Experience with Crab cavities

The 4th EIC workshop May 19-23, 2008

Experience with Crab cavities

The 4th Electron Ion Collider Workshop

19-23 May, 2008

Hampton University

Mika Masuzawa, KEK


Contents

1. Introduction• Where/What is KEKB?• Characteristics of KEKB

2. Machine performance before Crab cavities• Luminosity history• Machine parameters

3. Machine performance with Crab cavities• Crab crossing scheme• Cavity production & installation• Beam commissioning with crab cavities

• Can we confirm the prediction?

4. Summary


1. Introduction

• Where/What is KEKB?• Characteristics of KEKB

Mt. TsukubaNikko

KEKB tunnel ~11 m below Ground Level

Belle @IP

Linac

Aerial view of KEK

TokyoMt.Fuji

About KEKB

Where


Superconducting cavities (HER)

e-

e+

ARES cavities (LER)

8 GeV e- 3.5 GeV e+ Linac e+ target

ARES cavities (HER)

IR

Belle detectorKEKB B-FactoryBeam energy

– 8GeV (electron, “HER”)– 3.5GeV (positron, “ LER”)

Circumference– 3016 m– Uses TRISTAN tunnel

RF system

– fRF ~ 509MHz

– ARES (LER)– ARES+SCC (HER)

The construction of KEKB began in 1994,and was completed in November 1998.Commissioning started in Dec.1998.

What is KEKB

About KEKB


Characteristics of KEKB


Superconducting Cavities Storage of high current >1.4 A

ARES normal conducting cavities: Large storage cavity for stable acceleration

Finite crossing angleSuperconducting Q magnet for final focus.Small beam size achieved at IP

2.5 Cell Lattice: Low non-linearityhigh flexibility

J-LINAC: Efficient acceleration in a limited spaceTwo bunch positron injection

Bunch-by-bunch feedback system

Solenoids to reduce electron cloud effects

IR

SCC

Arc sectionmagnets

ARES

Solenoids on vacuum pipe



Interaction Region

• Vertical focusing by a pair of superconducting Q-magnets （ QCSL/QCSR ） .

• Extra vertical focusing by QC1L/R for the electron beam.

• One beam must go off axis due to the finite crossing angle at the IP.

• To minimize the flux of SR through the IP, the incoming positron (electron) beam orbit is set on the axis of QCSL (QCSR).

• Superconducting solenoid magnets SL/R used for compensating the detector solenoid field.


PEP-II

KEKB

KEKB has 22 mrad horizontal crossing angle at the IP:

•Easier beam separation

•Simpler design around the IP.

•Fewer components.

•Less synchrotron radiation.

•Less luminosity-dependent background.

•Space for compensation solenoid, etc.

More on Interaction Region



2. Machine performance before Crab cavities

• Luminosity history• Peak luminosity• Integrated luminosity

• Machine parameters (Design & Best)


Luminosity history

Peak

Daily

design

Peak luminosity



Highest luminosity ever achieved by a collider


1034

1033

1032

1031

1030

1029

Pea

k lu

min

osity

(cm

-2 s

-1)

1970 1975 1980 1985 1990 1995 2000 2005 2010 Year

Integrated luminosity



The best day (1.23 /fb/day) before Crab cavity installation

Design luminosity


Continuous injection scheme since 2004


Integrated luminosity


Machine parameters


11/15/2006 Design Unit

LER HER LER HER

Beam Current 1.65 1.33 2.6 1.1 A

# of bunches 1389 5000

Bunch current 1.19 0.96 0.52 0.22 mA

Emittance x 18 24 18 18 nm

*x 59 56 33 33 cm

*y 0.65 0.59 1.0 1.0 cm

*x 103 116 77 77 m

*y 1.9 1.9 1.9 1.9 m

x 0.115 0.075 0.039 0.039

y 0.101 0.056 0.052 0.052

Bunch length 7 6 4 4 mm

Luminosity 17.12 10 /nb/s

Luminosity/day 1232 ~600 /fb

Machine parameters (Best & Design)


More on the machine parameters of Nov.15, 2006

LER HER Unit

Circumference 3016 m

RF frequency 508.88 MHz

Horizontal Emittance 18 24 nm

Beam current 1662 1340 mA

# of bunches 1388

Bunch current 1.20 0.97 mA

Bunch spacing 2.1 m

# of bunch trains 1

Total RF voltage Vc 8.0 15.0 MV

Synchrotron tune s -0.0246 -0.0226

Betatron tune x/y 45.505/43.534 44.509/41.565

Beta’s at IP x*/y* 59/0.65 56/0.59 cm

Momentum compaction a 3.31 x 10 -4 3.38 x 10 -4

*y (estimated) 1.9 1.9 m

Beam-beam x/ y 0.115/0.101 0.075/0.056

Beam lifetime 110 @ 1600 180 @ 1340 min. @ mA

Luminosity 17.12 10 33/cm 2/sec

/ day / 7days / 30days 1.232 / 7.809 / 30.21 /fb


Machine performance Summary before Feb. 2007 (without crab cavities)

• Recorded highest luminosity of 17 /nb/sec (1.7x1034 /cm2/s),

with a crossing angle at the IP.


