Abstract
Microwave instability in the Super Proton Synchrotron (SPS) at CERN is one of the main limitations to reach the requirements for the High Luminosity-LHC project (increased beam intensity by a factor 2). To identify the impedance source responsible of the instability, beam measurements were carried out to probe the SPS impedance. The method presented in this paper relies on measurements of the unstable spectra of single bunches, injected in the SPS with the rf voltage switched off. The modulation of the bunch profile gives information about the main impedance sources driving microwave instability, and is compared to particle simulations using the SPS impedance model to identify the most important contributions. This allowed us to identify the vacuum flanges as the main impedance source for microwave instability in the SPS, and to evaluate possible missing impedance sources.
Highlights
Longitudinal instabilities in the Super Proton Synchrotron (SPS) at CERN are a major limitation for future projects, including the High Luminosity LHC (HL-LHC) [1] and the Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) [2]
The measurements of the bunch profile modulation by high frequency impedance sources with the rf voltage switched off allowed the identification of the main contributions driving microwave instabilities in the SPS
Measurements were performed in two optics available in the SPS, and a large bunch profile modulation was measured at 1.4 GHz
Summary
Longitudinal instabilities in the Super Proton Synchrotron (SPS) at CERN are a major limitation for future projects, including the High Luminosity LHC (HL-LHC) [1] and the Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) [2]. This instability is driven by high frequency impedance sources for which: frτL ≫ 1; ð1Þ where fr is the resonant frequency of the impedance and τL is the full bunch length (4σrms is the case of a Gaussian bunch profile). The method presented in this paper consists of measuring the density modulation of long bunches by high frequency impedances with the rf voltage switched off This method was used in the past to identify the main impedance sources responsible for microwave instability on the SPS flat bottom. The present SPS impedance model is evaluated by comparison of macroparticle simulations with measurements
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