Summary: Observations of Lepton Colliders

John Seeman March 22, 2013

[ICFA Mini-Workshop on Beam-Beam Effects in Hadron Colliders]

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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

e+e- Collider: New Collision Concepts

1.Round Beams

2.Crab Crossing

3.Large Piwinski Angle

4.Strong RF Focusing

5.Traveling Waist

6.Crab Waist

Tested at VEPP2000, CESR

Tested at KEKB

Tested at DAFNE

[M. Zobov]

SuperKEKB, DAFNE

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Funakoshi: Crab Cavities in KEKB at 2 x 11 mrad crossing

Crab On

Crab off

From simulation BB Parameter increases 0.06 0.15

(Ohmi)

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

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KEKB: Evidence of crabbing (2): Beam-beam deflection

Bunch Current: 0.73/0.42 mA Bunch Current: 0.64/0.47 mA

HER current was lost from 15 to 13.5 mA during scan.

Crab ON ( Vc = 1.31/0.92 MV)Crab OFF

Horizontal effective size at IP reduces to 72% by the crab.

Σx_x’=00=167 +/- 3 m (ON)

x L/H=18/24 nm

Σx_x’=11=230 +/- 3 m (OFF)Ratio ofSlope:

T. Ieiri

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.

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%.

KEKB: Chromaticity of x-y coupling at IP

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 >> 1F

3.Larger crossing angle q

4.Longer bunch length sz

5.Strong nonlinear elements (sextupoles)

x

y 2

x

y 2

Crabbed Waist Scheme

x

x

yy

K

*

*

1

2

1

Sextupole (Anti)sextupole

20 2

1yxpHH

Sextupole strength Equivalent Hamiltonian

IPyx , yx ,** ,

yx

*

2* /

yyy

xs

The DANE ColliderEnergy per beam 510 [MeV]

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[kV]

Harmonic number 120

Bunch spacing 2.7[ns]

Max ach. Luminosity(SIDDHARTA run) 4.51032 [cm-2s-1]

BTF

DANE Peak Luminosity

NEW COLLISION SCHEME

Des

ign

Goa

l

Crab Waist

Luminosity [1028 cm-2 s-1] Luminosity [1028 cm-2 s-1]

I I N harmonicN bunches

[A2]

I I N harmonicN bunches

[A2]

Comparison Among DAFNE Best Runs with and without Crab-Waist

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.

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

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BEPC-II

19

BEPC-II

20

BEPC-II

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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 xy > 0.04.

6) z location of collision point

Dmitry ShwartzBINP, Novosibirsk

March 18, 2013

ICFA Mini-Workshop on Beam-Beam Effects in Hadron Colliders

Recent Beam-Beam effects at VEPP-4M & VEPP-2000

VEPP-4M luminosity

VEPP-2000: Motivation of round beam use in an e+e- collider

Number of bunches (i.e. collision frequency) Bunch-by-bunch luminosity

2

*2

2

1

x

y

ye

xyx

r

fL

Round Beams:

Geometric factor:

Beam-beam limit enhancement:

IBS for low energy? Better life time!

21 / 4y x

0.1

2 2

2

4

e

fL

r

The Concept of Round Colliding Beams

x y

x y

x y

Head-on collisions

Small and equal β-functions at IP:

Equal beam emittances:

Equal fractional parts of betatron tunes:

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 1D.

V.V.Danilov et al., EPAC’96, Barcelona, p.1149, (1996)

Round beam

Mx = My

“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

Main parameters @ 1GeV

Circumference 24.388 m Energy 200 1000 MeV

Number of bunches 1 Number of particles 11011

Betatron tunes 4.1/2.1 Beta-functions @ IP 8.5 cm

Beam-beam parameter 0.1 Luminosity 11032 cm-2s-1

13 T final focusing solenoids

VEPP-2000: Beam current vs. energy

Luminosity & “real” bb-parameter

nom

*

*24e nom

nomnom

N r

*

*24e nom

lumilumi

N r

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: 11031 cm-2s-1 at φ-meson energy in 2008 run and 2.51031 cm-2s-1 at E=850 MeV in 2012.

• Potentially 21031 cm-2s-1 possible at φ and 1.61032 cm-2s-1 at 2 GeV.

• 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.

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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

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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.