High-brightness electron-beam production at the Brookhaven Accelerator Test Facility

The Accelerator Test Facility (ATF), under construction at Brookhaven National Laboratory, will consist of a high-brightness 50-MeV electron beam and a synchronizable 100-GW-peak-power CO{dollar}sb2{dollar} laser. It is designed to be a user facility for experiments that require a high-quality electron beam and that probe the interaction of electrons with strong electromagnetic fields.; The electron source is a radio-frequency (rf) gun incorporating a metal photocathode driven by a few-picosecond frequency-quadrupled Nd:YAG laser. The gun produces high-brightness pulses of few-picosecond duration and energy of up to 4.5 MeV. Upon exiting the gun the beam enters a low-energy transport line that allows for momentum selection, pulse compression, and diagnosis. The beam will be injected into a 50-MeV linear accelerator and transported to one of several experimental beamlines, where it will be brought into collision with the few-picosecond CO{dollar}sb2{dollar}-laser pulse.; We have commissioned the rf gun and low-energy beamline and have found all operational characteristics to be as designed. For this we have constructed an electron-beam diagnostic system consisting of a series of phosphor-screen beam-profile monitors and an rf-deflection cavity that provides spatial profiles of the electron beam with 45-{dollar}mu{dollar}m resolution and pulse-length measurements with picoseconds resolution. In the course of this work we have had a unique opportunity to study in detail the physical mechanisms of electron-beam production and preservation.; The electron beam was measured to have a normalized transverse emittance of 4 mm-mrad and an rms pulse length of 6 picoseconds with a bunch charge of up to 2 nanocoulombs. From this we infer a peak current of 130 A and a normalized brightness of 10{dollar}sp{lcub}11{rcub}{dollar} A/m{dollar}sp2{dollar}, making the ATF the highest-brightness electron-beam user facility in the world. A previously unobserved rf-gun excitation mode has been identified in which a sufficiently intense laser pulse creates a plasma on the photocathode surface. The result is a train of micropulses over many rf cycles containing a total charge at least two orders of magnitude higher than that of the normal photoemissive mode.; We have also constructed an x-ray-backscattering diagnostic that will allow the synchronized interaction of the 50-MeV-electron and CO{dollar}sb2{dollar}-laser beams to be established. Augmented by an x-ray spectrometer, this apparatus will be used to study nonlinear Compton scattering in the strong fields present at the CO{dollar}sb2{dollar}-laser focus. This experiment has been approved for the ATF and is in the design phase.