Single shot cathode transverse momentum imaging in high brightness photoinjectors

Abstract

In state of the art photoinjector electron sources, thermal emittance from photoemission dominates the final injector emittance. Therefore, low thermal emittance cathode developments and diagnostics are very important. Conventional thermal emittance measurements for the high gradient gun are time-consuming and thus thermal emittance is not measured as frequently as quantum efficiency during the lifetime of photocathodes, although both are important properties for the photoinjector optimizations. In this paper, a single shot measurement of photoemission transverse momentum, i.e., thermal emittance per rms laser spot size, is proposed for photocathode RF guns. By tuning the gun solenoid focusing, the electrons' transverse momenta at the cathode are imaged to a downstream screen, which enables a single shot measurement of both the rms value and the detailed spectra of the photoelectrons' transverse momenta. Both simulations and proof of principle experiments are reported.

Eliminating uncertainty of thermal emittance measurement in solenoid scans due to rf and solenoid fields overlap

“…Method Two uses the beam dynamics simulation to replace the transfer matrix calculation to obtain the thermal emittance [23]. A start-to-end simulation from the cathode to the screen is performed using a beam dynamics simulation code like ASTRA [24], OPAL [25] or GPT [26], etc.…”

Review of photocathodes for electron beam sources in particle accelerators

“…96 However, the thermal emittance of Cs 2 Te was measured by P. Huang and reached about 1.1 π mm mrad mm −1 in the warm RF gun at 40 MV m −1 and at 257 nm of the drive laser. 97…”

“…96 However, the thermal emittance of Cs 2 Te was measured by P. Huang and reached about 1.1 p mm mrad mm À1 in the warm RF gun at 40 MV m À1 and at 257 nm of the drive laser. 97 5.1.4 Lifetime. The vacuum conditions have a high impact on the lifetime of the cathodes.…”

Analysis of photoinjector transverse phase space in action and phase coordinates

“…The photoelectrons are generated by 257.5 nm UV photoemission from Cs 2 Te cathode. The typical quantum efficiency and intrinsic emittance of the fresh Cs 2 Te cathode are 10-30% and 1 µm/mm respectively [26][27][28]. The UV laser can be shaped both spatially and temporally for emittance optimization [29,30].…”

“…Without reducing the beam peak current in the injector, the beam brightness can be further enhanced by improving the cathode thermal emittance. Our Cs 2 Te cathode typically starts with a high fresh QE above 10%, and its life time above 1% QE was demonstrated to be more than years in the gun [26], but its typical thermal emittance is around 1 µm/mm [27], roughly a factor of 2 higher than that at SwissFEL, whose QE is roughly around 1% [39]. Assuming a 1% level QE, our laser pulse energy still supports 250 pC and 500 pC operation, but the total operation life time of the cathode may become tight compared to a cathode of 10% level QE, leading to more frequent cathode exchanges.…”