Implications of incomplete energy recovery in SRF-based energy recovery linacs

The choice of the loaded quality factor (QL) of a superconducting cavity is driven by many factors, including beam loading effects and microphonics. In accelerators with minimal beam loading, use of SRF cavities with relatively high loaded-Q allows one to employ lower power RF sources. Many individuals are therefore considering energy recovered linac designs making use of SRF cavities with loaded-Q values that are primarily limited by microphonic effects. While this is valid for machines which have near-ideal energy recovery, many applications do not necessarily fit this model. In some applications the second pass, energy recovered beam experiences a phase shift between one state of machine operation and a second state. One complication in this process is that the cavity resonance control algorithms are influenced by this phase shift. With respect to RF power requirements, this is a positive interaction inasmuch as the tuner partially compensates for the phase shift of the recovered beam. This work will go through the implications of partial energy recovery on the selection of the loaded-Q for cavity fundamental power couplers