Investigation of Beam Alignment Monitor Technologies for the LCLS FEL Undulator

To maintain gain in the 100 m long linac-driven Linac Coherent Light Source (LCLS) FEL undulator, the electron and photon beams must propagate colinearly to within ~5 μm rms over distances comparable to the 11.7 m FEL gain length in the 6 mm diameter undulator vacuum chamber. We have considered a variety of intercepting and non-intercepting position monitor technologies to establish and maintain this beam alignment. We present a summary discussion of the applicability and estimated performance of monitors detecting synchrotron radiation, transition and diffraction radiation, fluorescence, photoemission or bremsstrahlung from thin wires, Compton scattering from laser beams, and image currents from the electron beam. We conclude that: 1) non-intercepting RF cavity electron BPMs, together with a beam based alignment system, are best suited for this application; and 2) insertable intercepting wire monitors are valuable for rough alignment, for beam size measurements, and for simultaneous measurement of electron and photon beam position by detecting bremsstrahlung from electrons and diffracted x-rays from the photon beam