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Journal of Directed Energy
Volume 5, Number 3 Fall 2014

The papers listed below constitute Volume 5, Number 3 of the Journal of Directed Energy.
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Performance of the Mark II Quarter-Wave SRF Photoinjector
John W. Lewellen and others; Naval Postgraduate School and other affiliations

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

The Application of SCOS for HPM Field Measurement
Bradley M. Whitaker and others, Brigham Young University and other affiliations

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

MATILDA: A Military Laser-Range Safety Tool Based on Probabilistic Risk Assessment Techniques
Brian K. Flemming, Paul K. Kennedy, Daniel F. Huantes, and Matthew D. Flower; SELEX ES Ltd. and other affiliations

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

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

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

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

Volume 5, Number 3, Journal of Directed Energy

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