Abstract
High current storage rings, such as the Z-pole operating mode of the FCC-ee, require accelerating cavities that are optimized with respect to both the fundamental mode and the higher order modes. Furthermore, the cavity shape needs to be robust against geometric perturbations which could, for example, arise from manufacturing inaccuracies or harsh operating conditions at cryogenic temperatures. This leads to a constrained multi-objective shape optimization problem which is computationally expensive even for axisymmetric cavity shapes. In order to decrease the computation cost, a global sensitivity analysis is performed and its results are used to reduce the search space and redefine the objective functions. A massively parallel implementation of an evolutionary algorithm, combined with a fast axisymmetric Maxwell eigensolver and a frequency-tuning method is used to find an approximation of the Pareto front. The computed Pareto front approximation and a cavity shape with desired properties are shown. Further, the approach is generalized and applied to another type of cavity.
Highlights
Accelerating cavities are metallic chambers with a resonating electromagnetic field that are used to impart energy to charged particles in many particle accelerators
In Eq (3), the objective functions corresponding to the minimization of the local sensitivities of frequency of the FM (fFM), fTE111 and fTM110 against geometric changes are denoted by F 6, F 7 and F 8, respectively
The implementation of the optimization algorithm from the previous section is based on a combination of a massively parallel implementation of an evolutionary algorithm (EA) [31,32], written in C þþ and parallelized using MPI, with the axisymmetric Maxwell eigensolver [43]
Summary
Accelerating cavities are metallic chambers with a resonating electromagnetic field that are used to impart energy to charged particles in many particle accelerators. In addition to the maximum achievable Eacc, the surface losses of the cavity should be minimized, which can be achieved by maximizing G · R=Q (where G is the geometry factor) It was shown in [2] that high G · R=Q typically goes along with low Bpk=Eacc, so either of the two can be considered in the optimization. Reaching higher Eacc is precluded due to limitations on the fundamental power coupler in providing high input power per cavity (e.g., for FCC-ee-Z this is around 1 MW per cavity) In such low Eacc and highcurrent operations, minimizing surface peak fields should not be the primary goal. The focus of optimization is on axisymmetric single-cell cavities used in high-current accelerators, e.g., FCC-ee-Z, which unlike the multicell cavities have not been extensively studied before.
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