Abstract

The high intensity proton beam for LHC accelerated in the CERN SPS is stabilised against coupled-bunch instabilities by a 4th harmonic RF system in bunch-shortening mode. Bunch-lengthening mode, which could also be useful to reduce peak line density and alleviate problems from e-cloud and kicker heating, does not give desirable results for beam stability. In this paper an analysis of the limitations of these two different modes of operation is presented together with measurements of the Beam Transfer Function for the double RF system. As predicted by theory, for sufficiently long bunches with the same noise excitation, the measured amplitude of the beam response in bunch-lengthening mode is an order of magnitude higher than that for bunch-shortening mode or for a single RF system.

Highlights

  • The high intensity proton beam for LHC accelerated in the CERN SPS is stabilised against coupled-bunch instabilities by a 4th harmonic RF system in bunch-shortening mode

  • Double RF systems are widely used for high intensity beams to modify the particle or synchrotron frequency distribution inside the bunch

  • In recent years a high intensity beam was accelerated in the CERN SPS in preparation for both the LHC and CNGS projects

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Summary

BEAM STABILISATION

Double RF systems are widely used for high intensity beams to modify the particle or synchrotron frequency distribution inside the bunch. In low energy proton rings the higher harmonic RF system is usually added to flatten bunches to reduce the space charge effects This mode of operation gives an increase in synchrotron frequency spread ∆ωs by decreasing the zero-amplitude frequency ωs(0) (for maximum effect to zero) [1], and is often called bunch-lengthening (BL) mode. Despite the fact that for the LHC beam the instability threshold is five times below the nominal bunch intensity, in 2003 the nominal longitudinal parameters were obtained at top energy, 450 GeV [2]. At the end of the 2004 beam run we used the 26 GeV/c flat bottom in the SPS, where accurate adjustment of the phase shift Φ2 between the two RF systems could be made, to study the effect of BL-mode on the 75 ns spaced LHC beam which suffers less from beam loading than the nominal 25 ns spaced beam. Measurements of BTF, presented point to a local loss of Landau damping, typical for BL-mode, as a probable explanation for this reduction of beam stability threshold

BEAM TRANSFER FUNCTIONS
Bunch Length ns
Phase deg
Findings
BL mode
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