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Operational experience with Crab Cavities at KEKB

Operational experience with Crab Cavities at KEKB. Y. Funakoshi KEK. Superconducting cavities (HER). Belle detector. e-. KEKB B-Factory. e+. ARES copper cavities (HER). ARES copper cavities (LER). TRISTAN tunnel. 8 GeV e- 3.5 GeV e+ Linac. e+ target. KEKB B-Factory. Crab cavities

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Operational experience with Crab Cavities at KEKB

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  1. Operational experience with Crab Cavities at KEKB Y. Funakoshi KEK

  2. Superconducting cavities (HER) Belle detector e- KEKB B-Factory e+ ARES copper cavities (HER) ARES copper cavities (LER) TRISTAN tunnel 8 GeV e- 3.5 GeV e+ Linac e+ target KEKB B-Factory Crab cavities 1 for each ring • ♦World-highest Peak Luminosity • 2.11 x 1034cm-2s-1 • Twice as high as design value • ♦World-highest Integrated Luminosity • Total: 1041fb-1as of June 30th 2010 ♦Crab crossing (f = 11 mrad) ♦Skew-sextupole magnets The KEKB operation was terminated at the end of June 2010 for theupgrade toward SuperKEKB. Operation of SuperKEKB will start in Jan. 2015.

  3. Motivation of crab cavity at KEKB • Crab Crossing can boost the beam-beam parameter higher than 0.15 ! (K. Ohmi) Head-on (crab) Strong-strong beam-beam simulation 22mrad crossing angle Head-on } y ~0.15 (mA) nx =.508 Luminosity would be doubled with crab cavities!!! • After this simulation appeared, the development of crab cavities wasrevitalized. First proposed by R. B. Palmer in 1988 for linear colliders.

  4. Finally two crab cavities were installed in KEKB,one for each ring in January 2007 HER (e-, 8 GeV) LER (e+, 3.5 GeV) …..after 13 years’ R&D from 1994

  5. Structure of KEKB Crab Cavity Top View Input coupler Frequency Tuning by Adjusting Distance Magnetic Shield　（ Jacket Type） RF Absorber RF Absorber Stub Support I.D. 240 Coaxial Coupler I.D.100 Crab mode: TM110: By on beam axis Lower mode: TM010: dumped through coaxial coupler Squashed cell shape to split TM110 modes Crab Mode Reject Filter Liq. He Bellows Monitor Port Liq. He 80 K LN2 Radiation Shield

  6. Skew-sextupoles Beam lifetime problem

  7. Evidence of crabbing motion (1): Streak camera

  8. Crab OFF Crab ON ( Vc = 1.31/0.92 MV) Evidence of crabbing (2): Beam-beam deflection x L/H=18/24 nm Bunch Current: 0.73/0.42 mA Bunch Current: 0.64/0.47 mA Σx_x’=11=230 +/- 3 m (OFF) Σx_x’=00=167 +/- 3 m (ON) Ratio ofSlope:  Horizontaleffective size at IP reduces to 72% by the crab. • HER current was lost from 15 to 13.5 mA during scan. T. Ieiri

  9. ( ): design of KEKB

  10. Horizontal Tune close to half-integer

  11. Efforts to improve performance with crab cavities Beam lifetime problem Introduction of skew-sextupole magnets

  12. Beam lifetime problem • Observation • When the HER bunch current increased, the LER beam lifetime decreased. The HER bunch current was limited by this phenomena. • It turned out that the lifetime decrease was caused by dynamic beam-beam effects related to the physical aperture at the crab cavity. • Cures • Change linear optics near crab cavities. • Enlarge IP horizontal beta functions which enabled us to decrease horizontal beta functions at crab cavities. • bx*: 0.8 or 0.9m to 1.2 or 1.5m

  13. Dynamic-b and dynamic emittanceby beam-beam (calculation) LER The focusing force of the beam-beam interaction not only squeezes the beam at the interaction point, but increases the emittance drastically.

