Investigation of impact of Alvarez-type cavity RF-field tuning on final beam quality using a longitudinal tracking method

Tuning of the RF-field quality of accelerating cavities aims firstly at matching the cavity’s eigen-frequency to the one of the high frequency power amplifiers. Additionally, it is required to assure that the individual gap voltages match to those anticipated by the beam dynamics design. Tuning both cavity properties is a must-have during RF-commissioning of each linear accelerator section. Although voltage gap tuning has been investigated w.r.t. its impact on the final output beam quality, the variation of the gap-transient factors through the tuning has not been taken into account for the time being. Beam dynamics codes implicitly assume that for the mis-tuned cavity the gap transient-time factors are equal to those for the designed cavity. Since the factors depend on the actual RF-phase and particle energy, this assumption is too strong and may lead to considerable differences in simulated and finally achieved longitudinal beam parameters. Applying a novel longitudinal tracking method (Xiao et al., 2019), the effect of proper cavity tuning on the output longitudinal emittance has been investigated quantitatively for an Alvarez-type drift tube LINAC, being currently constructed at GSI. It revealed that according proper tuner tuning can avoid longitudinal emittance growth of 39 %.