Formation and acceleration of uniformly filled ellipsoidal electron bunches obtained via space-charge-driven expansion from a cesium-telluride photocathode

Abstract

We report the experimental generation, acceleration, and characterization of a uniformly filled electron bunch obtained via space-charge-driven expansion (often referred to as ''blow-out regime'') in an L-band (1.3-GHz) radiofrequency photoinjector. The beam is photoemitted from a cesium-telluride semiconductor photocathode using a short (< 200 fs) ultraviolet laser pulse. The produced electron bunches are characterized with conventional diagnostics and the signatures of their ellipsoidal character are observed. We especially demonstrate the production of ellipsoidal bunches with charges up to $0:5 nC corresponding to a $20-fold increase compared to previous experiments with metallic photocathodes.

Bunch shaping experiments at the high brightness electron beam source laboratory (HBESL)

“…The deflecting mode cavity is a normal conducting, water cooled, 3.9 GHz copper cavity that is powered by a 50 kW klystron. While our experiment used a UV laser (∼100fs) illuminating a Cs 2 Te photocathode, several experiments involving multiphoton emission [5], ellipsoidal bunch generation [6], diamond field emitter array cathode [7], carbon nanotube cathodes, channelling radiation [8]and inverse compton scattering [9] have been done or proposed at HBESL. The alpha-BBO crystal is placed right before the UV enters the cathode and the amount of UV sent to the crystal is controlled using a remote-controllable aperture.…”

Generation of sub-fs electron beams at few-MeV energies

“…The advantage of the ellipsoidal distribution is that also the transverse space charge field is linear for all aspect ratios of the bunch; its disadvantage is that is not easily generated by a laser pulse. (The so-called blow-out concept [7,8] is not discussed at this point for its unfavorable transverse emittance results [9,10].) Thus for the following calculations the parabolic distribution has been chosen.…”

Quasi-half-cycle pulses of light from a tapered undulator

“…The generation of prebunched beams with the required characteristics has not been demonstrated yet, but the theoretical studies [29] suggest that 100 pC ellipsoidal bunches are compressible to 30 fs, and recent experimental results [30,31] give evidence that high-quality ellipsoidal bunches can be produced. Space-charge effects in 200 MV=m X-band rf guns are greatly reduced.…”