Summary Observations of Lepton Colliders ICFA MiniWorkshop on

Summary: Observations of Lepton Colliders [ICFA Mini-Workshop on Beam-Beam Effects in Hadron Colliders] John Seeman March 22, 2013

Topics Yoshihiro Funakoshi: KEKB Crab Cavities Experience Mikhail Zobov: DAFNE Large Piwinski Angle and Crab Waist Yuan Zhang: Beam-Beam in BEPC-II Dimitry Shwartz: Beam-Beam in VEPP-4 and VEPP-2000 Summary: Overview Observations 2

e+e- Collider: New Collision Concepts 1. Round Beams Tested at VEPP 2000, CESR 2. Crab Crossing Tested at KEKB 3. Large Piwinski Angle Super. KEKB, DAFNE 4. Strong RF Focusing 5. Traveling Waist 6. Crab Waist [M. Zobov] Tested at DA NE

Funakoshi: Crab Cavities in KEKB at 2 x 11 mrad crossing (Ohmi) Crab On Crab off From simulation BB Parameter increases 0. 06 0. 15 4

KEKB: Evidence of crabbing motion (1): Streak camera

KEKB: Evidence of crabbing (2): Beam-beam deflection Crab OFF Crab ON ( Vc = 1. 31/0. 92 MV) x L/H=18/24 nm Bunch Current: 0. 73/0. 42 m. A Bunch Current: 0. 64/0. 47 m. A Σx_x’=11=230 +/- 3 m (OFF) Σx_x’=00=167 +/- 3 m (ON) Horizontal effective size at IP reduces to 72% by the crab. Ratio of Slope: HER current was lost from 15 to 13. 5 m. A during scan. T. Ieiri 6

KEKB: Beam lifetime problem from crab cavities Observation • When the HER bunch current increased, the LER beam lifetime decreased. The HER bunch current was limited by this phenomena physical aperture at the crab cavity. Cures • Change linear optics near crab cavities. And enlarge IP horizontal beta functions.

KEKB: Chromaticity of x-y coupling at IP Ohmi, Cai, 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%.

Skew-sextupoles Beam lifetime problem

KEKB Summary The crab cavities were installed in KEKB in Feb. 2007 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). The skew-sextupole magnets are also effective to increase the luminosity when the crab cavities were switched off.

Zobov (INFN LNF): DAFNE Crab Waist: 1. Small emittance ex 2. Large Piwinski angle F >> 1 3. Larger crossing angle q 4. Longer bunch length sz 5. Strong nonlinear elements (sextupoles)

Crabbed Waist Scheme Sextupole strength IP (Anti)sextupole Equivalent Hamiltonian

The DA NE Collider Energy per beam 510 [Me. V] Machine length 97 [m] Max. beam current (KLOE run) 2. 5(e-) 1. 4(e+) [A] N of colliding bunches 100 RF frequency 368. 67 [MHz] RF voltage 200[k. V] Harmonic number 120 Bunch spacing 2. 7[ns] Max ach. Luminosity (SIDDHARTA run) 4. 5 1032 [cm-2 s-1] BTF

Crab Waist NEW COLLISION SCHEME Design Goal DA NE Peak Luminosity

Comparison Among DA NE Best Runs with and without Crab-Waist Luminosity [1028 cm-2 s-1]

DAFNE: Some Next Steps 1. Mitigate the e-cloud instability by increasing the voltage on clearing electrodes. New power supplies have been acquired. 2. Increase (remove) the single bunch instability threshold in the electron ring - Install modified IR with new bellows Further vacuum chamber check RF voltage variation (decrease? ) Consider lattice with higher momentum compaction 3. Study nonlinear beam dynamics to be able to increase the crab waist sextupoles strength to the nominal values.