For even higher luminosity

• Achieve a head-on collision while keeping the crossing angle at the IP.Crab Crossing Scheme


3. Machine performance with Crab cavities

• What is Crab crossing scheme?• Cavity production and installation• Beam commissioning

What is Crab crossing scheme?


Input Coupler Liq. Helium Vessel

Stub Support

Coaxial Coupler

Copper Bellows80 K Liq. Nitrogen Shield

Notch Filter

RF Absorber

Aluminum End Plate

Aluminum End Plate

SUS Support Pipe

Crab Crossing Scheme

Crossing angle 22 mrad

Head-on (crab)(Strong-strong simulation)

Simulation by K. Ohmi

K. Hosoyama, et al.

RF Deflector( Crab Cavity )

Head-onCollision

Crossing Angle (11 x 2 m rad.)

Electrons PositronsLERHER

1.41 MV

1.41 MV

1.44 MV

1.44 MV

First proposed by R. B. Palmer in 1988 for linear colliders.

Need two RF deflectors for each ring


Crab crossing scheme (our choice)

Install one crab cavity per ring, at Nikko straight

section where superconducting acceleration cavities

(SCC)are located.• Saves on cost of cavities and cryogenic systems.• Avoids synchrotron radiation hitting the cavity.


Crab crossing scheme Comparison between two cavities/ring

and one cavity/ring

IPIP

beam beam

crabcavity

crabcavity

bunch tailbunch head

crabcavity

1 cavity per ring 2 cavities per ring

orbits of bunch head and tail

Different COD between head and tail Same COD


Crab Crossing Scheme (KEKB)

•Beam tilts all around the ring.

•z-dependent horizontal closed orbit.

• tilt at the IP (head on):Crab cavities

Streak cameras to observe the tilt

LER HER

x 22 mrad

*x 80 80 cm

cx 73 162 m

x /2 0.505 0.511

cx/2 ~0.25 ~0.25

Vc 0.95 1.45 MV

rf /2 509 MHz

x2

xcx

*cos xc x 2

2sin x 2

VcrfEc

Typical parameters for Crab crossingTilt angle depends on crab voltage, x *and x(Crab).


What crab cavity looks like

Crab Cavity & Coaxial Coupler in Cryomodule

Support rod

Jacket type main He vessel(SUS316L)

Jacket type sub He vessel

Coaxial beam pipe (Nb)

Stub support

Crab cavity cell

Notch filter

Support pipe tuning rod

RF absorber(Ferrite)

Extract TM010, TE111 modefrequency tuning

Input coupler

Bellows

RF absorber(Ferrite)

K. Hosoyama et al


Schematic Drawing of Cryostat


Cavity production and installation

Crab Cavity Fabrication Procedure

Nb Sheet

Half Cell Hydro-forming

Mechanical Polishing & Trimming

Electron Beam Welding

Crab Cavity Cell

Grinding of Welding Part

Barrel Polishing

Electro-Polishing

High Pressure Water Rinsing

Annealing

Electro-Polishing

Beam Pipe & Flange

Nb Sheet

Rolle

High Pressure Water Rinsing

Assembling for Cold Test

Cold Test in Vertical Cryo.

MHI Kobe

KEK Tsukuba

Nomura Plating Kanuma

Kinzoku Giken Mito

Tokyo Denkai5 mm t RRR = 180

Cell Equator

~ 100 mm

EP 1~ 100 mm

EP 2~ 5 mm

700 oC x 3 hr

80 bar. 60 min.

Forming and barrel polishing 　

Forming of 4 Half-Cells for Crab Cavity for LER and HER Feb. 14, 2005 at Mitsubishi Heavy Industries, LTD. Kobe

Barrel PolishingNov. 11, 2005 at KEK

Polishing Time 312 hours

High Pressure Rinsing and Assembling for RF Cold Test

Nozzle

High pressure water rinsing by 80 bar Ultra-Pure water

Rotation & up-down motion

Set flanges of beam pipes and ports in Class 100 clean room

Electro Polishing & Annealing

Cathode: Aluminum Straight Pipe

Crab Cavity: Rotating ~ 1 rpm

Annealing at 700oC for 3 hours at Kinzoku Giken Ltd.

Titanium Box

EP 1 ~ 100 mEP 2 ~ 5 m

Electro Polishing at Nomura Plating Ltd.

Alignment of coaxial coupler

Determine the axis of the coaxial couplerset in the cryostat by using transit.

Align the axis of the coaxial couplerwhich will be connected to the coaxial coupler on cryostat side.

