Recent work at Fermilab in collaboration with the Advanced Photon Source and members of other national labs, designed an experiment to study the relationship between the RF repetition rate and the average current per RF pulse. While existing models anticipate a direct relationship between these two parameters we observed an inverse relationship. We believe this is a result of damage to the barium coating on the cathode surface caused by a change in back-bombardment power that is unaccounted for in the existing theories. These observations shed new light on the challenges and fundamental limitations associated with scaling an un-gated thermionic cathode RF gun to high average current machines.
Undesirable electron field emission (a.k.a. dark current) in high gradient RF photocathode guns deteriorates the quality of photoemission current and limits the operational gradient. To improve the understanding of dark current emission, a high-resolution (~100 um) dark current imaging experiment has been performed in an L-band photocathode gun operating at ~100 MV/m of surface gradient. Dark current from the cathode has been observed to be dominated by several separated strong emitters. The field enhancement factor, beta, of selected regions on the cathode has been measured. The post scanning electron microscopy (SEM) and white light interferometer (WLI) surface examinations reveal the origins of ~75% strong emitters overlap with the spots where rf breakdown have occurred.
KEK-ATF is studying the low-emittance multi-bunch electron beam for the future linear collider. In ATF, thermionic gun is used to generate 20 bunches electron beam with the bunch spacing of 2.8 ns. Due to a distortion of the gun emission and the beam loading effect in the bunching system, the intensity for each bunch is not uniform by up to 40 % at the end of the injector. We have developed a system to correct the gun emission by precisely controlling the cathode voltage with a function generator. For the beam loading effect, we have introduced RF amplitude modulation on Sub Harmonic Buncher, SHB. By these technique, bunch intensity uniformity was improved and beam transmission for later bunches was recovered from 67% to 91%, but intensity for first five bunches is still lower than others.
CW photoinjectors operating at high accelerating gradients promise to revolutionize many areas of science and applications. They can establish the basis for a new generation of monochromatic X-ray free electron lasers, high brightness hadron beams, or a new generation of microchip production. In this letter we report on the record-performing superconducting RF electron gun with $textrm{CsK}_{2}textrm{Sb}$ photocathode. The gun is generating high charge electron bunches (up to 10 nC/bunch) and low transverse emittances, while operating for months with a single photocathode. This achievement opens a new era in generating high-power beams with a very high average brightness.
Proposed fourth generation light sources using SASE FELs to generate short pulse, coherent, X-rays require demonstration of high brightness electron sources. The Gun Test Facility (GTF) at SLAC was built to test high brightness sources for the proposed Linac Coherent Light Source at SLAC. The transverse emittance measurements are made at nearly 30 MeV by measuring the spot size on a YAG screen using the quadrupole scan technique. The emittance was measured to vary from 1 to 3.5 mm-mrad as the charge is increased from 50 to 350 pC using a laser pulse width of 2 ps FWHM. The measurements are in good agreement with simulation results using the LANL version of PARMELA.
Future colliders that require low-emittance highly-polarized electron beams are the main motivation for developing a polarized rf gun. However there are both technical and physics issues in generating highly polarized electron beams using rf guns that remain to be resolved. The PWT design offers promising features that may facilitate solutions to technical problems such as field emission and poor vacuum. Physics issues such as emission time now seem to be satisfactorily resolved. Other issues, such as the effect of magnetic fields at the cathode-both those associated with the rf field and those imposed by schemes to produce flat beams-are still open questions. Potential solution of remaining problems will be discussed in the context of the PWT design.