Y. Zhang: Beam-Beam Effects in BEPC-II 17

BEPC-II 18

BEPC-II 19

BEPC-II 20

BEPC-II Present Study Topics 1) Lower alpha-p lattice 2) Injection bunches every 2 or 3 buckets versus the design of 4 buckets. 3) Try IP chromaticity improvements 4) Optimize parasitic crossing effects in the non-collision IR. See below. 5) Resolve multi-bunch reduction of luminosity; Try for higher y > 0. 04. 6) z location of collision point 21

Recent Beam-Beam effects at VEPP-4 M & VEPP-2000 Dmitry Shwartz BINP, Novosibirsk March 18, 2013 ICFA Mini-Workshop on Beam-Beam Effects in Hadron Colliders

VEPP-4 M luminosity

VEPP-2000: Motivation of round beam use in an e+e- collider q Number of bunches (i. e. collision frequency) q Bunch-by-bunch luminosity Round Beams: üGeometric factor: üBeam-beam limit enhancement: üIBS for low energy? Better life time!

The Concept of Round Colliding Beams Axial symmetry of counter beam force together with x-y symmetry of transfer matrix should provide additional integral of motion (angular momentum Mz = x’y - xy’). Particle dynamics remains nonlinear, but becomes 1 D. Head-on collisions Small and equal β-functions at IP: Round beam Equal beam emittances: Equal fractional parts of betatron tunes: V. V. Danilov et al. , EPAC’ 96, Barcelona, p. 1149, (1996) Mx = M y

“Weak-Strong” Beam-Beam Simulations I. Nesterenko, D. Shatilov, E. Simonov, in Proc. of Mini-Workshop on “Round beams and related concepts in beam dynamics”, Fermilab, December 5 -6, 1996.

VEPP-2000 layout & parameters 13 T final focusing solenoids Main parameters @ 1 Ge. V Circumference 24. 388 m Energy 200 1000 Me. V Number of bunches 1 Number of particles 1 1011 Betatron tunes 4. 1/2. 1 Beta-functions @ IP 8. 5 cm Beam-beam parameter 0. 1 Luminosity 1 1032 cm-2 s-1

VEPP-2000: Beam current vs. energy

Luminosity & “real” bb-parameter nom

Swartz: VEPP-4 and VEPP-2000 Summary • Round beam scheme is not a bad idea! • Round beam coupling done with the coupling resonance and not skew quadrupoles. • Maximum luminosity achieved: 1 1031 cm-2 s-1 at φ-meson energy in 2008 run and 2. 5 1031 cm-2 s-1 at E=850 Me. V in 2012. • Potentially 2 1031 cm-2 s-1 possible at φ and 1. 6 1032 cm-2 s-1 at 2 Ge. V. • More positrons are required. VEPP-5 injection complex will supply them in the near future. • The weak-strong simulation clearly predicts better lifetime for lower tunes. Dynamic aperture enhancement required to move working point lower as well as to squeeze * at low energy.

Seeman: Overall Observations Lepton e+e- Colliders (1) Tunes: For best collisions: nx = ~0. 505, ny = 0. 512 -0. 538, Crab cavities: Crab cavities tilt bunches as expected at IP. Expected luminosity gains not, so far, fully achieved. Must include dynamic beta effects with respect to ring apertures. Crab cavity trip rates need some additional study. Large Piwinski Angle: Works in a collider. Allows nx >0. 505 Crab waist: Crab waist can improve the luminosity. Effects of crab sextupoles on dynamic aperture needs work. Round beams: Initial beam tests look promising. Additional tolerance studies are needed. IP Design needs more work with round beam constraints 31

Seeman: Overall Observations Lepton e+e- Colliders (2) Lattices: x-y chromatic coupling in the IR is important: skew sextupoles. Sextupole and skew quadrupole coupling corrections in IR More studies of IR error tolerances needed. Instabilities: More work on e-cloud to allow more bunches. Beam-Beam Calculations: Need mores studies of non-linear beam dynamics. Parasitic crossing studies Beam lifetimes: Short beam lifetimes expected in the next collider (~10 minutes) with continuous top-off needed. 32