Beam breakup current limit in multiturn energy recovery linear accelerators

Abstract

CBETA, the Cornell BNL Energy-Recovery-Linac (ERL) Test Accelerator, is the first multiturn ERL with superconducting radio frequency (SRF) cavities cavities and will therefore be the first ERL whose current could be limited by the beam break-up (BBU) instability. To investigate the threshold current, we calculate HOMs in CBETA's cavities, including their construction errors, and simulate the BBU threshold current with the established simulation code BMAD. For some construction errors, this current limit is not sufficiently large and means of increasing it are investigated. We find that some means that have been shown effective in single turn ERLs are not practical for multiturn ERLs. To make sure that simulations are trustworthy for CBETA, we revisit the BBU theory for multiturn configurations, and compare to simulation results. We also further explore the scaling law of the BBU threshold current for the case with symmetric ERLs, and a new scaling factor has been found. BBU suppression with lattice chromaticity has been investigated assuming bunches with a Gaussian energy spread. The ability to simulate BBU effect with multiple particles per bunch has been added into BMAD, and agreement with the derived theoretical formula is found. CBETA is currently under beam commissioning at Cornell University. Our simulations show that an optimized optics will push the BBU threshold current beyond the design current, for all realistic cavity production errors.

Implications of beam filling patterns on the design of recirculating Energy Recovery Linacs

“…Sequence Angal-Kalinin et al proposed [10] a similar filling pattern and delay mechanism for the purpose of separating low energy bunches to minimize Beam-Breakup (BBU) instability [28]. BBU is a main limiting factor for the ERL beam current [29] and we will investigate it further in a future study.…”

Implications of beam filling patterns on the design of recirculating energy recovery linacs

“…BBU is a major limiting factor for the ERL beam current [28] and we will investigate it further in a future study.…”

Energy Recovery Linac Based Fully Coherent Light Source

“…The high-quality electron beams with low emittance, low energy spread and especially ultra-short duration (100 fs to several ps) in ERL make it possible to open up new scientific frontiers such as ultrafast dynamics, which is not suitable with storage-ring sources. However, the achieved average current of the ERL (tens mA) [16], mainly limited by the photoinjector and the beam break up criteria in superconducting RF linac (HOM effects) [17,18], is almost one order of magnitude lower than that of conventional storage-ring light source (about several hundred mA) currently achieved, restricting its application at x-ray regime. Different types of light sources have their own pros and cons.…”