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DIRECTED
ENERGY
PROFESSIONAL
SOCIETY
Journal of Directed Energy
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Volume 5, Number 3 |
Fall 2014 |
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The papers listed below constitute Volume 5, Number 3 of the Journal of Directed Energy.
Print copies of this, and other issues of the Journal of Directed Energy are available through the
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DEPS members enjoy access to the complete technical paper(s) through links in the paper titles. Members should sign in to their online account and return to this page to access this additional content. | Join DEPS |
Performance of the Mark II Quarter-Wave SRF Photoinjector
John W. Lewellen, C. Wayne Bennett, John R. Harris and Richard L. Swent, Naval Postgraduate School;
Chase H. Boulware and Terry L. Grimm, Niowave, Inc.; Mark Curtin and Daniel Sox, Boeing; and Todd I. Smith, Stanford University
The Naval Postgraduate School and Niowave, Inc., in collaboration with The Boeing
Company, have undertaken a design-build-measure-refine approach in the development of
superconducting high-average-current electron beam sources. The Mark I, a super -
conducting radio-frequency (SRF) electron beam source, was based around a quarter-wave
RF structure. At the time of first operation, the Mark I represented the fastest time to first
beam of any SRF beam source in the world. The Mark II, an evolution of the original Mark
I design, went from design concept to first beam in less than 2 years and in the process
broke the record set by the Mark I. In addition to addressing issues found during Mark I
testing, the Mark II also incorporated a number of new features, such as a cavity frequency
tuner and field probe, which make it suitable as both a stand-alone test bed and beam source
for a higher-energy accelerator. Liquid helium was supplied to the Mark II via dewars in
its initial operation. More recently, the Mark II has also been supplied by a helium
refrigerator. This paper presents RF and electron beam measurements made on the Mark
II in both modes of operation.
KEYWORDS: Superconducting radio-frequency, Electron gun, Photoinjector, Cryoplant
PAGES 207-218
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The Application of SCOS for HPM Field Measurement
Bradley M. Whitaker, Jonathan R. Noren, Daniel T. Perry, Stephen M. Schultz, and Richard H. Selfridge,
Brigham Young University; Richard Forber and Wen C. Wang, IPITEK; and Jeffrey S. Schleher, SAIC
This paper presents a small, completely dielectric, directional sensor built on an optical
fiber platform. The sensor is intended to detect high power electric fields and functions up
to microwave frequencies. The paper also discusses a method of combining single-axis
sensing units to create a multiaxis sensor capable of measuring both the magnitude and
direction of an electric field. Through experimental tests with a high power microwave
(HPM) source, we confirmed the reliability and reproducibility of detecting large electric
fields with our sensor. In the tests, the sensor detected a 1.3-GHz, 45-kV/m electric field.
After applying post-processing filters, our sensor revealed a detected field with precision
comparable to a larger, nondielectric sensor. In addition, our sensor distinguished the vector
components of the electric field created by the HPM source.
KEYWORDS: Fiber optics, Electric field sensor, High power microwave, Dielectric sensor
PAGES 219-236
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MATILDA: A Military Laser-Range Safety Tool Based on Probabilistic Risk Assessment Techniques
Brian K. Flemming, SELEX ES Ltd.; Paul K. Kennedy and Daniel F. Huantes, JBSA; and Matthew D. Flower, UK Military Laser Safety Committee
Recently, the use of probabilistic risk assessment (PRA) techniques to perform laser safety
and hazard analysis for high output lasers in outdoor environments has become an
increasingly accepted alternative to standard risk analysis methods, based on maximum
permissible exposure (MPE) limits. Over the past 10 years, the United Kingdom Ministry
of Defence (MoD) and the United States Air Force Research Laboratory (AFRL) have
collaborated to develop a jointly owned, PRA-based, laser range safety tool, the Military
Advanced Technology Integrated Laser Hazard Assessment (MATILDA) system. The UK
MoD has been developing PRA-based, laser hazard analysis models for nearly four decades,
and using them to assess laser irradiation risks to unprotected persons from laser test and
training operations on UK military ranges. The AFRL wishes to develop PRA-based hazard
analysis models for outdoor high energy laser applications, and began the collaboration
to leverage the PRA modeling expertise of the UK. Initial MATILDA code development was
based on the PRA "partition" model developed to perform range safety clearances for the
UK Thermal Imaging Airborne Laser Designator (TIALD) system. MATILDA is the first
military software tool to contain a complete end-to-end laser PRA model, crafted for range
applications, and with generalized terrain modeling. In the future it will provide a starting
point for development of more advanced laser PRA models and tools.
