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Scientific Assessment of High-Power Free-Electron Laser Technology 1 Introduction and Principal Findings INTRODUCTION The National Academy of Sciences was asked to perform a scientific assessment of free-electron laser technology for naval applications. The specific Office of Naval Research charge was to assess whether the desired performance capabilities are achievable or whether some fundamental limitations will prevent them from being realized. The speed-of-light delivery of energy from a high-energy laser has the potential to provide the Navy with a ship defense capability against a class of threats not available to conventional defenses. Starting in the late 1960s, the Navy embarked on significant high-energy laser development, looking first at gas dynamic CO2 lasers and then at deuterium fluoride chemical lasers, demonstrating megawatt-level power output in the early 1980s. The Navy successfully engaged a supersonic target in a crossing pattern, but after tests against a target in a head-on engagement, it was determined that the potential utility of the deuterium fluoride chemical laser was severely limited by the propagation issue of thermal blooming. At that point, the Navy discontinued the chemical laser program but continued technology studies to look for a laser that would produce wavelengths that optimized propagation. The free-electron laser, which could produce a continuum of wavelengths and was an all-electric device (preferable to energetic chemicals like deuterium fluoride), was considered an attractive alternative but was only at the tens-of-watts level in its development when the Navy program was initiated in the mid-1990s. Since that time, through a series of scale-ups, 14 kilowatts of continuous-wave power has been demonstrated. In 2008, the Navy issued a Broad Agency Announcement to design and fabricate a 100 kilowatt free-electron laser for the purpose of developing the technologies required for a megawatt-class free-electron laser. The free-electron laser is currently seen as a potential way for the Navy to achieve megawatt-class output power levels, good optical beam quality, and wavelengths of interest from an all-electric device. The specific statement of task for Phase 1 of the study is as follows: 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 FELs. The Navy provided the following performance characteristics and considerations for the study:
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Scientific Assessment of High-Power Free-Electron Laser Technology Output power. Approximately 1 megawatt class at the aperture (also address the 100 kilowatt step); Wavelength. Three atmospheric windows (reduced absorption) at 1.04, 1.62, and 2 micrometers (1-2 micrometers); and Power to the free-electron laser. Approximately 20 megawatts. It is important to realize that although it may be possible to design and build a free-electron laser with the desired high levels of output power, that does not necessarily mean that an effective weapon system that uses the free-electron laser as a component can be built and operated in a naval environment of interest. To properly understand and interpret the meaning and applicability of the results of this study, it is critical to identify the factors it does not address. It does not address whether a megawatt-class free-electron laser will be an effective weapon in a naval context nor does it address operational lethality factors, such as duration of the beam on target or repetition rate. More specifically, the study does not address: The effectiveness of the device to perform Navy missions of interest or The physics associated with atmospheric propagation of the laser beam (thermal blooming, aerosols, weather effects, etc.). This study and report also do not address the realistic constraints of shipboard operation and installation such as those that follow. These constraints are not insignificant and should be addressed in a follow-on study: Sizing the free-electron laser beam generation system and engineering it to operate in a shipboard environment, including the following associated factors: Inherent ship vibration and motion; Radiation safety and shielding; Protection of the free-electron laser system from warfighting damage; Power conditioning; Support for cryosystem operation; Provision of vacuum; Transmission of the beam between the free-electron laser and the beam director; Engineering of the beam director; and Manpower, personnel, and knowledge-base issues related to the operability, maintainability, and repairability of the system by sailors. This study identifies the highest-priority scientific and technical gaps that will need to be overcome along the development path to achieve a megawatt-class free-electron laser. The development of a 100 kilowatt device is considered an interim step to demonstrate the scalability of component technologies to the megawatt class. While a 100 kilowatt device may exhibit naval utility in its own right, component-level scalability to the megawatt class is considered essential to this study. The committee’s principal findings are provided in the following section. The information that follows this chapter is organized into two chapters. Chapter 2 describes the state of the art with free-electron lasers. It provides a history of free-electron lasers for Navy applications, gives an overview description of free-electron lasers, discusses the trade-offs between free-electron lasers and other types of high-energy lasers, and describes the relationship of free-electron lasers to scientific applications. Chapter 3 provides a detailed assessment of free-electron laser technologies and challenges. It begins with a general discussion of how we get from the current state-of-the-art free-electron lasers to free-electron lasers in the 100 kilowatt class and 1 megawatt class. The discussion that follows is organized around the components and major operational issues of a free-electron laser and addresses the technical operation, state of the art, and challenges to progress associated with each aspect of an overall free-electron laser system. Following Chapter 3, the appendixes include the statement of task for the study and report, agendas for the committee meetings, biographies of the committee members and staff, and a combined glossary and acronyms list.
