Electron beam and rf characterization of a low-emittance X-band photoinjector

Detailed experimental studies of the first operation of an X-band (8.547 GHz) rf photoinjector are reported. The rf characteristics of the device are first described, as well as the tuning technique used to ensure operation of the 11 / 2-cell rf gun in the balanced π-mode. The characterization of the photoelectron beam produced by the rf gun includes: measurements of the bunch charge as a function of the laser injection phase, yielding information about the quantum efficiency of the Cu photocathode ( 2×10^-5 for a surface field of 100 MV/m); measurements of the beam energy (1.5–2 MeV) and relative energy spread ( Δγ/γ₀  =  1.8±0.2%) using a magnetic spectrometer; measurements of the beam 90% normalized emittance, which is found to be ɛ_n  =  1.65π mm mrad for a charge of 25 pC; and measurements of the bunch duration ( <2 ps). Coherent synchrotron radiation experiments at Ku-band and Ka-band confirm the extremely short duration of the photoelectron bunch and a peak power scaling quadratically with the bunch charge.