Progress in crab
crossing
Mac 2009 Feb. 09
Y. Funakoshi

last MAC

change of correction
factor for on-line CsI
luminosity monitor(+4.5%)

2S
1S

5S

V. Zhilich

Machine operation in
FY 2008

Physics Run (73.5%)
Machine Study (9.8%)
Machine Tuning (1.7%)
Beam Tuning (8.5%)
Troubles (3.5%)
Maintenance (2.7%)
Others (0.3%)

total operation period: 161 days
(3849 hours)

Integrated luminosity: 98.5 fb-1 (FY2008)
Total Integrated luminosity: 894.8 fb-1
needed to be updated

Annual integrated luminosityAveraged Daily Luminosity

Machine parameters
Date

Current

Nov.15 2006
before crab

May 19 2008
with crab

LER

HER

LER

HER

LER

HER

1.65

1.33

1.58

0.839

1.60

0.933

A

0.590

mA

Bunches
Bunch current

Nov. 28 2007
with crab (Last MAC)

1389
1.19

spacing

1584
0.96

0.998

2.10

1584
0.530

1.01

1.84

1.84

mA

emittance εx

18

24

18

24

15

24

nm

βx*

59

56

90

90

90

90

cm

βy*

6.5

5.9

5.9

5.9

5.9

5.9

mm

σx @IP

103

107

127

147

116

147

µm

σy @IP

1.8

1.8

1.1

1.1

1.1

1.1

µm

νx

45.50543.53445.50644.51145.50544.509νy44.50941.56543.57041.59043.56741.596νs-0.0246-0.0226-0.0246-0.0204-0.0240-0.0204

beam-beam ξx

0.117

0.070

0.089

0.098

0.099

0.119

beam-beam ξy

0.108

0.058

0.096

0.089

0.101

0.096

Luminosity

17.6

15.1

16.8

1033cm-2s-1

What are the problems?
Two serious problems(1) Bunch current limitaIon(2) Low speciﬁc luminosity at high bunch currents
βx* = 0.8m
βx* = 1.5m
κ = 1%
βx* = 1.5m
κ = 1.3%

w/o crab
βx* = 0.8m

Machine study
βx* = 1.5m
2SuperKEKB design

Cause of bunch current limitaIon• Physical aperture at the crab caviIes?

–Dynamic beam‐beam eﬀects in the horizontal direcIon• Possible cures–LER:• Reduce the βx around the crab by changing wiring of quadrupole
magnets (actually done in summer break)–Both rings• Raise the crab Vc by lowering crab cooling temperature– We gave up the trial in the autumn run for some technicalreasons (‐> Nakai’s Talk ).
– Raised the HER crab Vc w/o changing temperature (1.343 ‐> 1.5MV)• Increase at βx at IP• Realize the e+/e‐ simultaneous injecIon ( ‐> Iida’s Talk)– enables us to operate the machine with shorter beam lifeIme

Beta’s at LER
crab cavity(w/o beam‐beam)

before summer
βx*=0.9m

crab

crab

this fall
βx*=0.9m

this fall
βx*=1.5m

Beta’s withdynamic beam‐
beam eﬀect

crab

before summer βx*= 0.9m

with/without beam-beam effects
crab

this fall βx*= 0.9m

HER/LER Fast Alternating Beam Injection

N. IidaOn Dec. 12 2008, HER/LER injection could be successfully switched in 2 sec.



The injection rates were as good as usual.

The beam current variations were by one order decreased in both rings.

1.7%

0.19%

2.5%

0.25%

HER
LER

What is the origin of steep slope of
specific luminosity?
• Short beam lifetime– Horizontal offset at IP• Beam current dependent emittance growth in a single beam mode?
• Machine errors– Usual knob tuning is not enough to compensate the machine errors?
• Too many knobs?
– Side effects of large knobs?
• Beam-beam simulation misses something?
– Cross-check the beam-beam simulation code
– Wakefield effect + beam-beam?
– Off-momentum optics play some role to decrease the luminosity?

Horizontal
offset scan
• The (HER) beam current
seems to be limited by
the short life time of
the LER beam.

LER lifetime
Collision center given by thebeam-beam kick

• The (LER) beam lifetime is very asymmetric with respect to H
offset.