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Scientific Assessment of High-Power Free-Electron Laser Technology In preparing this report, the committee was aware that the audience comprises two general groups of readers. One group is composed of decision makers and other readers who are not experts in free-electron laser technology and operation. The other group is composed of those who are deeply knowledgeable in the technical details associated with free-electron lasers. The report attempts to address the needs of both groups. The preface, executive summary, and introduction and principal findings chapters are written to be easily understandable by all readers. The state-of-the-art and technical assessment chapters of the report provide sufficient technical detail to give the free-electron laser community a good grounding in the information base and extrapolations employed by the committee in performing this study. For easy reference, the principal findings of the present study are listed below. The technical basis and context for these findings are provided in Chapters 2 and 3 of this report. PRINCIPAL FINDINGS There have been significant engineering and technological advances in the 30 years since free-electron lasers (FELs) were first considered for directed-energy applications. The most notable technical advance is the development of energy recovery technology using a superconducting radio-frequency (RF) linac, but other advances in the understanding and management of both high-peak-power and high-average-power beams are also significant. These include the modeling and mitigation of the beam breakup instability, beam halo production and associated scraping and loss, emittance preservation in the gun and magnetic optics, the modeling and mitigation of coherent synchrotron radiation in bends, and the microbunching instability, to mention a few examples. The combination of classification and subsequent funding reductions has also led to the loss of high-average-power free-electron laser development capabilities in certain critical areas. The committee notes that the unintended effect of prior stewardship of free-electron laser research has been to reduce rather than protect the nation’s valuable advantage in some key areas of technology. The combination of classification and inconsistent funding of free-electron laser research and development has led to advances that were neither sustained in the laboratory nor preserved in the open literature and are for all intents and purposes lost from the national science base. This is particularly evident in the case of high-damage-threshold, free-electron-laser-unique optical coatings. In some cases, the key investigators have since left the field and the knowledge base has been lost. By providing consistent and sustained support to early-career scientists participating in free-electron laser research and development programs, the ongoing transfer of key technologies can be assured. The primary advantages of free-electron lasers are associated with their energy delivery at the speed of light, selectable wavelength, and all-electric nature, while the trade-offs for free-electron lasers are their size, complexity, and relative robustness. Like other high-energy laser systems, free-electron lasers offer extremely fast tracking and response compared to ballistic devices for engaging maneuvering targets. Unlike other laser systems, they offer the freedom to choose wavelengths to match propagation windows in the region of maritime interest, and the free electrons that are their lasing medium facilitate removal of waste heat as well as electric power recovery. Since they could be powered by a ship’s own fuel supply, they offer a deep magazine. They have the potential to scale to high power and the optical beam quality is high. On the other hand, free-electron lasers require high-current accelerators and cryogenic coolers of substantial size, significant mechanical isolation from vibration and shock, hard vacuum, and radiation shielding.
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Scientific Assessment of High-Power Free-Electron Laser Technology Despite the significant technical progress made in the development of high-average-power free-electron lasers, difficult technical challenges remain to be addressed in order to advance from present capability to megawatt-class power levels. In particular, in the committee’s opinion, the two “tall poles” in the free-electron laser development “tent” are these: An ampere-class cathode-injector combination. Radiation damage to optical components of the device. In both cases, the most well-developed approach (demonstrated in a 14 kilowatt free-electron laser) does not scale in a straightforward manner to the parameters needed for megawatt-class average power levels. However, there are several options in each case that appear to be promising research directions for addressing the critical technology gaps. 4a. Drive-laser-switched photocathodes are the likely electron source for megawatt-class free-electron lasers. Photocathodes have been used in accelerator applications for more than 2 decades; however, they have not reached the level of performance in terms of quantum efficiency and robustness that will likely be required for a reliable megawatt-class free-electron laser. Drive-laser technology appears to be approaching the level required for megawatt-class free-electron laser operation. There are some promising photocathode approaches under investigation; however, there are still considerable basic physics and engineering issues that must be resolved. 4b. High-performance optical resonators and coatings that operate successfully with megawatt-class lasers have existed for 2 decades. However, free-electron lasers uniquely generate harmonic radiation in the ultraviolet region, which has been shown to fatally damage many of the existing high-performance coatings. There were promising approaches under development during the Strategic Defense Initiative (SDI) era, and additional research is ongoing that has been making substantial advances. There are a number of components for which the extrapolation to megawatt-class power levels represents an experience/predictive gap rather than a physics or technology gap. The committee notes that in some areas there appears to be no fundamental showstopper to achieving the parameters described in Chapter 1 of this report; rather, there is a lack of experience or predictive modeling capability, which makes it difficult to quantify how challenging the technology gap will be to address. The committee refers to these as “gray poles,” which include ring and high-gain oscillator configurations (lack of experience, very few technical papers), beam halo production and control (lack of benchmarked predictive models), amplifier configurations, coherent synchrotron radiation, and the development of diagnostic techniques and algorithms for measuring experimental beam distributions with sufficient accuracy to provide realistic input to modeling. There are other potential, difficult technical challenges (“tall poles”) not addressed in the present phase of the free-electron laser study that may be important to future realization of naval applications. These challenges include tight constraints on the allowable shipboard vibration (less than 10 nm radio-frequency accelerator cavity deformation), atmospheric propagation issues, and automated (sailor-friendly) controls and readiness challenges.