Abstract
Higher order modes (HOMs) may affect beam stability and refrigeration requirements of superconducting proton linacs such as the Superconducting Proton Linac, which is studied at CERN. Under certain conditions beam-induced HOMs can accumulate sufficient energy to destabilize the beam or quench the superconducting cavities. In order to limit these effects, CERN considers the use of coaxial HOM couplers on the cutoff tubes of the 5-cell superconducting cavities. These couplers consist of resonant antennas shaped as loops or probes, which are designed to couple to potentially dangerous modes while sufficiently rejecting the fundamental mode. In this paper, the design process is presented and a comparison is made between various designs for the high-beta SPL cavities, which operate at 704.4 MHz. The rf and thermal behavior as well as mechanical aspects are discussed. In order to verify the designs, a rapid prototype for the favored coupler was fabricated and characterized on a low-power test-stand.
Highlights
A bunch of charged particles propagating through a cavity will absorb energy from the rf field in order to be accelerated, but it creates so-called wakefields
We have presented a detailed analysis of Higher order modes (HOMs) couplers foreseen for the high-beta Superconducting Proton Linac (SPL) cavities
Several measurements have been performed on copper and niobium cavities in order to characterize and compare their HOM spectra
Summary
A bunch of charged particles propagating through a cavity will absorb energy from the rf field in order to be accelerated, but it creates so-called wakefields. Often it is necessary to install dampers, waveguides, or coaxial couplers to reduce higher order mode (HOM) effects The latter option has been chosen for the Superconducting Proton Linac (SPL) at CERN, Geneva [1]. One of the most important design parameters of a HOM coupler is the external quality factor Qext, which describes the coupling to the modes or their damping by the coupler, respectively. Fundamental frequency f0 Nominal gradient Eacc Required field flatness ΔV=V Quality factor Q0 at nominal gradient Active length L (R=Q) Epeak=Eacc Bpeak=Eacc Geometry factor Number of HOM couplers beta 1⁄4 1, 5-cell standing wave TM010;π mode 704.4 MHz 25 MV=m < Æ2.5% 1010. The recommended damping for monopole HOMs is Qext ∼ 105 [8] This parameter plays an important role for the adjustment (penetration depth of the coupling antenna into the cutoff tube) and will be discussed in Secs.
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