Beam-based alignment of SRF cavities in an electron injector linac

Proper alignment of accelerating cavities is an important issue concerning beam quality of linear rfaccelerators. In particular, SRF cavities of injector linacs using high accelerating gradients on low-velocity electron beams can deteriorate the beam quality significantly when not aligned sufficiently. On the other hand, knowing the exact position of every cavity after several cool-down cycles of a cryomodule can be difficult depending on the particular module design. We will report on operational experience on the SC injector of the Darmstadt superconducting recirculating linac and ERL (S-DALINAC) showing unwanted effects on beam dynamics and beam quality due to cavity misalignment, such as transverse beam deflections by changing accelerating phases and a growth of transverse emittance. These effects could already be observed at small beam currents of a few nA, so spacecharge effects were eliminated to be the reason for these observations. In this work we will report on the possibility to realign SRF cavities after cool-down by measuring the transverse deflection of the beam and compare results with beam dynamics simulations.