Pulse stacking in the SCA/FEL external cavity

The External Cavity System (ECS) is a passive optical resonator into which we couple the SCA/FEL optical pulse train. We describe measurements of the energy stored in the ECS cavity, where we have routinely observed pulses with 70 times the energy of the incident FEL pulses, near the theoretical maximum for our cavity configuration. We also describe measurements of the stored energyʹs dependence on cavity length and beam steering. Using these results, we evaluate the potential for increasing the energy of optical pulses delivered by the SCA/FEL to experimenters.