Empirical considerations for laser guided electron beams in the advanced test accelerator

Relativistic electron beam currents in excess of 10 kA have been propagated successfully for 64 m through the Advanced Test Accelerator (ATA), and up to 31 m beyond that, by means of an electro-static column of benzene ions generated by a KrF laser pulse. The immediate consequence was a dramatic reduction in transverse rf beam displacements to less than 0.1 mm. The absence of any magnetic focusing or steering necessitated stringent controls on the pointing, focusing, spatial uniformity and temporal stability of the ionizing laser. The control and monitoring of a benzene pressure profile in a hostile radiation environment have been addressed but still require constant attention. The subtleties of matching the electron beam between ion columns and magnetic transport out of the injector, at bends in the beam line, and at other transition zones are not yet fully understood. The consequences of temporal, spatial, and angular mismatch impact the electron beam output rise time, pulse shape, pulse length, total current, energy spread and emittance.