Scientific Assessment of High-Power Free-Electron Laser Technology (2009)

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Committee on a Scientific Assessment of Free-Electron Laser Technology for Naval Applications Board on Physics and Astronomy Division on Engineering and Physical Sciences 

THE NATIONAL ACADEMIES PRESS  500 Fifth Street, N.W.  Washington, DC 20001 NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance. This study is based on work supported by Contract N00014-05-G-0288, T.O. 18, between the National Academy of Sciences and the Department of the Navy. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the agency that provided support for the project. International Standard Book Number 13:  978-0-309-12689-2 International Standard Book Number 10:  0-309-12689-4 Copies of this report are available free of charge from Board on Physics and Astronomy National Research Council 500 Fifth Street, N.W. Washington, DC 20001 Additional copies of this report are available from the National Academies Press, 500 Fifth Street, N.W., Lockbox 285, Wash- ington, DC 20055; (800) 624-6242 or (202) 334-3313 (in the Washington metropolitan area); Internet, http://www.nap.edu. Copyright 2009 by the National Academy of Sciences. All rights reserved. Printed in the United States of America 

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COMMITTEE ON A SCIENTIFIC ASSESSMENT OF FREE-ELECTRON LASER TECHNOLOGY FOR NAVAL APPLICATIONS THOMAS C. KATSOULEAS, Duke University, Chair RICARDO ALARCON, Arizona State University JOHN ALBERTINE, Independent Consultant ILAN BEN-ZVI, Brookhaven National Laboratory SANDRA G. BIEDRON, Argonne National Laboratory CHARLES A. BRAU, Vanderbilt University WILLIAM B. COLSON, U.S. Naval Postgraduate School RONALD C. DAVIDSON, Princeton University PAUL G. GAFFNEY II, Monmouth University LIA MERMINGA, TRIUMF JOEL D. MILLER, Johns Hopkins University Applied Physics Laboratory BRIAN E. NEWNAM, Los Alamos National Laboratory (retired) PATRICK O’SHEA, University of Maryland C. KUMAR N. PATEL, Pranalytica, Inc. DONALD PROSNITZ, RAND Corporation ELIHU ZIMET, Independent Consultant Staff DONALD C. SHAPERO, Director, Board on Physics and Astronomy CY L. BUTNER, Senior Program Officer ROBERT L. RIEMER, Senior Program Officer (until March 2008) CARYN J. KNUTSEN, Program Associate Dr. Patel resigned from the committee on March 14, 2008.  

BOARD ON PHYSICS AND ASTRONOMY MARC A. KASTNER, Massachusetts Institute of Technology, Chair ADAM S. BURROWS, Princeton University, Vice-Chair JOANNA AIZENBERG, Harvard University JAMES E. BRAU, University of Oregon PHILIP H. BUCKSBAUM, Stanford University PATRICK L. COLESTOCK, Los Alamos National Laboratory RONALD C. DAVIDSON, Princeton University ANDREA M. GHEZ, University of California at Los Angeles PETER F. GREEN, University of Michigan LAURA H. GREENE, University of Illinois, Urbana-Champaign MARTHA P. HAYNES, Cornell University JOSEPH HEZIR, EOP Group, Inc. MARK B. KETCHEN, IBM Thomas J. Watson Research Center ALLAN H. MacDONALD, University of Texas at Austin PIERRE MEYSTRE, University of Arizona HOMER A. NEAL, University of Michigan JOSE N. ONUCHIC, University of California at San Diego LISA RANDALL, Harvard University CHARLES V. SHANK, Howard Hughes Medical Institute, Janelia Farm MICHAEL S. TURNER, University of Chicago MICHAEL C.F. WIESCHER, University of Notre Dame Staff DONALD C. SHAPERO, Director MICHAEL H. MOLONEY, Associate Director ROBERT L. RIEMER, Senior Program Officer JAMES LANCASTER, Program Officer DAVID LANG, Program Officer CARYN J. KNUTSEN, Program Associate ALLISON McFALL, Senior Program Assistant BETH DOLAN, Financial Associate vi 