Horizontal offset at IP and crossing angle
Beam‐beam simulaIon

Horizontal oﬀset scan:
experiment with relaIvely small beam current

Beam life

Beam size
Luminosity

horizontal offset

 Luminosity boost by crab crossing disappears
with 2 mrad crossing angle.
 Luminosity boost by crab crossing disappears
with ~40 µm horizontal offset.
 Typical value of horizontal offset in physics
experiment is ~15 µm, which is obtained by offset
scan.
 This kind of offset depending on beam current
can degrade the specific luminosity.
 Some luminosity boost by the crab crossing is
actually observed by crab Vc scan.

Crab Vc scan (experiment in physics run)

Bunch current dependenceof vertical beam sizes

Bunch current dependence of vertical beam size was observed in
LER in Apr. 2008.
This can explain the bunch current dependence of the specificluminosity?
More studies were conducted in autumn 2008.

Lifetime issue• Can we store more bunch currents and increase theluminosity by enlarging physical aperture around thecrab cavities?
• βx*=0.9m– The LER beam lifetime seemed to be longer than before
summer.
– The HER beam lifetime was short and the beam loss monitor
near crab had a response to the HER beam life.
– At nominal operation currents, both LER and HER beam
lifetime became short depending on IP horizontal offset.
• We decided to go to βx*=1.5m.

– Trial of larger βy*• βy*=5.9mm -> 7mm: No significant difference was observed.

Lifetime issue [cont’d]• βx*=1.5m–We could successfully store the high bunch
currents corresponding to the SuperKEKB design.
–At I+ x I- ~1.1mA2, no beam lifetime decrease wasobserved. However, the achieved luminosity wasmuch lower than the simulation.
–At I+ x I- ~1.5mA2, beam lifetime decrease in HER
was observed depending on IP horizontal offset.

Aperture survey around HER crab
• Scan of HER Crab Alignment Bump
• Original bump height: ~-6.5 mm necessary to minimize the beam loading• Higher bumps made the lifetime longer (peak at a bump with Δx = ~-4mm).
• Mis-alignment of HER crab cavity?
• After the machine was shutdown, we actually found a mis-alignment of the
HER crab cavity which is consistent with the original bump height.
• Necessity of the additional 4mm bump is still a mystery.
Alignment Bump

I+ x I- ~1.5mA2

Crab Cavity

10

~ 4mm
Life peak original heightHorizontal offset target scanI+ x I- ~1.5mA2

Original crab bump
Could not go to the right direcIon.

crab bump ‐5mm
in addiIon to original crab bumpPhysical aperture around the crab is responsible to the short beam lifeIme
11
andrestricted the luminosity.

ξy (HER)=0.08
ξy (HER)=0.09
βx*=0.8m

Geometrical luminosity (κ=1%)
with dynamic beam-beam
βx* = 0.8m
κ = 1%
βx*=1.5m

βx* = 1.5m
κ = 1%

w/o crab

βx* = 1.5m
κ = 1.3%

green: βx*=1.5m (crab on)
blue: βx*=1.5m (crab off)
cyan: before crab (βx*=0.59/0.56m)
others: βx*=0.8m or 0.9m

w/o crab
βx* = 1.5m
κ = 0.5%/0.3%

w/o crab
βx* = 1.5m
κ = 1%/1.3%

w/o crab
βx* = 0.8m
κ = 1%

Geometrical loss due to crossing angle: ~11%

Vertical beam size issues• Direct measurement– LER: κ=1.3~2.0%, HER: κ=1.0% (2008/4/8)– LER: κ=0.9~1.0%, HER: κ=1.3% (2008/11/28)• The achieved luminosity with crab off is by far higher
than the simulation with κ=1.0%, 1.3%(LER,HER).!
– Consistent with κ=0.5%, 0.3% (LER, HER) !
• Recalculated beam sizes from the luminosity–~60% of direct measurement (Y. Ohnishi)– Consistent with κ=<0.5%

VerIcal beam size measurement(2008/4/8)• The beam size seems to depend on the bunch
current in LER.

