Operational experience with Crab Cavities at KEKB
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Transcript of Operational experience with Crab Cavities at KEKB
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Operational experience with Crab Cavities at KEKB
Y. FunakoshiKEK
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KEKB B-FactorySuperconducting
cavities (HER)
e-
e+
ARES copper cavities (LER)
8 GeV e- 3.5 GeV e+ Linac e+ target
ARES copper cavities (HER)
Belle detector
KEKB B-Factory
TRISTAN tunnel
♦World-highest Peak Luminosity• 2.11 x 1034cm-2s-1
• Twice as high as design value
♦World-highest Integrated Luminosity• Total: 1041fb-1 as of June 30th 2010
Crab cavities1 for each ring
♦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.
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(mA)
Motivation of crab cavity at KEKBHead-on (crab)
First proposed by R. B. Palmer in 1988 for linear colliders.
Crab Crossing can boost the beam-beam parameter higher than 0.15 ! (K. Ohmi)
22mrad crossing angle
Strong-strong beam-beam simulation
nx =.508
Head-on } y ~0.15
After this simulation appeared, the development of crab cavities wasrevitalized.
Luminosity would be doubled with crab cavities!!!
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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
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Input couplerMagnetic Shield ( Jacket Type )
80 K LN2 Radiation Shield
Coaxial Coupler
Stub Support
BellowsLiq. HeMonitor Port
RF Absorber
Frequency Tuningby Adjusting Distance
Crab Mode Reject Filter
RF Absorber
I.D. 240 I.D.100
Liq. He
Structure of KEKB Crab Cavity
Top View
Crab mode: TM110: By on beam axisLower mode: TM010: dumped through coaxial couplerSquashed cell shape to split TM110 modes
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Skew-sextupoles
Beam lifetime problem
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Evidence of crabbing motion (1): Streak camera
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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 of
Slope:
T. Ieiri
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( ): design of KEKB
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Horizontal Tune close to half-integer
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Efforts to improve performance with crab cavities
1. Beam lifetime problem2. Introduction of skew-sextupole
magnets
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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
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Dynamic-b and dynamic emittanceby beam-beam (calculation)
LER
The focusing force of the beam-beaminteraction not only squeezes thebeam at the interaction point, butincreases the emittance drastically.
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Optics deformation with dynamic beam-beam effect (LER)
Black: w/o beam-beamRed: with beam-beam
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Beta’s with dynamic beam-
beam effect(LER)
before optics change bx*= 0.9m
After optics change bx*= 0.9m
crab
crab
with/without beam-beam effects
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Tuning with skew-sextupole magnets
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• 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%.
Chromaticity of x-y coupling at IP
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D. Zhou, K. Ohmi, Y. Seimiya,
Luminosity
Horizontal size
Vertical size
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Definition of x-y coupling parameters (SAD notation)
Normal (decoupled)coordinate
Usual coordinate
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Examples of scan of chromatic x-y coupling at IP
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2005/June/06
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Measurement on chromaticity of x-y coupling at IP (HER)
blue: without skew-sextuples
red: with skew-sextuples (after luminosity tuning)
Y. Ohnishi
dotted line: model opticswithout machine errors
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Effectiveness of skew-sextupole magnets (crab on)
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Effectiveness of skew-sextupole magnets (crab off)
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Machine parameters (before/after crab)
Date Nov.15 2006before crab
Jun. 17 2009with crab
LER HER LER HERCurrent 1.65 1.33 1.64 1.19 ABunches 1389 1584
Bunch current 1.19 0.96 1.03 0.750 mAspacing 2.10 1.84 mA
emittance εx 18 24 18 24 nmβx* 59 56 120 120 cmβy* 6.5 5.9 5.9 5.9 mm
σx @IP 103 107 147 170 μmσy @IP 1.8 1.8 0.94 0.94 μm
νx 45.505 43.509 45.506 44.511νy 44.534 41.565 43.561 41.585νs -0.0246 -0.0226 -0.0246 -0.0209
beam-beam ξx 0.117 0.070 0.127 0.102beam-beam ξy 0.108 0.058 0.129 0.090
Luminosity 17.6 21.08 1033cm-2s-1
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Effect of the crab cavities on the luminosity and the beam-beam
parameter
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Specific luminosity (crab on/off)
Luminosity improvement by crab cavities is about 20%.Geometrical loss due to the crossing angle is about 11%.
