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.