1. J. Madey, "Stimulated Emission of Bremsstrahlung in a Periodic Magnetic Field," J. Appl. Phys. 42, 1906 (1971).

2. D.A.G. Deacon et al., "First Operation of a Free-Electron Laser". Phys. Rev. Lett., vol. 38, No. 16, Apr. 1977, pp. 892–894.

3. R.M. Phillips, "The Ubitron, a High-power Traveling-Wave Tube based on a Periodic Beam Interaction in Unloaded Waveguide," IRE Transaction on Electron Devices, vol. 7, No. 4, Oct. 1960, pp. 231–241.