Beam simulations with initial bunch noise in superconducting rf proton linacs

Abstract

Circular machines are plagued by coupled bunch instabilities (CBI), driven by impedance peaks, where then all cavity higher order modes (HOMs) are possible drivers. Limiting the CBI growth rate is the fundamental reason that all superconducting rf cavities in circular machines are equipped with HOM dampers. The question arises if for similar reasons HOM damping would not be imperative also in high current superconducting rf proton linacs. Therefore we have simulated the longitudinal bunched beam dynamics in such machines, also including charge and position noise on the injected bunches. Simulations were executed for a generic linac with properties close to the planned SPL at CERN, SNS, or Project X at FNAL. It was found that with strong bunch noise and monopole HOMs with high Q ext large beam scatter, possibly exceeding the admittance of a receiving machine, cannot be excluded. A transverse simulation shows similar requirements. Therefore including initial bunch noise in any beam dynamic study on superconducting rf cavities in high current proton linacs is advisable before envisaging to omit all HOM dampers.

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

“…In this section a brief summary is given of the basics of higher order mode excitation and beam interaction based on [8,[16][17][18]. For simplification, a complex notation for the voltage is used.…”

“…Concerning the induced longitudinal voltages, Eqs. (6)- (8) are also valid for dipole modes under the assumption that x is constant.…”

“…In a real machine bunch noise, such as arrival time errors, energy errors, transverse displacement and tilt, and charge jitter, has to be considered in this context, leading to a much higher voltage as one would expect from assuming a perfect beam off resonance. This can even drive instabilities as shown in [8]. That is why numerical studies have to be performed to estimate the effect of HOMs.…”

“…The particle velocity is assumed constant inside the cavity and the interaction between particle and cavity takes place in the cavity midplane. In [8] the implementation of the beam-HOM interaction used in SMD is described in detail. It is assumed that there is no coupling between the longitudinal and the transverse plane so they can be treated independently.…”

“…Beam instabilities caused by higher order modes (HOMs) are of concern in high power superconducting linacs and have already been treated in various publications [5][6][7][8]. The experience gained at SNS [9,10] raises the question, if dedicated HOM couplers are really needed in machines like SPL, ESS [11,12], or Project X [13,14].…”