“…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.…”