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E-Cloud Effects in the Proposed CERN PS2 Synchrotron

This workshop at Cornell University presents the results of recent studies on e-cloud build-up and its effects on single-bunch instability in the proposed CERN PS2 synchrotron. The workshop discusses the motivations, outline, and parameters for e-cloud simulations, as well as the effects of e-cloud on beam stability. The conclusions suggest the need for mitigation measures to decrease the effective peak secondary to below 1.3.

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E-Cloud Effects in the Proposed CERN PS2 Synchrotron

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  1. ECLOUD10 Workshshop, Oct. 8-12 Cornell University E-Cloud Effects in the Proposed CERN PS2 Synchrotron M. Venturini, M. Furman, and J-L Vay (LBNL) Work supported by the US DOE and LHC Acceleration Research Program (LARP)

  2. Motivations & Outline • One of the upgrade options considered for future LHC injector complex upgrades entails replacing PS with PS2. • Larger (1.34km circumference) • Higher energy (in@4GeV, extr@ 50GeV) • E-cloud identified as a possible factor limiting machine performance. • We present results of recent studies of • E-cloud build-up (POSINST) • Simulation of effect of e-cloud on single-bunch instability (WARP)

  3. Parameters for E-Cloud Build-up Simulations (POSINST)

  4. BUILD-UP IN FIELD-FREE REGION E-density vs. bunch population for various vacuum chamber sizes • Note roll-over of e-density with increasing bunch-population E-density vs. bunch population for various choices of max energy of yield curve BUILD-UP IN DIPOLE REGION

  5. E-density vs. peak secondary yield for various regions, train structures (extraction) E-density vs. peak secondary yield for various regions, train structures (injection)

  6. Summary of E-cloud Build-upStudies • Results obtained for 6D gaussian beams. Alternate bunch shape (parabolic transverse, flat longitudinal) yields build-up results 5-10% lower

  7. Effects of Ecloud on Beam • Focus on single-bunch e-cloud driven instability • Simulations carried out with WARP • Quasi-static approximation • Transversly uniform ecloud localized at finite no. of stations distributed along the ring (refreshed after each bunch passage) • Smooth approx. for lattice • 6D gaussian beam Selected parameters used in WARP simulations

  8. An fast head-tail like instability develops for sufficiently high e-cloud density • Detect instability by monitoring evolution of transverse emittances and amplitude of transverse centroid offset Vertical dipole moment a long the beam as the instability develops. Snapshots are taken for 5 successive bunch passage starting from turn no. 200 and 600 Evolution of vertical and horizontal emittances for three values of the e-cloud density

  9. Identify instability threshold by recording max. values of bunch emittances, & amplitude of centroid oscillations over 1000 turns (4.5ms). Start simulations with small initial transverse offset x0, y0. Bunch population: 5.9*1011. Negative chromaticities stabilize the motion (PS2 lattice has negative mom. compaction).

  10. To model electrons dynamics in dipoles use Warp option to pin electrons to vertical lines. • This suppresses instability in horizontal plane • Instability threshold in vertical plane somewhat higher (left Fig.) • Reducing bunch population is found to affect instability in vertical plane minimally while increasing stability in horizontal plane (middle Fig.) • At injection instability occurs at higher e-cloud densities (right Fig.)

  11. Conclusions • For vanishing chromaticities instability thresholds in terms of e-density are found to be about 0.4 in the vertical and 0.5 in the horizontal plane (units of 1012 m-3) • Chromaticities=-3 increase threshold by about 50% • Pinning electrons to vertical lines (dipoles) suppresses instability in the horizontal plane. Instability threshold in the vertical plane at re=0.8*1012m-3 • Above values lie in the mid-range of POSINST estimates of e-cloud build-up, suggesting that mitigation measures would have to be taken to decrease effective peak secondary to below 1.3

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