N. IidaVerIcal beam size measurement
The bunch current dependenceof the vertical beam size isvery weak.

(∼1.3%)(∼1%)(∼0.9%)

N. Iida18

Beam-beam simulations• Cross-check the beam-beam simulation code
– We invited Prof. Yunhai Cai from SLAC who is the head of beam physics department.
– He made a beam-beam simulation with a different code from Ohmi’s. The result wasconsistent with Ohmi’s.

• Y. Cai studied the wake field effect on the beam-beam performance.
• No remarkable effects were found.
• As a byproduct of the study, he showed a possibility that the microwave
instability already occurs in the present LER.
• M. Tawada simulated the knob tuning method in the computer by using Ohmi’scode. The result is very interesting.
• K. Ohmi and his student (Y. Seimiya) are studying effects of momentum
dependent optics.
• Chromaticity of R parameters at IP may induce the degradation of specificluminosity.

beam-beam simulation
with wakefield (Y. Cai)LRC Broadband Impedance Model
For q<0, wakefield is given by

where
It’s integral:

Conversion of LRC parameters:

Yunhai Cai

Impedance Model and Measurement
Q=1, x(ωRσz/c)=3, w0=5x105(m-1)

For the LER, we have L=116.8 nH, R=22.94 KΩ, C=22.1 fF
Yunhai Cai

The threshold of 0.5mA is 5 Imes
smaller than the value in KEKB
design report.

Yunhai Cai

Y(2S) Run Summary

800mA

1600mA‐30MeV

0.857

0.777

9.3%

+30MeV

0.810

0.739

9.8%

J. RorieBeam-beam simulations to investigate
effectiveness of knob tuning• Computer simulations have been done on knob tuning (Downhill
Simplex Method plus Manual Scan) by M. Tawada.
• Start with 4 or 5 units of machine errors on 12 coupling anddispersion parameters at IP, with which the luminosity was about
35% of that w/o the errors.
• With the Downhill Simplex method in the computer, the luminosity weachieved was only around 60% of that w/o the errors.
• We could not increase the luminosity with the manual scan after this.
• We tried with another set of initial errors having a similar size. But
the resultant luminosity was almost the same.
• These simulations indicate a possibility that we can not reach as thehigh luminosity as the beam-beam simulation predicts with the usual
tuning methods, if the machine errors have some sizes.

LER (1unit)

HER (1unit)

r1 (mrad)15.71 (3.17)‐3.16 (0.53)

r2 (mm)‐1.34 (0.22)‐1.97 (0.43)

r3 (/km)‐341 (59.38)

374 (48.72)

r4 (mrad)‐149 (25.02)

215 (36.85)

ey (mm)‐1.91 (0.36)

2.17 (0.59)

eyp (mrad)‐62.6 (18.98)

94.4 (21.65)

Downhill simplex method

LER (1unit)

HER (1unit)

r1 (mrad)‐24.94 (3.17)‐22.377 (0.53)

r2 (mm)‐1.51 (0.22)‐1.73 (0.43)

r3 (/km)‐651 (59.38)

1176 (48.72)

r4 (mrad)‐21.3 (25.02)‐20.9 (36.85)

ey (mm)‐0.314 (0.36)‐0.114 (0.59)

eyp (mrad)‐25.3 (18.98)‐1.455 (21.65)

DSM fell into a local maximum.

Initial errors

Luminosity without errors

Beam‐beam simulaIon with the resultant errors
aier the tuning in the computer• With the errors, the steep slope of thespeciﬁc luminosity is reproduced.
Lspec
30

Lsp

25

20

15

10

Error
No error
0

1.25

2.5
I+I-

3.75

5

K. Ohmi, Y. Seimiya

Beam size and luminosity simulation
K. Ohmi, Y. Seimiya
with BB

Other trials• Horizontal emittance– LER 15, 18, 24 nm HER 18, 24 nm– The highest luminosity with crab (16.8 /nb/s) was achieved with
15nm(LER) and 24nm(HER).

• Negative α (May 2008)– 49 spacing– LER -6.7e-4, HER +3.4e-4 �• The microwave instability was successfully suppressed but the beam
condition was not very stable.

– LER 3.3e-4, HER -3.4e-4 �• We couldnʼt lower the horizontal tune below the SB resonance line thistime.

