Design of an x-ray FEL undulator

An undulator designed to be used far an x-ray free electron laser has to meet a set of stringent requirements. With no optical cavity, an x-ray FEL operates in the single pass Self Amplified Spontaneous Emission (SASE) mode; an electron macropulse is microbunched by an undulator and the radiation it creates. The microbunched pulse emits spontaneous radiation and coherent FEL radiation, whose power may reach saturation in a sufficiently long and perfect undulator. The pulse must have low emittance and high current, and its trajectory in the undulator must keep the radiation and the pulse together with a very high degree of overlap. In this paper we use the case of the Linear Coherent Light Source (LCLS) x-ray free electron laser to illustrate design concepts for long free electron laser undulators. [1,2] The LCLS is intended to create 1.5 Angstrom x-rays using an electron beam with 15 GeV energy, 1.5 pi mm-mrad normalized emittance, 3400 A peak current, and 280 fsec FWHM bunch duration. According to our simulations, this 2 sigma(r) = 65 mm rms diameter beam must overlap its radiation with a walkoff of no more than 5 mu m RMS per 11.7 meter field gain length for efficient gain. The energy spread of the beam is sigma(E) = 0.0002 E-L. This places severe limitations on the magnetic field errors and on mechanical tolerances. In this paper we shall discuss how to meet these requirements.