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 rf-accelerators. 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 space-charge 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.