Studies of Photo-emission and Field Emission in an Rf Photo-injector with a High Quantum Efficiency Photo-cathode

An 18 MeV electron photo-injector facility was constructed at Fermilab in collaboration with UCLA, INFNMilano, University of Rochester, and DESY. The photoinjector is now being operated in partnership with the Northern Illinois Center for Accelerator and Detector Development. The facility consists of an RF gun [1, 2] with a cesium telluride (Cs2Te) photo-cathode [3] excited by a pulsed ultra-violet (UV) laser [4], a 9-cell superconducting cavity, a magnetic chicane, and a beam line for experiments with the electron beam. The original goal was to produce a beam with high charge per bunch (8 nC) and low transverse emittance (< 20 mm mrad) in long bunch trains (800 bunches, 1 μs apart). The facility at Fermilab was used to develop, fabricate, and commission an RF gun for the TeSLA Test Facility Linac. This gun has been in use at DESY since November 1998, and its behaviour is under study [5]. A second RF gun was subsequently installed at Fermilab. Recent activities at Fermilab include studies of photo-injector dynamics [6, 7] and various experiments with the electron beam: electro-optic sampling of wake fields [8], plasma wakefield acceleration, channeling radiation studies [9], and production of flat beams [10]. Another major goal for the photo-injector was high QE and low field emission (“dark current”) from the gun (the latter being a source of unwanted radiation and unwanted background signals). Hence, in between the work outlined above, studies of the photo-emission and field emission behaviour of the gun were carried out. The preliminary results will be summarised in this paper.