KEYWORDS: Laser safety, Range safety, Laser hazard analysis, Probabilistic risk assessment
PAGES 237-259
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Removing Linewidth Limitations for Spectrally Combined Lasers
R. Andrew Motes, Schafer Corporation
There have always been challenges associated with spectral beam combining (SBC) of fiber
lasers. It is commonly believed that combined beam quality is highly dependent on laser
linewidth where larger linewidths produce lower beam quality or higher M2. And, because
the stimulated Brillouin scattering (SBS) threshold goes down as linewidth declines, these
lower thresholds result in lower output powers per laser module. Therefore, with existing
designs, the tradeoff is between combined beam quality and output power per laser module.
Or, since output power per module is a function of laser linewidth, the tradeoff is between
beam quality and laser linewidth. Here I show further understanding of SBC and a design
where beam quality can be made independent of laser linewidth. With this new design, the
tradeoff is between beam quality and irradiance levels on the optical elements, and it opens
up the possibility of scaling to higher powers with fewer fiber lasers while maintaining
relatively good beam quality.
KEYWORDS: Incoherent beam combining, Spectral beam combining, Fiber lasers
PAGES 260-267
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Analysis of Three-Dimensional Heat Flow in the Laser Heating of Materials
Chuck LaMar; U.S. Army Space and Missile Defense Command
The problem of an intense laser beam irradiating a thin metal plate and permanently damaging,
drilling, or welding the plate is well studied. It is highly nonlinear and difficult to address with
analytical methods. However, there are distinct advantages to developing analytical solutions
for such problems. The advantages include insight into the physics of the laser interaction and,
in particular, insight into the dimensionality of the problem. If the dimensionality of the problem
is understood, then a greatly simplified physical understanding of the problem is possible. In
this article, the dimensionality of the problem is shown to be strictly dependent on a spot-size
parameter, thermal diffusivity, and time. Provided that the melt removal temperature and
absorption stay constant during the interaction, the lateral heat losses stay constant regardless
of spot size. There is no limiting, or asymptotic, behavior in the time to penetrate.
KEYWORDS: Analytical solutions, Laser material interactions, Gaussian beams, Fourier series
PAGES 268-284
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Calculating Beam Quality Using Power in the Bucket Curves
Brian Strickland, U.S. Army Space and Missile Defense Command
This paper discusses the various methods currently in use to characterize laser performance
by a single number with emphasis on calculation from power in the bucket (PIB) curves.
The significant variation in single-number laser BQs calculated from these methods and
the problems associated with using such numbers are highlighted. A method to approximate
the PIB curve for a real non-Gaussian beam using a single-Gaussian and a two-Gaussian
model, and how to use these numbers in system performance codes like HELCoMES is
detailed. This paper focuses on laser performance; effects of the beam control system (BCS)
(jitter, BCS wavefront error, focus errors, and beam director obscuration), atmospheric
effects, and the accuracy of the PIB curve (simulation vs. measurement or measurement
errors) are not included.
KEYWORDS: Laser, Beam quality, Laser characterization, Laser standards, Gaussian beam, Non-Gaussian beam, M2
PAGES 285-307
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Volume 5, Number 3, Journal of Directed Energy
Last updated: 13 May 2016
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