  14. Optics deformation with dynamic beam-beam effect (LER) Black: w/o beam-beam Red: with beam-beam

  15. Beta’s with dynamic beam-beam effect(LER) crab before optics change βx*= 0.9m with/without beam-beam effects crab After optics change βx*= 0.9m

  16. Tuning with skew-sextupole magnets

  17. Chromaticity of x-y coupling at IP • Ohmi et al. showed that the linear chromaticity of x-y coupling parameters at IP could degrade the luminosity, if the residual values, which depend on machine errors, are large. • To control the chromaticity, skew sextupole magnets were installed during winter shutdown 2009. • The skew sextuples are very effective to increase the luminosity at KEKB. • The gain of the luminosity by these magnets is ~15%.

  18. D. Zhou, K. Ohmi, Y. Seimiya, Vertical size Horizontal size Luminosity

  19. Definition of x-y coupling parameters (SAD notation) Normal (decoupled)coordinate Usual coordinate

  20. Examples of scan of chromatic x-y coupling at IP

  21. 2005/June/06

  22. Measurement on chromaticity of x-y coupling at IP (HER) blue: without skew-sextuples red: with skew-sextuples (after luminosity tuning) dotted line: model opticswithout machine errors Y. Ohnishi

  23. Effectiveness of skew-sextupole magnets (crab on)

  24. Effectiveness of skew-sextupole magnets (crab off)

  25. Machine parameters (before/after crab)

  26. Effect of the crab cavities on the luminosity and the beam-beam parameter

  27. Specific luminosity (crab on/off) Luminosity improvement by crab cavities is about 20%. Geometrical loss due to the crossing angle is about 11%.

  28. Beam-beam parameter (crab on/off)

  29. Calculation of beam-beam parameter • Reduction factor for beam-beam parameter • 2 sources of reduction • hourglass effect and finite crossing angle Montague’s factor

  30. Calculation of beam-beam parameter [cont’d] • Reduction factor for luminosity • Luminosity • We use calculated values for x* and calculatey* and y0 from observed luminosity.

  31. Beam-beam parameters crab on/off

  32. Possible causes of lower luminosity with crab than simulations

  33. Possible causes of lower luminosity with crab than simulations • Short beam lifetime prevents us from approaching a better parameter set? • Synchro-betatron resonance lines near half-integer? • Vertical crab? • Beam-beam + impedance? • Some fast noise? • Too many tuning parameters to find out an optimum set of parameters? • Crosstalk between beam-beam and lattice nonlinearity?

  34. LER crab phase ± 1 deg HER crab phase Crab phase error • Spectrum of pick up signal is consistent with phase detector data. • Phase fluctuation faster than 1 kHz is less than ± 0.01°, and slow fluctuation from ten to several hundreds of hertz is about ± 0.1°. • They are much less than the allowed phase error obtained from the beam-beam simulations for the crabbing beams in KEKB. According to b-b simulation by Ohmi-san, allowed phase error for N-turn correlation is 0.1×√N (degree). Span 500 Hz Sideband peaks at 32, 37, 46, 50, 100 Hz. Span 10 kHz Span 200 kHz Sideband peaks at 32kHz and 64kHz. Phase detector signal. Beam current was 385mA (HER) and 600 mA (LER). Spectrum around the crabbing mode measured at a pick up port of the LER crab cavity. Beam current was between 450 and 600 mA. K. Akai

  35. 2005/June/06

  36. Too many tuning parameters? • Many tuning parameters used to be tuned one-by-one by scans with only observing the luminosity and the beam sizes. • There was a possibility that we could not reach an optimum set of parameters with this method. • A method of parameter search based on downhill simplex method was developed in 2007, which can search 12 parameters simultaneously. However, with this method the achievable luminosity was not improved.

  37. IP coupling and dispersion tuning knob

  38. Side effect of a large tuning knob R2=37.29 units R2=-21.36 units LER R2 10 units Vertical beam size [mm] iSize Bump 3 mm to check response of size monitor Knob zero Original setting R2=7.99 • The vertical beam size is enlarged due to the side effect of a large tuning know such as 30 units of R2 knob. • If a large x-y coupling remains at IP, the luminosity degradation may not be recovered by the tuning knobs.

