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Dependence of e-Cloud on the Longitudinal Bunch Profile Studies in the PS & Extension to the HL-LHC

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Chandrashekhara Bhat
Updated by:
Chandrashekhara Bhat
Document Created:
18 Jul 2012, 11:29
Contents Revised:
18 Jul 2012, 11:29
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18 Jul 2012, 11:54
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Even with the nominal LHC beam parameters the e-cloud related issues are currently prohibiting one to go to 25 ns bunch spacing. During the HL-LHC era the bunch brightness will be 4 x the LHC nominal value. Mitigation of the e-cloud effect in the LHC is highly critical. Some early simulation studies on LHC indicated that the longitudinal profiles of the bunches may have effect on growth of e-cloud and bunch lengthening may be a possible mitigation technique. In this regard we have carried out an experiment in the PS and Bench-marking with e-cloud simulations using ECLOUD and PyECloud. Here we clearly see e-cloud growth dependance on the bunch profile; the bunches in double harmonic rf buckets with harmonic radio of two and V2/V1= 0.5 produce about 2.7 times more electrons than those with V2/V1= -0.5 (bunch lengthening mode). A good agreement is seen between PS e-cloud measurements and the simulations for SEY=1.55-1.6 and R=0.55-0.6. Results are extended to the e-cloud scenarios for the HL-LHC. Our study suggest that the e-cloud dependence on the bunch profile is very small (<20%), unlike in the case of PS. For example bunch profile in bunch lengthening mode with the 400MHz+800MHz rf with V2/V1= -0.5 or water-bag distribution (LARP-doc-1065, version 1) yield similar heat-load, about 20% smaller than LHC nominal bunch shapes or bunch shortening mode with V2/V1= 0.5. Hence, shaping the bunch by adding 800 MHz rf in the LHC may not help with mitigation of e-cloud during the HL-LHC. However, this weak dependence of e-cloud on bunch profile is a good news for the foreseen use of Landau cavity to stabilize high intensity beam in the HL-LHC. (The differences in the e-cloud dependence on the bunch-profile between PS and the LHC may be due to the difference in the mechanisms of e-cloud formation; in LHC all of the primary electrons are produced by synchrotron radiation hitting the beam pipe, while, in the PS they are from the gas ionization.) These studies are in progress.
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