New J. Phys. 17, 063036 (2015)

Optical power can be used to drive electron motion — as in inverse free-electron lasers, which do so via undulator magnets. Researchers from the University of California Los Angeles and Radiabeam Technologies, Santa Monica, now show that by reversing this process and decelerating an electron beam it is in principle possible to extract 50% of the electron beam power into coherent radiation with a wavelength of 13.5 nm using a 23-m-long tapered undulator. The team call their scheme TESSA (tapering-enhanced superradiant amplification) due to the use of a strongly tapered undulator; the deceleration is optimized by balancing the tapering with pondermotive gradients. The authors of the study believe that it should be possible (assuming 1 GeV electron energy, 20 kHz repetition rate, 4 kA peak current and 500 fs root-mean-square bunch length) to achieve 20 kW average power at 13.5 nm wavelength, which could be useful for extreme-ultraviolet lithography. In another example the team highlight the possibility of using TESSA to generate hard X-rays with ∼6 TW output, only limited by sideband instability in their analysis.