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.
Highlights
Energy recovery linacs (ERLs) open up a new regime of beam parameters with large current and simultaneously small emittances, bunch lengths, and energy spread
The Cornell BNL ERL Test Accelerator (CBETA) is the first accelerator that is constructed to analyze the potential of multipass ERLs with superconducting superconducting radio frequency (SRF) accelerating cavities [1]
Since the beam break-up (BBU) theory derived in [6] assumes a thin-lens cavity, the T12 in the formulas corresponds to the T12 of the recirculation beamline
Summary
Energy recovery linacs (ERLs) open up a new regime of beam parameters with large current and simultaneously small emittances, bunch lengths, and energy spread. CBETA is currently under construction at Cornell University’s Wilson Laboratory This is a collaboration with BNL, and will be the first multipass ERL with a fixed field alternating (FFA) lattice. CBETA will be 4-pass ERL with maximum electron beam energy of 150 MeV This is achieved by first accelerating the electron beam to 6 MeV by the injector (IN). While the beam current in ERLs is no longer limited by the power consumption in the cavities, there will be new, higher limits to the current These are higher order modes (HOMs) heating and the recirculative beam breakup (BBU) instability. With a larger beam current the effect becomes stronger, so BBU is a limiting factor on the maximum achievable current, called the threshold current Ith. With multiple recirculation passes, bunches interact with cavities for multiple times, and the Ith can significantly decrease [6]. Simulations are required to check whether the Ith is above these target values
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