Accelerator, Laser, and Coherent Optoelectronics R&D Lab (ALCOL)

Wake fields are normally produced when a high power particle beam passes through a discontinuous point of the beamline (e.g. stepped beampipe, cavity, cross, bellows, etc). An abrupt change of physical dimension or material property in the beamline can cause impedance mismatch and generate wakefields. If the discontinuity like a cavity strongly resonates with the wake fields with very large Q value, then a significant amount of the wakefield is trapped in the cavity and it will make a serious impact such as emittance growth, phase-space dilution, or even beam breakup (BBU) to subsequent bunches in next cycles if the wakefield does not fully decay before a next bunch arrives at the cavity. It is thus quite essential to estimate line impedances of the beamline components in an accelerator facility before starting high rep. operation. The ASTA 50 MeV injector beamline includes many of beamline components with impedance mismatch. We have analyzed wake potentials and wake impedances of 6-way vacuum cross and bellows with CST Particle Studio, including Wakefield and Particle-In-Cell solvers. We already produced wakefield data out of a bellows model and a simple model like a pillbox cavity is currently being tested with the simulation code for benchmark.