Influence of higher order modes on the beam stability in the high power superconducting proton linac

Abstract

Higher order modes (HOMs) can severely limit the operation of superconducting cavities in a linear accelerator with high beam current, high duty factor, and complex pulse structure. The full HOM spectrum has to be analyzed in order to identify potentially dangerous modes already during the design phase and to define their damping requirements. For this purpose a dedicated beam simulation code simulation of higher order mode dynamics (SMD) focused on beam-HOM interaction was developed, taking into account important effects like the HOM frequency spread, beam input jitter, different chopping patterns, as well as klystron and alignment errors. Here, SMD is used to investigate the influence of HOMs in detail in the superconducting proton linac at CERN and their potential to drive beam instabilities in the longitudinal and transverse plane.

Superconducting Hadron Linacs

“…where Q l,n is the loaded Q of mode n. By adding up the excitation caused by the bunches of a complete bunch train, and by taking into account the voltage decay over time, one can find the maximum possible HOM voltage per mode and cavity [78]. If the HOM frequency of a mode with high R/Q coincides with or is close to a machine line (such as a multiple of the bunch frequency or a multiple of any frequency, which is introduced through low-energy beam chopping or beam pulsing), it can yield a significant blowup of the effective beam emittance and even lead to complete beam loss.…”

“…Monopole modes have field components on the cavity axis and are therefore always excited by the beam. In the following we will explain the formalism to understand their excitation, and we refer to the literature for the excitation of dipole modes by off-axis particles [78]. The voltage per HOM (monopole), which is excited by a point charge q, is given by the fundamental theorem of beam loading as…”

Upgrade Possibility of the ESS Linac for the ESSnuSB Project

“…This will decrease the demands for the RF power in each module, results in lower space charge tune shift in the ring and reduces the strain on the accumulator ring design. Also stripping of H⁻ ions due to intra-beam scattering will be reduced, leading to lower beam loss, which has been a problem at SNS [4]. The layout of the accelerator is shown in Figure 2 with different sections of the accelerator indicated, including an upgrade section in the high energy section to 2.5 GeV.…”

Comparison of coaxial higher order mode couplers for the CERN Superconducting Proton Linac study

“…Preceding studies [4,5] of the influence of HOMs on the beam stability in the SPL identified the modes shown in Fig. 2 for the high-beta cavities, which are the focus of this paper.…”

“…2 for the high-beta cavities, which are the focus of this paper. Information about the influence of HOMs with respect to the medium-beta cavities can be found in [4,6].…”