Houjun Qian

Cesium telluride cathodes for the next generation of high-average current high-brightness photoinjectors

We report on the performances of a Cs2Te photocathode under extreme conditions of high peak time-dependent accelerating fields, continuous wave operations, and MHz pulse extraction with up to 0.3 mA average current. The measurements, performed in a normal conducting cavity, show extended lifetime and robustness, elucidate the main mechanisms for cathode degradation, and set the required system vacuum performance for compatibility with the operations of a high average power X-ray free electron laser user facility, opening the doors to the next generation of MHz-scale ultrafast scientific instruments.

Tests of photocathodes for high repetition rate x-ray FELs at the APEX facility at LBNL

After the formidable results of X-ray 4th generation light sources based on free electron lasers around the world, a new revolutionary step is undergoing to extend the FEL performance from the present few hundred Hz to MHz-class repetition rates. In such facilities, temporally equi-spaced pulses will allow for a wide range of previously non-accessible experiments. The Advanced Photo-injector EXperiment (APEX) at the Lawrence Berkeley National Laboratory (LBNL), is devoted to test the capability of a novel scheme electron source, the VHF-Gun, to generate the required electron beam brightness at MHz repetition rates. In linac-based FELs, the ultimate performance in terms of brightness is defined at the injector, and in particular, cathodes play a major role in the game. Part of the APEX program consists in testing high quantum efficiency photocathodes capable to operate at the conditions required by such challenging machines. Results and status of these tests at LBNL are presented.

The VHF-Gun, the LBNL High-Brightness Electron Photo-Injector for MHz-Class Repetition-Rate Applications

Science needs are pushing the development of MHz-class repetition-rate linac-based facilities generating high-brightness electron beams. The successful lower repetition-rate RF gun schemes cannot be scaled up to MHz rates. At LBNL, we developed the VHF-Gun, a room-temperature RF gun designed for CW operation and high-brightness beam performance.

APEX Present Experimental Results

The APEX electron source at LBNL combines highrepetition-rate and high beam brightness typical of photoguns, delivering low emittance electron pulses at MHz frequency. Proving the high beam quality of the beam is an essential step for the success of the experiment. It would enable high repetition rate operations for brightness-hungry applications such as X-Ray FELs, and MHz ultrafast electron diffraction. A full 6D characterization of the beam phase space at the gun beam energy (750 keV) is foreseen in the first phase of the project. Diagnostics for low and high current measurements have been installed and tested, measuring the performances of different cathode materials in a RF environment with mA average current. A double-slit system allows the characterization of beam emittance at high charge and full current (mA). An rf deflecting cavity is being installed, and a high precision spectrometer allow the characterization of the longitudinal phase space. Here we present the latest results at low and high repetition rate, discussing the tools and techniques used.