Abstract

An important advantage of the free-electron laser (FEL) is that the fundamental mechanism is tunable. The undulator wavelength λ₀ is twice Doppler shifted to a much shorter electromagnetic wavelength λ ~ λ₀/2γ² using a relativistic electron beam of energy γmc². Realizable undulator designs limit λ₀ to a few centimeters so that a large electron energy (γ ~ 2000) is required to reach λ ~ 100 Å. The fundamental electron-photon interaction remains classical, and the statistical nature of electrons (shot noise) and photons (quanta) is negligible except for the initial stages of start-up. To evaluate the performance of an FEL system, the dimensionless current density, j = 8N(πeKL)²pF/ γ³mc² is useful; N is the number of undulator periods, L is the undulator length, K ~ 1 is the undulator constant, p is the electron beam particle density, and F describes the coupling to the transverse optical mode. An FEL oscillator in this wavelength range uses / > 10 to compensate for large mirror losses, and an FEL single-pass amplifier uses y ~ 10⁴ to reach high power starting from spontaneous emission.

© 1986 Optical Society of America