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Contributions to the Beam Lifetime

Contributions to the Beam Lifetime. John T. Seeman SLAC Hawaii Super-B-Factory Workshop January, 2004. Lifetime Effects. Quantum lifetime (Not a problem as the RF voltage is high.) Residual gas Bremsstrahlung Elastic scattering Small change as the gas is only a few times worse.

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Contributions to the Beam Lifetime

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  1. Contributions to the Beam Lifetime John T. Seeman SLAC Hawaii Super-B-Factory Workshop January, 2004

  2. Lifetime Effects • Quantum lifetime • (Not a problem as the RF voltage is high.) • Residual gas • Bremsstrahlung • Elastic scattering • Small change as the gas is only a few times worse. • Touschek effect • Luminosity • Bhabha (e+e-  e+e-) • Radiative Bhabha (e+e-  e+e-g) • Beam-beam tune shift

  3. Present PEP-II B Factory with 476 MHz RF Frequency • E- = 9 GeV • E+ = 3.1 GeV • I- = 1.2 A • I+ = 1.8 A • by* = 12 mm • bx* = 30-50 cm • Bunch length = 12 mm • Crossing angle = 0 mrad • Beam-beam parameters = 0.06-0.09 • N = 1317 bunches • L = 7.2 x 1033 cm-2s-1

  4. Advanced B Factory with 952 MHz RF Frequency • E+ = 8 GeV • E- = 3.5 GeV • I+ = 10.1 A • I- = 23.0 A • by* = 1.5 mm • bx* = 15 cm • Bunch length = 1.8 mm • Crossing angle = ~15. mrad • Beam-beam parameters = 0.115 • N = 6900 bunches • L = 1 x 1036 cm-2s-1 • Site power with linac and campus = ~120 MW.

  5. Vacuum lifetimes • At PEP-II the LER vacuum lifetime is about 6 hours at 2 amps. The HER vacuum lifetime is about 12 hours at 1 amp. • Extrapolating the lifetime to a Super B-Factory, we assume ten times the radiation and, thus, ten times the gas generation and three times the pumping/outgassing rate. • Therefore, the vacuum lifetimes will be about three times worse or 2 hours in the LER and 4 hours in the HER.

  6. PEP-II LER Touschek Estimate

  7. Recent: PEP-II LER Touschek Lifetime U. Wienands [J. LeDuff formula]

  8. Touschek lifetime versus RF voltage Smaller bunches Quantum lifetime

  9. Touschek lifetime • Large angle-single-scattering events which change the scattered particle momentum sufficiently to make it fall outside the momentum acceptance of the accelerator. • Changed issues: • Higher bunch currents I (x10) • More bunches n (x4) • Shorter bunches sz (x0.2) • Energy change g (3.1 3.5) (x1.37) N increases from 2A to 20 A in the LER and 1 A to 10 A in the HER. The number of bunches increase a factor of 4. The bunch lengths are reduced from 10 mm to 2 mm. Thus, the Touschek lifetimes decrease by a factor of 10 to 13. The LER Touschek is reduced from 7 hours to 46 mins. The HER Touschek lifetime is reduced from 100 hours to 6 hours.

  10. Luminosity lifetime (e+e- --> e+e-g) • Luminosity lifetime from particle losses from collisions. s = 3 x 10-25 cm-2. • At L = 1036, dN/dt=3.0 x 1011/s = 2.2 x 106/turn. • N = 9 x 1014 or 1.3 x 1011/bunch for 20 A. • At 10A the lifetime is 25 min. At 20 A, 50 min.

  11. Fast Luminosity Monitor Luminosity signal e+e--> e+e-g g Beam loss monitors placed here have measured the luminosity.

  12. Can lifetime be traded for beam-beam parameter? Beam-beam lifetime • The accelerator operators often trade beam lifetime for luminosity (i.e. tune shift) by pushing the tunes. • Here the horizontal tune was changed by -0.002 which lowered the LER lifetime from 165 minutes to 65 minutes but gained over 3% in luminosity. The tune was then restored before the fill was lost. • This effect has not been studied in detail. More studies to follow. Luminosity Time (10 min)

  13. PEP-II Betatron Tune Locations LER HER New tunes Old tunes Old tunes New tunes

  14. PEP-II operates in a beam-beam limited regime Luminosity vs I+I- Specific Luminosity vs I+I- I+I- I+I- Beam-beam parameter limit Background/lifetime limit

  15. Lifetime for 1035 luminosity

  16. Lifetime for 1036 luminosity

  17. Lost particles per second and power loss Without beam-beam tune shift lifetime.

  18. Summary of contributions to the beam lifetime • Touschek, luminosity losses, and beam-beam effects will dominate the lifetimes in the Super-B-Factories. • 1035 beam lifetimes will be about 45 to 60 minutes without beam-beam. • 1036 beam lifetimes will be about 20 minutes without beam-beam. • The lifetimes will be reduce another factor of two if the beam-beam parameters are pushed hard.

  19. Backup slides • Interaction region beta functions • Luminosity gain from low crossing angles.

  20. IR betas for HER and LER LER beta functions HER beta functions

  21. Luminosity enhancement at very small crossing angles PEP-II simulation (Cai) Enhancement only appears at high beam-beam parameters. 0 and 0.75 mrad L Lsp KEKB simulation (Ohmi) I+*I- Experiments on PEP-II planned but not yet done.

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