Element Fiber Array
(350 KB)
J. Anderegg and others, Northrop Grumman Space and Technology
A four-element fiber array has been constructed to yield 8 W of coherently phased, linearly polarized light energy in a single far-field spot.
Each element consists of a 2-W, single-mode fiber-amplifier chain. Phase control of each element is achieved with a lithium-niobate phase
modulator. A master laser provides a linearly polarized, narrow-linewidth signal that is split into five channels. Four channels are
individually amplified by using polarization maintaining fiber power amplifiers. Frequency broadening of the signal is necessary to avoid
stimulated Brillouin scattering. The fifth channel is used as a reference arm. It is frequency shifted and then combined interferometrically
with a portion of each channel's signal. Detectors sense the heterodyne modulation signal, and an electronics circuit measures the relative
phase for each channel. Compensating adjustments are then made to each channel's phase modulator. The stability of the optical train is an
essential contributor to its success. A state-of-the-art interferometer was built with mountless optics. A lens array was constructed by
using nanopositioning tolerances, where each lens was individually aligned to its respective fiber to collimate its output and point it at a
common far-field spot. This system proved to be highly robust and handled any acoustic perturbations.
KEYWORDS: Fiber amplifier, Phase control, Phased array
PAGES 275-281
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Analysis of Thermal Effects Association with High-Energy Radiation in Reducing Scintillation of a Coaxial Beam
(350 KB)
L.C. Andrews, R.L. Phillips, H. Izadpanah, and M. Mokhtari; Florida Space Institute and other affiliations
The purpose of this theoretical study is to examine atmospheric effects on an optical propagation path using two coaxial beams, one a high-energy
beam intended to modify the atmospheric channel for the other optical beam. The high-energy beam created local heating of the atmosphere that
tends to reduce the effective structure parameter of the refractive index, thereby reducing some of the deleterious effects of
turbulence-induced scintillation. For the high-energy beam we considered millimeter waves (63 and 183 GHz) and an infrared wave (10.6 µm)
propagating through extended atmospheric turbulence and a thin random-phase screen. We found that the use of a coaxial high-power millimeter
beam with an optical beam can reduce the effective refractive-index structure parameter C2n for the low-power optical beam in the atmospheric
channel, but mostly for the initial portion of the propagation path. Greater reduction in C2n can be realized with the use of an infrared wave
(10.6 µm) in place of a milimeter wave, because the absorption is greater than that of the millimeter beam. Overall, the most favorable
atmospheric channel for the use of such coaxial beams appears to be one in which the atmospheric effects are limited primarily to a thin
turbulent layer (phase screen) between the transmitter and receiver.
KEYWORDS: Atmospheric optics, High-energy beam, Scintillation
PAGES 283-292
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Thermal Energy Storage for Solid-State Laser Weapon Systems
(1,250 KB)
C.B. Baxi and T. Knowles, General Atomics and other affiliations
The challenge to high-power solid-state laser (SSL) heat rejection is the very high heat loads during lasing. A thermal energy storage (TES) system
capable of rapid heat absorption followed by heat rejection over a longer period can reduce the thermal management system size by over a
factor of 10. The most compact system approaches to thermal storage use phase change materials (PCMs) such as ice or paraffin. Because of use
of latent heat of fusion of these materials, a large amount of energy can be stored in a small mass of the storage material. The major
drawback with this approach is the relatively low thermal conductivity of PCMs (ice at 1.9 and paraffin at 0.15 W/m-K compared to copper at
386 W/m-K). A new PCM system design approach is proposed that uses a "carbon fiber composite" material manufactured by Energy Science
Laboratories Inc. (ESLI), which dramatically increases the effective thermal conductivity of the PCM. The material consists of high
conductivity carbon fibers, 2-5 mm long, bonded to a thin, flexible metal substrate. Water or paraffin is infiltrated into the carbon fiber
structure, which can be constructed as circular tubes or flat plates. A high-heat-flux low-weight (HHF-LW) thermal storage system would
consist of a number of plates in a heat exchanger configuration. The fluid used to cool the laser components is on one side of the heat
transfer surface, and the recharge fluid from the chiller system is on the other side. The PCM material is "sandwiched" in between. The
achievable value of keff (effective thermal conductivity) for the proposed HHF-LW depends on the density of fibers and the type of fiber used.
With carbon fibers of thermal conductivity 1,100 W/m-K, a keff of ~100 W/m-K could reasonably be achieved. Preliminary analysis indicates that
a keff of 60-80 W/m-K results in an optimum weight reduction in a TES system. A compact high-power thermal storage system using this
technology can be fabricated to reduce the weight of the heat refection system for SSHCL (solid-state heat capacity laser) by 90%. The
estimated weight of the TES for 100 kW SSHCL is 180 kg
KEYWORDS: Lasers, Thermal storage
PAGES 293-308
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Multipass, Multibench Beam Breakup Suppression in Two-Pass Recirculating Accelerators
(250 KB)
E. Pozdeyev and others, Thomas Jefferson National Accelerator Facility and other affiliations
At sufficiently high currents beam breakup (BBU) occurs in all accelerators. In recirculating accelerators, such as the energy recovery linacs used
for high-power free-electron lasers (FELs), the maximum current has historically been limited by multipass, multibunch BBU, a form that occurs
when the electron beam interacts with the high-order modes (HOMs) of an accelerating cavity on one pass and then again on the second pass.