Preface The National Research Council was asked by the U.S. Navy’s Office of Naval Research (ONR) to assess the current capabilities of free-electron lasers (FELs) to deliver large amounts of energy; assess the prospects for developing such devices with megawatt average power capabilities; identify the key technical problems that must be solved to achieve such performance; and evaluate the feasibility of achieving power, energy, and other technical parameters specified by the Office of Naval Research. The request did not include a charge to make a determina- tion of the requirements for effective directed-energy weapons. The National Research Council responded by forming the Committee on a Scientific Assessment of Free- E ­ lectron Laser Technology for Naval Applications to perform the requested study. As described below, this study will be performed in two phases. For Phase 1, covered in the present report, the committee has performed a tech­ nology assessment of the state of the art across the free-electron laser community in order to evaluate the feasibility of achieving power and other technical parameters specified by the Office of Naval Research and to identify the technical gaps that must be overcome to achieve such performance. Directed-energy weapons have been pursued by the U.S. military for decades; these weapons use very-high- power beams to disable or destroy targets. They typically use a single optical system both to track a target and to focus the beam on the target. The Air Force has sponsored research using chemically powered lasers, the Army has researched the use of solid-state laser technologies, and the Navy has developed free-electron lasers through programs at the Office of Naval Research. A free-electron laser is an accelerator-based device that causes stimulated emission of radiation to occur from an electron beam. It generates tunable, coherent, highly collimated, high-power radiation, currently ranging in wavelength from microwaves to x-rays. While a free-electron laser beam shares to some degree the same optical properties as optically or chemically pumped lasers (such as coherence), the operation of a free-electron laser is quite different. Unlike gas or diode lasers, which rely on transitions between bound atomic or molecular states, free-electron lasers use a relativistic electron beam as the lasing medium, hence the term “free electron.” Today, a free-electron laser requires the use of an electron accelerator with its associated ionizing-radiation shielding and other support systems. The electron beam must be maintained in a vacuum, which requires the use of numerous pumps along the beam path. Free-electron lasers can achieve extremely high peak powers without damage to the laser medium. The Navy has chosen to pursue the free-electron laser route to a directed-energy weapon, in part because free-electron lasers offer the advantage of being design-wavelength-selectable, allowing them to be designed to vii 

viii PREFACE operate at wavelengths that are optimal for maritime environments. The free-electron laser’s relatively efficient conversion of “wall-plug power” to “beam power” would make it attractive for use on a mobile platform such as a ship. However, there are still problems that need to be resolved. Supported by the Office of Naval Research, researchers at the U.S. Department of Energy’s Thomas ­Jefferson National Accelerator Facility (TJNAF) delivered the first light from their free-electron laser on June 17, 1998. Only 2 years after ground was broken for the free-electron laser, infrared light of more than 150 watts was d ­ elivered—15 times the power of free-electron lasers existing at that time. On July 15, 1999, the free-electron laser exceeded its design goal of 1,000 watts by producing 1,720 watts of infrared light. The current development effort at TJNAF has now achieved average beam powers of 14 kilowatts. Recent advances in accelerator science and technology using superconducting radio-frequency cavities in an energy recovery linear accelerator (linac) suggest that the necessary optical cavity could be contained within a 20-meter-long structure. The Office of Naval Research program in free-electron-laser research is currently classified as an applied research program (budget category 6.2). The Office of Naval Research is considering an expansion of the research effort in the form of an advanced technology development program (budget category 6.3). In order to ultimately design and build a ship-based, directed-energy weapon, the next step proposed by the Navy program is to dem- onstrate and study a 100 kilowatt free-electron-laser system to establish the technology needed for scaling to the megawatt level in the infrared wavelength region. To assist the Navy in planning its next steps, the committee embarked upon this study. As originally envisioned and contracted, the study included the following three tasks: 1. Review the current state of the art and anticipated advances for high-average-power free-electron lasers (FELs). Using performance characteristics defined by the Navy for directed-energy applications, analyze the capabilities, constraints, and trade-offs for free-electron lasers. 2. Evaluate the scientific and technical development path from current demonstrated capabilities toward the eventual goal of achieving megawatts of radiated power at wavelengths suited to naval applications; consider the realistic constraints of shipboard installation. 3. Identify the highest-priority scientific and technical gaps along the development path from present-day capabilities through a 100 kilowatt test facility to a megawatt demonstration project. Recommend a phased approach for this development path using staged milestones with explicit performance and success criteria at each stage. However, the committee believed that a fourth task should be added to the study: 4. Assess the capabilities and constraints related to beam steering and atmospheric propagation at wavelengths suited to naval applications for a free-electron-laser-based system. The committee viewed the fourth task as essential for giving the Navy appropriate advice on a free-electron- laser-based “system.” The committee’s intent was to address this task at a high level, touching on factors that are critical to the successful operation and feasibility of a free-electron-laser-based weapon system. The effort was not, however, intended to amount to an in-depth examination, but rather to provide a contextual summary based on information in the open literature. The addition of the fourth task was discussed with the Office of Naval Research in the initial planning phase of the study, and it was generally agreed that this was acceptable to the Office of Naval Research. Subsequently, however, the Office of Naval Research expressed its desire to not add the fourth task to the statement of task. At the committee’s first meeting (January 17-18, 2008, at the Keck Center of the National Academies in Washington, D.C.), the then Chief of Naval Research, RADM William E. Landay III, presented the charge that the Office of Naval Research wished the committee to pursue, which did not include the fourth task. The context for the Office of Naval Research’s desire for this study is the Navy’s view of what it will need to prevail in the anticipated conditions of future naval warfare. The Navy anticipates threats different from those it faced during the days of the Strategic Defense Initiative (SDI). To counter these new threats, the Navy wants to be able to 