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Beam-beam parameter (crab on/off)
Crab on Crab off
RL 0.828 0.763
Ry(HER) 1.15 0.993
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Calculation of beam-beam parameter
• Reduction factor for beam-beam parameter
– 2 sources of reduction• hourglass effect and finite crossing angle
Montague’s factor
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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.
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Beam-beam parameters crab on/off
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Possible causes of lower luminosity with crab than simulations
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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?
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Crab phase error
Span 200 kHzSideband peaks at 32kHz and 64kHz.
Span 10 kHz Span 500 HzSideband peaks at 32, 37, 46, 50, 100 Hz.
• 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).
Spectrum around the crabbing mode measured at a pick up port of the LER crab cavity. Beam current was between 450 and 600 mA.
LER crab phase
HER crab phase
± 1 deg
Phase detector signal. Beam current was 385mA (HER) and 600 mA (LER).
K. Akai
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2005/June/06
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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.
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IP coupling and dispersion tuning knob
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Side effect of a large tuning knob
iSize Bump 3 mm to check response of size monitor
R2=37.29 units
R2=-21.36 units
10 units
Original settingR2=7.99
Knob zero
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.
LER R2
Vertical beam size [m
]
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Strong-weak beam-beam simulation with lattice nonlinearity
K. Ohmi
The strong-weak beam-beam simulation was done using SAD with full lattice (head-on).Sextupole mis-alignments were introduced so that the emittance ratio (V/H) of ~ 1% was created.
Strong-strong (head-on)
Strong-strong (22mrad crossing angle)
experiment
With IP x-y coupling and vertical dispersionCorrect IP coupling and dispersion
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Strong-weak beam-beam simulation with lattice nonlinearity
K. Ohmi
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.
Head-on
Crab crossing
Crab crossing(vertical crab)
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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.
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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.
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Spare slides
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Phase stability
Span 200 kHzSideband peaks at 32kHz and 64kHz.
Span 10 kHz Span 500 HzSideband peaks at 32, 37, 46, 50, 100 Hz.
• 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).
Spectrum around the crabbing mode measured at a pick up port of the LER crab cavity. Beam current was between 450 and 600 mA.
LER crab phase
HER crab phase
± 1 deg
Phase detector signal. Beam current was 385mA (HER) and 600 mA (LER).
K. Akai
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LER Nikko New Optics
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
crab crabβx @ crab ~85 m as is before
βx
βy
H. Koiso
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QC2L
QC2R
QW4NP.1
QW4NP.2
QW4OP.2
QW4OP.1
MD06H1
MD06H2
MD06H3
MD06H4
MD03H1
MD03H2
MD03H3
MD03H4
Beam size calculation with dynamic beam-beam effects
@crab (aperture < 5 x)
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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
2nx + 2ns = integer
LERHER
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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 = integer2νx + 2νs = integer
2νx - νs = integer2νx - 2νs = integer
νx: .5112, .5224with given νs ~ -.0224
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Comparison of beam lossKEKB (design) KEKB (achieved) SuperKEKB
LER HER LER HER LER HER
Radiative Bhabha 21.3h 9.0h 6.6h 4.5h 28min. 20min.
Beam-gas 45ha) 45ha) 24.5min.b) 46min. b)
Touschek 10h - 10min. 10min.Total 5.9h 7.4h ~133min. ~200min. 6min. 6min.Beam current 2.6A 1.1A 1.6A 1.1A 3.6A 2.6A
Loss Rate 0.12mA/s 0.04mA/s 0.23mA/s 0.11mA/s 10mA/s 7.2mA/s
a) Bremsstrahlungb) Coulomb scattering, sensitive to mask setting
Beam loss accompanied with the beam injection should be added.