• Fill pattern– Almost all cases: 3.06 spacing– We once tried 2.88 spacing : V size blowup just after the 2-bucket
spacing in LER.

3.06

2.88

2.88 spacing speciﬁc luminosity

Just aier 2‐bucket spacing~15 % lower

2008/3/21

Other resonances
Υ(1S)

Υ(2S)

Υ(4S)

Υ(5S)

Mass [GeV]

9.460310.0233

10.5794

10.865

Radiation
damping rate
(relative)

0.715

0.850

1

1.083

Achieved
specifcluminosity

~70%

~80%

~105%

~105%

Difference in damping rate is not nought to explain difference inluminosity (K. Ohmi)
Residual x-y coupling at IP in the optics model is not zero at theenergy other than Υ(4S) (A. Morita)

Operation statistics

FY2007 (5256 hours)

FY2008 (3849 hours)

Operation statistics (troubles)

FY2007

FY2008LER weekly aborts

crab installation

LER
before crab

LER

after crab

HER weekly aborts

crab installation

HER
before crab

HER

after crab

Causes of beam loss
abortsInstability
coupled bunch oscillation
Malfunction of tune control
Vacuum trouble
X-aborts
Dust trapping?

Ratio of presenceof betatron oscillation

Summary• We finally confirmed that physical aperture around crab
cavities is responsible to the beam lifetime decrease at highbunch currents (LER, HER).
– We could successfully store the design bunch currents of SuperKEKB.

• This lifetime decrease brings some loss in the luminosity. But
its effect does not seem as large as initial expectations,although we need further confirmation with βx*=0.9m optics.
• The luminosity with crab off was unexpectedly high. The
difference between crab on and off is about 20%. There is apossibility that the actual vertical beam sizes (w/o beambeam) are much smaller than the measurements.
• If this is the case, the luminosity predicted by the simulation
with crab on becomes much higher the present one.

Summary [cont’d]• The achieved specific luminosity seems to be on the line of aconstant beam-beam parameter (ξy (HER)) of 0.08 or 0.09.
• There is 10% ~ 20% difference in the specific luminosity between
a fewer number collision (24.5 bucket spacing) and the usual
multibunch collision (3.06 or 3.5 bucket spacing).
• The beam current dependence of vertical beam size in LER, which
we once believed, was maybe fake by the vertical oscillation.
• Efforts to explain the steep slope of the specific luminosity by thebeam-beam simulation are still going on.
• Some realistic machine errors seem to explain why we can notreach the high luminosity predicted by the beam-beam simulation.

What is the origin of steep slope of specificluminosity?

xx• Short beam lifetime– Horizontal offset at IP• Beam current dependent emittance growth in a single beammode?
• Machine errors– Usual knob tuning is not enough to compensate the machine errors?
• Too many knobs?

– Side effects of large knobs?

• Beam-beam simulation misses something?

xx– Cross-check the beam-beam simulation code
– Wakefield effect + beam-beam?
– Off-momentum optics play some role to decrease the luminosity?

Remaining problems
Method for knob search
Can we measure x-y coupling at IP with independent
methods?
Artificial noise to crab? (-> Y. Morita’s talk)Tuning with the simultaneous injection scheme
Chromaticity of R parameters at IP
We are preparing tuning knobs for R chromaticity. (->Koiso’s talk)Vertical beam size problem
Estimation of vertical beam size by the beam-beam scan?
Can we get higher luminosity with a higher HER beam
current (for exceeding the luminosity record before crab)?

spare slides

2008/12/12

下流側のビームラインは

M. Masuzawa

レーザーで外装。
Crab barrel上の罫書線が太いので+/‐ 0.5~1 mm
下流ゲートバルブ
目印なし
ケースの中点では合っている（Qmagで決めた）
ビームライン精度は上流の方がよいと思われる。
セオドライトで読む。

QY2E.1QY2E.2上流ゲートバルブ
目印なし
ケースの中点ではむしろ外側数ミリ

下流側円形フランジ上の罫書線

上流側円形フランジ上の罫書線

4.4 mm内側

7‐8? mm内側

上流側矩形フランジ上の罫書線

5.0 mm内側

フランジ罫書線
大体合っているY. Yamamoto