This effect is of particular concern in the designs of modern high-average-current energy recovery accelerators utilizing superconducting
technology. In such two-pass machines rotation of the betatron planes by 90 deg, first proposed by Smith and Rand in 1980, is expected to
significantly increase the threshold current of the multibunch BBU. The effect of rotation on the threshold current of the Jefferson
Laboratory FEL Upgrade is being studied experimentally and with a newly developed four-dimensional tracking code. Several optical rotator
schemes based on quadrupoles and solenoids are being evaluated for their performance in terms of the instability threshold current increase
and their effect on FEL optics. Results of experiments and simulations are presented.
KEYWORDS: BBU, Energy recovery, ERL, Threshold current
PAGES 309-316
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The Alpha Program
(650 KB)
M. Wacks, L. Ryan, D. Johannsen, and R. Geopfarth; Northrop Grumman space Technology and other affiliations
In the late 1970s DARPA initiated a program to develop the technologies necessary to fly a space-based laser weapon. The encompassing effort was
know as Triad, and the three programs were the Alpha Program to develop the laser source, the Large Optics Demonstration Experiment (LODE) to
develop the beam control, and the Talon Gold Program to develop the precision pointing capability. This paper provides an overview of the
Alpha Program from its antecedents through its final, most successful, tests. Alpha was intended to develop a hydrogen fluoride (HF) laser to
megawatt-class levels in a configuration compatible with space operation. We begin with a brief history of the development of HF [and
deuterium floride(DF)] lasers up through the demonstration of megawatt-class performance with MIRACL, a DF laser designed for sea-level
operation. We then cover the program goals, design, and hardware of the Alpha Program. The early tests led to some hardware modifications,
following which Alpha achieved megawatt-class performance. We then discuss the diagnostics suite used with Alpha, which recorded the data
that fed into a data review task under the Alpha Laser Optimization program. The results of this review allowed for the refinement of Alpha
test operations and allowed the program to culminate with a series of highly successful tests. Although Alpha is now being dismantled, and no
direct descendant is planned, what we have learned from the Alpha program will be very useful to future high-energy laser programs.
KEYWORDS: Alpha laser, HF lasers, High-energy lasers, Missile defense
PAGES 317-331
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High-Energy Hydrogen Fluoride/Deuterium Fluoride Laser Beam Correction: History and Issues
(1,250 KB)
D. Johannsen, L. Ryan, and J. Albertine; The Aerospace Corporation and other affiliations
We begin with a discussion of the history of attempts to correct combustion-driven high-energy laser (HEL)beams; these attempts have not tended to
be as successful as hoped. After discussing beam control tests on MIRACL, a megawatt-class deuterium fluoride (DF) laser, we consider the
conclusions of a Beam Control Maturity Assessment Panel (BCMAP)--convened by the now-terminated Space Based Laser Integrated Flight Experiment
program--concerning the history of HEL beam control. We then discuss the results of the Alpha LAMP Integration (ALI) tests and what was and
was not learned from those tests. Next we evaluate what was known from the wavefront sensor available as part of the Alpha Laser diagnostics
suite and from high-frequency subaperture intensity data available from a mode beating experiment performed with Alpha. Taken together, these
sources defined the need for a high-bandwidth measurement of Alpha subaperture intensity and tilt behavior, the defining requirements for the
MHz Intensity and Tilt Sensor (MITS).
KEYWORDS: Deuterium fluoride laser, High-energy laser, Hydrogen fluoride laser, Laser beam control, Laser beam correction
PAGES 333-345
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The HELEEOS Atmospheric Effects Package: A Probabilistic Method for Evaluating Uncertainty in Low-Altitude, High-Energy Laser Effectiveness
(1,350 KB)
S.T. Fiorino and others, Air Force Institute of Technology and other affiliations
The Air Force Institute of Technology's (AFIT's) Center for Directed Energy, sponsored by the High Energy Laser Joint Technology Office(JTO), has
developed the High Energy Laser End-to-End Operational Simulation (HELEEOS) parametric one-on-one-engagement-level model. HELEEOS incorporates
scaling laws tied to respected wave optics codes and all significant degradation effects to include thermal blooming due to molecular and
aerosol absorption, scattering extinction, and optical turbulence. The HELEEOS model enables the evaluation of uncertainty in low-altitude,
high-energy laser (HEL) engagement due to all major clear-air atmospheric effects. Atmospheric parameters investigated include profiles of
temperature, pressure, water vapor content, and optical turbulence as they relate to layer extinction coefficient magnitude. Worldwide
seasonal, diurnal, and geographical spatial-temporal variability in these parameters is organized into probability density function (PDF)
databases using a variety of recently available resources to include the Extreme and Percentile Environmental Reference Tables (ExPERT), the
Master Database for Optical Turbulence Research in Support of the Airborne Laser, the Global Aerosol Data Set (GADS), and the Directed Energy
Environmental Simulation Tool (DEEST) in conjunction with Air Force Weather Agency MM5 numerical weather forecasting data. Updated ExPERT
mapping software allows the HELEEOS operator to choose from specific site or regional surface and upper air data to characterize correlated
molecular absorption, aerosol absorption and scattering, and optical turbulence by percentile. The PDF nature of the HELEOS atmospheric
effects package enables realistic probabilistic outcome analyses that permit an estimation of the level of uncertainty in the calculated
probability of kill (Pk).
KEYWORDS: Aerosol extinction, Correlated atmosphere, HEL propagation, Percentile, Probabilistic
PAGES 347-360
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Journal of Directed Energy, Volume 1, Number 4
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