Abstract
Beam breakup instability is a potential issue for all particle accelerators and is often the limiting factor for the maximum beam current that can be achieved. This is particularly relevant for Energy Recovery Linacs with multiple passes where a relatively small amount of charge can result in a large beam current. Recent studies have shown that the choice of filling pattern and recirculation scheme for a multi-pass energy recovery linac can drastically affect the interactions between the beam and RF system. In this paper we further explore this topic to study how filling patterns affect the beam breakup instability and how this can allow us to optimise the design in order to minimise this effect. We present a theoretical model of the beam-RF interaction as well as numerical modeling and show that the threshold current can vary by factors of 2-4, and potentially even more depending on the machine design parameters. Therefore a judicious choice of filling pattern can greatly increase the onset of BBU, expanding the utility of future ERLs.
Highlights
In the 2020 European Strategy for Particle Physics [1], superconducting energy recovery linacs (ERLs) [2] were identified as a key accelerator technology requiring priority research and development to underpin the future anticipated needs of the community
There are several factors in multi-turn ERLs that can affect beam breakup (BBU), which are not present in circular machines: bunches at different turns have different energies so under the same higher order mode (HOM) voltage higher energy bunches are deflected less than low energy ones; bunches transitioning from accelerating phase to decelerating phase need to be delayed by at least half an rf cycle and have a changed revolution time; bunches accumulate offset and kicks within the recirculating process which become amplified as the bunch energy decreases, this is termed adiabatic antidamping
We have investigated the impact of filling pattern choice in a multipass ERL on the regenerative BBU instability
Summary
In the 2020 European Strategy for Particle Physics [1], superconducting energy recovery linacs (ERLs) [2] were identified as a key accelerator technology requiring priority research and development to underpin the future anticipated needs of the community. There are several factors in multi-turn ERLs that can affect BBU, which are not present in circular machines: bunches at different turns have different energies so under the same HOM voltage higher energy bunches are deflected less than low energy ones; bunches transitioning from accelerating phase to decelerating phase need to be delayed by at least half an rf cycle and have a changed revolution time; bunches accumulate offset and kicks within the recirculating process which become amplified as the bunch energy decreases, this is termed adiabatic antidamping All these effects must be taken into account when determining filling pattern dependent BBU threshold currents. We are able to show that with the correct choice of filling pattern, the threshold current can be increased significantly with appropriate optimization
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