High current cavity design at BNL

In order to meet the requirements of high average current accelerators, such as the Superconducting Proton Linac (SPL) at CERN and the electronion collider, (MeRHIC) at BNL, a high current 5-cell Nb superconducting cavity, called BNL3 cavity, was optimized and designed. The optimization process aimed at maximizing the R/Q of the fundamental mode and the geometry factor G under an acceptable RF field level of B_peak/E_acc or E _peak/E_acc. In addition, a pivotal consideration for the high current accelerators is efficient damping of dangerous Higher-Order Modes (HOM) to avoid inducing emittance degradation, cryogenic loading or Beam-Break-Up (BBU). To transport the HOMs out of the cavity, the BNL3 cavity employs a larger beam pipe, allowing the propagation of HOMs but not the fundamental mode. Moreover, concerning the BBU effect, the BNL3 cavity is aimed at low (R/Q)Q_ext for dangerous modes, including dipole modes and quadrupole modes. This paper presents the design of the BNL3 cavity, including the optimization for the fundamental mode, and the BBU limitation for dipole and quadrupole modes. The BBU simulation results show that the designed cavity is qualified for high-current, multi-pass machines such as MeRHIC and by-product light source.