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

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Executive Summary This report presents a scientific assessment of free-electron laser technology for naval applications. The charge from the Office of Naval Research was to assess whether the desired performance capabilities are achievable or whether fundamental limitations will prevent them from being realized. The statement of task for the present study, Phase 1, 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: • 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. To properly understand and interpret the meaning and applicability of the results of this study, it is critical to identify the factors that it did not address. The present study did not address whether a megawatt-class free-electron laser will be an effective weapon in a naval context, nor did it address operational lethality factors, such as duration of the beam pulse on target or the repetition rate. More specifically, the study did 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.). In addition, the study did not address the realistic constraints of shipboard operation and installation, such as sizing the beam generation system or engineering it to operate in a shipboard environment. These specific issues are not insignificant and should be addressed in a follow-on study. The present study identifies the highest-priority scientific and technical issues that must be resolved along the development path to achieve a megawatt-class free-electron laser. In this regard, the development of a ­scalable 100 kilowatt device is considered an important interim step. In accordance with the charge, the committee consid- ered (and briefly describes) trade-offs between free-electron lasers and other types of lasers and weapon systems to show the advantages free-electron lasers offer over other types of systems for naval applications as well as  

 SCIENTIFIC ASSESSMENT OF HIGH-POWER FREE-ELECTRON LASER TECHNOLOGY their drawbacks. The characteristics of different types of free-electron lasers are discussed and compared in detail throughout the report. Following a description of the state of the art of free-electron laser technology (Chapter 2), particularly as it relates to Navy interests and applications, this report presents a detailed assessment of the scientific and tech­ nological challenges that must be addressed before the current state of the art (14 kilowatt output power) can advance to the 100 kilowatt and 1 megawatt-class output power levels (Chapter 3). The principal findings of the present study are summarized below: 1. There have been significant engineering and technological advances in the 30 years since free-electron lasers were first considered for directed-energy applications. 2. 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. 3. 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. 4. 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. 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. 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. 5. 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. 6. 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. The technical basis and the context for these findings are elaborated in Chapters 2 and 3 of this report.