  39. K. Ohmi Strong-weak beam-beam simulation with lattice nonlinearity With IP x-y coupling and vertical dispersion Correct IP coupling and dispersion Strong-strong (head-on) Strong-strong (22mrad crossing angle) experiment The strong-weak beam-beam simulation was done using SAD with full lattice (head-on). Sextupolemis-alignments were introduced so that the emittance ratio (V/H) of ~ 1% was created.

  40. Strong-weak beam-beam simulation with lattice nonlinearity K. Ohmi Head-on Crab crossing Crab crossing(vertical crab) The crab cavities were implemented in the strong-weak beam-beam simulation (crab crossing). We found a remarkable difference between the head-on and crab crossing.

  41. Summary • The crab cavities were installed in Feb. 2007 at KEKB and worked very well until the end of the KEKB operation. • The highest luminosity with the crab cavities is about 23% higher than that before crab (prediction by b-b simulation: ~100% increase). • The tuning with skew-sextupole magnets were effective to increase the luminosity w/ crab (~15% gain). • We found that the skew-sextupole magnets are also effective to increase the luminosity when the crab cavities were switched off.

  42. Summary [cont’d] • The luminosity difference between the crab on and off was about 20% and is larger than the geometrical loss of the luminosity due to the crossing angle (~11%). • The achieved vertical beam-beam parameter was ~ 0.09. This value is rather high but is lower than the predicted value by the beam-beam simulation (~0.15). • Possible causes for this discrepancy: • Large machine errors which can not compensated by the usual tuning knobs? • Lattice nonlinearity + beam-beam? • We are still studying this issue.

  43. Spare slides

  44. LER crab phase ± 1 deg HER crab phase Phase stability • Spectrum of pick up signal is consistent with phase detector data. • Phase fluctuation faster than 1 kHz is less than ± 0.01°, and slow fluctuation from ten to several hundreds of hertz is about ± 0.1°. • They are much less than the allowed phase error obtained from the beam-beam simulations for the crabbing beams in KEKB. According to b-b simulation by Ohmi-san, allowed phase error for N-turn correlation is 0.1×√N (degree). Span 500 Hz Sideband peaks at 32, 37, 46, 50, 100 Hz. Span 10 kHz Span 200 kHz Sideband peaks at 32kHz and 64kHz. Phase detector signal. Beam current was 385mA (HER) and 600 mA (LER). Spectrum around the crabbing mode measured at a pick up port of the LER crab cavity. Beam current was between 450 and 600 mA. K. Akai

  45. LER Nikko New Optics H. Koiso βx βy crab crab βx @ crab ~85 m as is before Before 2/21(maintenance) βx max 199 m (νx,νy)=(45.505,43.59) After 2/21(maintenance) βx max 91 m (νx,νy)=(44.505,43.59) Large β distortion in wiggler section

  46. Beam size calculation with dynamic beam-beam effects @crab (aperture < 5 sx) QW4NP.1 QC2R QW4OP.2 QC2L QW4NP.2 QW4OP.1 MD03H1 MD03H3 MD06H3 MD06H4 MD03H2 MD06H1 MD03H4 MD06H2

  47. Synchro-betatron resonance • The horizontal tune is set nearby the half integer resonance and its synchrotron side bands. • On the resonances, some harmful effects are observed. • Single-beam beam size blowup • Tow-beam beam size blowup • Beam lifetime reduction (or beam loss) • The resonance is stronger in HER where no local chromaticity correction is installed. • Strength of the resonance is strongly dependent on a choice of sextupole setting. The luminosity also changes by changing the sextupole setting. • Even in the off resonance tunes, it affects the luminosity due to the tune footprint by the beam-beam? 2nx + ns = integer HER LER 2nx + 2ns = integer

  48. Negative- Optics • Motivation • To weaken the synchro-betatron resonance particularly in HER • To shorten the bunch length • Results • We have succeeded to weaken the synchro-betatron resonance line in HER. We could operate the machine with nx below the resonance line. • We have successfully shorten the bunch length of both beam. • ~6mm -> ~4.5mm • However, we found unexpectedly large synchrotron oscillation in LER (microwave instability) and gave up the trial of the negative- optics. 2νx + νs = integer 2νx + 2νs = integer 2νx - νs = integer 2νx - 2νs = integer νx: .5112, .5224 with given νs~ -.0224

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