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 technological 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.

  1. 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.

  2. 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.

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