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UBC Theses and Dissertations

The TRIUMF nine-cell SRF cavity for ARIEL Kolb, Philipp Ulrich


Modern physics research relies on particle accelerators and available beam time is a very limited resource. The ARIEL eLINAC will strengthen the rare isotope program at TRIUMF by providing an alternative way to create rare isotope beams (RIB). A possible way to add additional use to this machine is to create a return beam line and use the beam to excite a free electron laser (FEL). The remaining beam can be used to drive fields in the SRF cavities to reduce the required RF power. One limitation of these energy recovery LINACs (ERL) is beam-break up. Higher order modes (HOM), especially dipole modes, have a negative influence on the beam which can lead to beam loss. The design of the SRF cavity has to accommodate this to make sure a beam current of up to 10mA can be used for both RIB production and ERL operation. This thesis will go through the design process of the ARIEL 1.3 GHz nine-cell cavity. The design relies on simulations to calculate the fields inside the cavity and with it the shunt impedance of HOMs. The investigations showed that resistive beam line absorbers can be used to reduce the shunt impedance of HOMs sufficiently without interfering with the accelerating mode. The performance of the absorber material has been verified in dedicated low temperature measurements, while the HOM field distribution has been measured via beadpulling on a copper model of the cavity. These measurements showed good agreement with the simulations. The power dissipation in the SRF cavities is of vital importance. The cryogenic system is a signicant part of the capital investment for the accelerator and sets the power budget for each cavity to around 10 W. This corresponds to a Q₀ value of 1 x 10¹⁰ at an operational temperature of 2 K. The gradient goal is 10 MV/m to reach the design energy of 50 MeV with five cavities. Both Q₀ and Eacc specifications have been met in the first two cavities that are installed in cryomodules. Two more cavities have been built and are in their qualification phase.

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