Beyond the RF photogun

Laser-triggered switching of MV DC voltages enables acceleration gradients an order of magnitude higher than in state-of-the-art RF photoguns. In this way ultra-short, high-brightness electron bunches may be generated without the use of magnetic compression. The evolution of the bunch during the critical initial part of the acceleration trajectory, the 'pancake' regime, where the space-charge induced deterioration is most severe, is investigated using a simple, but effective analytical model. We find an expression for the maximally achievable peak current that does not depend on the bunch charge. An expression for the normalized emittance is derived, which allows us to calculate the optimal beam radius. It is shown that both the peak current and the transverse emittance required for the most challenging applications can be attained without magnetic compression, if acceleration gradients of 1 GV/m can be realized. The results are confirmed by simulations with the GPT code, assuming a 1 GV/m acceleration field and a 50 fs laser pulse, generating 100 pC of charge. The model is complementary to simulations in the sense that it supplies useful scaling laws and improved understanding of the physics involved. Interestingly, we find that the highest brightness is achieved with the shortest photoemission laser pulses.