PREFACE ix fight at the speed of light, with all-electric systems. In accordance with this view, the Office of Naval Research is interested in exploring the potential of free-electron lasers to serve as the basis of effective weapon systems and in achieving a megawatt of power at the aperture of a free-electron laser. Its main interests in this study are how much free-electron-laser power and what size would be possible—that is, its interest is in the free-electron laser “box” rather than what happens past the free-electron-laser aperture. The Office of Naval Research’s view is that the committee would help the most by identifying the “tall poles” in the free-electron-laser development “tent”—the key technical challenges that must be overcome to achieve significantly higher power output from a shipboard free-electron laser. As the study progressed from its initial stages, it was decided that the full study would be conducted in two phases. Phase 1 (covered in this report), conducted under the auspices of the National Research Council’s Board on Physics and Astronomy, addressed the first element of the statement of task. The information that was used in performing Phase 1 was limited to that obtainable in the open literature. Phase 2 of this study will commence, at the option of the Office of Naval Research, upon completion of Phase 1. The responsibility for Phase 2 has been assigned to the National Research Council’s Naval Studies Board, and the work in Phase 2 will be based on the results of Phase 1. The plan is for Phase 2 to address tasks 2-4 of the statement of task or modifications of them subject to agreement between the Office of Naval Research and the National Research Council. Based on the negotiated statement of task for Phase 2, the committee’s composition will be reevaluated by the National Research Council. In addition, Phase 2 may require that the committee have access to restricted, limited-distribution information or, possibly, classified information. The formation of this committee drew on the expertise of the Naval Studies Board in naval matters and on that of the Board on Physics and Astronomy in the relevant technical matters. Committee members were selected on the basis of demonstrated intellectual and technical leadership and familiarity with the policy aspects of the Navy’s research programs. Some are expert in the science and technology of free-electron lasers and the enabling accelerator technology, and some are expert in military science and technology, especially naval architecture and seafaring performance constraints. The committee was not asked to directly address the general issue of directed- energy weapons, but a few of its members were familiar with this issue. To ensure balance, the committee included a mix of experts on military and civilian research on free-electron lasers. Most members were from the university and national laboratory communities; many were familiar with Navy research and applications needs. The committee responded to its charge with sincere dedication and a desire to perform a valuable service to the free-electron-laser policy and science communities. It believes it has succeeded in its goal. Thomas C. Katsouleas, Chair Committee on a Scientific Assessmen of Free-Electron Laser Technology for Naval Applications 

Acknowledgment of Reviewers This report has been reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the National Research Council’s (NRC’s) Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the institution in making its published report as sound as possible and to ensure that the report meets institutional s ­ tandards for objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We wish to thank the following individuals for their review of this report: Martin Breidenbach, Stanford Linear Accelerator Center, David H. Dowell, Stanford Linear Accelerator Center, Nathaniel Fisch, Princeton University, Donald L. Hartill, Cornell University, Jay Marx, California Institute of Technology, Carmen S. Menoni, Colorado State University, C. Kumar N. Patel, Pranalytica, Inc., Claudio Pellegrini, University of California at Los Angeles, Triveni Rao, Brookhaven National Laboratory, and Jonathan Wurtele, University of California at Berkeley. Although the reviewers listed above have provided many constructive comments and suggestions, they were not asked to endorse the conclusions or recommendations, nor did they see the final draft of the report before its release. The review of this report was overseen by Elsa Garmire, Dartmouth College. Appointed by the NRC, she was responsible for making certain that an independent examination of this report was carried out in accordance with institutional procedures and that all review comments were carefully considered. Responsibility for the final content of this report rests entirely with the authoring committee and the institution.  

Contents EXECUTIVE SUMMARY 1 1 INTRODUCTION AND PRINCIPAL FINDINGS 3 Introduction, 3 Principal Findings, 5 2 STATE OF THE ART 7 A Brief History of the Free-Electron Laser for Navy Applications, 7 Free-Electron Laser Descriptions, 10 High-Energy Laser Trade-offs, 11 Relation to Scientific Free-Electron Lasers, 12 Notes, 13 3 TECHNICAL ASSESSMENT: SCALABILITY TO ONE-MEGAWATT POWER LEVELS 14 How to Achieve 100 Kilowatts and One Megawatt, 14 End to End by System Blocks, 16 Electron Gun Systems, 16 Photocathodes, 17 Photocathode Drive Lasers, 18 Electron Guns, 18 Booster, 18 Merger Optics, 19 Energy Recovery Linac, 19 Radio-Frequency Couplers and Power Handling, 20 Energy Recovery Linac Lattice and Peripherals: Transport Challenges, 20 Undulator and Associated Pinch for Amplifiers, 21 Optical System Issues, 21 Introduction, 21 Coatings, 22 xi 

xii CONTENTS Oscillators, 24 Single-Pass Free-Electron Lasers, 25 Regenerative Amplifier Alternative to the Master Oscillator Power Amplifier (MOPA), 26 Beam Dump, 27 Tunability, 27 Controls, 28 Simulation and Modeling, 29 Injectors, 29 Coherent Synchrotron Radiation, 30 Beam Halo, 30 Beam Breakup, 31 FEL Simulation Codes, 31 FEL Start-to-End Simulation Codes, 32 Notes, 32 APPENDIXES A Statement of Task 39 B Committee Meeting Agendas 40 C Biographies of Committee Members and Staff 43 D Acronyms and Glossary 50