Move to Test Stand for Cool-down & High Power Test

Mt. Tsukuba

Crab cavity for HER

April 26, 2006 1stOct. 16, 2006 2nd

Crab cavity for LER

Dec. 6, 2006

Q0 vs. Esp Curve

LER Crab Cavity HER Crab Cavity

shows the target value in the operation.

These figures show the comparison of Q0 vs. Esp curve between Vertical and Horizontal Test for the both Crab Cavities.


Two crab cavities were installed in KEKB in January 2007.

HER (e-, 8 GeV) LER (e+, 3.5 GeV)


Beam commissioning with crab cavities

Be very careful as we have no spare cavities

Will the beam receive the kick that we want?

Can we collide the crabbed beams?

No extra heating due to crabbing in the entire ring?


2007.2.13 - 3.19 (first beam operation with Crab cavities)

Collision tuning with crab on

Tuning (SR monitors, streak cameras and so on). Crab cavitiy aging without beams.

Feb. 19 First beam with crab kicks (No collision when Crab on).

Feb. 21 First collision with Crab crossing.


Is the beam really crabbed (tilted)?


Tilt confirmed!

LER HER

inside of the rings

outside of the rings

Observation with Streak Cameras (H. Ikeda et al, FRPMN035)

The streak camera

longitudinal

horizontal

Crab Phase Scan (LER)

0

36

72108

144

180

216

252288

324

360

Crab voltage Vcrab set was 1.0MV Obtained from the data was 0.987MVAgrees very well.

crabHorizontal orbit by crab kickcrab

Horizontal kick by crab cavity (rad)(Estimated by orbit fit)


One Day History of collision tuning with Crab On March.11

Luminosity 0.55/nb/s y~0.078 preliminary


Specific Luminosity

Crab Crossing

22 mrad crossing

•y 0.078 in HER was achieved.

•This is higher than our record in Nov.

•But not as high as we expected (a factor of 2 was predicted by simulation).

12th KEKB Accelerator Review March 19, 2007Haruyo Koiso


For higher luminosity!luminosity

specific luminositybeam-beam tune shift

Can we confirm the prediction?


Crab detuned Feb.2007-Apr.2008

Summershutdown

Wintershutdown

Lpeak~15/nb/sIHER = 0.7 A, ILER = 1.3 A,

L > 1034 with crab crossing.

Warm up300K


Oct.-Dec. 2007

3.5 3.06 buckets

Peak: 14.7 /nb/s

xcm

We trieddifferent opticsdifferent fill patternHigher HER/LER currents


Specific luminosity with crab crossing

•Simulation (no Crab)

•Simulation (Crab)

Higher than without Crab,but not as high as prediction.

22 mrad crossing

3.06 bucket spacing

Specific Luminosity

★ A number of measurements indicate effective head-on collision.

★ The vertical tune shift became higher than 0.088. Before crab, it was 0.055.

★ The specific luminosity / bunch was improved more than the geometrical gain.

★ Need more time to achieve the goal (X2 specific luminosity).

the highest vertical beam-beam tune shift was about 0.088.

before crab, tune shift was 0.055

Simulation22 mrad

x*=90 cm

x*=68 cm

x*=80 cm

x*=100 cm

Simulationhead-on

Crab Crossing•49 sp. x*=80, 84cm


Specific Luminosity

Crab crossing•49-sp x*=80, 84cmx=18, 24 nm•3.5-sp x*=80cm•3.06-sp x*=80cm•3.06-sp x*=90cm

22 mrad crossing

y=-16.35x+26.54 Green Ratio=100%

Green line

y~0.093 (HER) (4/3)


Beam-beam parameter

[mA]

:experiments

Crab crossing

Crossing angle 22mrad


Why specific luminosity not doubled?Possibilities

• We can not find a parameter set which gives a higher specific luminosity, even if such a parameter set exists?

– Too large a parameter space?

• Faster parameter search, more efficient method of parameter search

– Short beam lifetime prevents us from reaching a better parameter set?

• Identify the mechanism to determine the beam lifetime

• Implement e- and e+ simultaneous injection （ autumn 2008 ）• Some unknown effects are responsible for the low specific luminosity?

– Synchro-betatron resonance?

– Vertical crab?

– Fast noise?

– Bunch-by-bunch orbit difference?

– Lattice non-linearity?

– Global x-y coupling?

– Others?


Tuning parameters

22-241-122

16106

42

•Many knobs are determined by scans only on the luminosity, beam sizes,

and the lifetime.

•Scan is slow, each takes about 30 minutes.

•Problem in multi-dimensional nonlinear optimization.

IP Coupling

LER HER LER HER (set by IP knob tuning) (measured at the optics correction)R1 4.12 4.16 0.01 0.17 mradR2 2.01 -3.96 0.001 -0.07 mmR3 63.90 0 -98.5 72.3 /kmR4 -64.69 128.52 -150. -23.7 mrad

X

PXY

PY

0 r4 r20 r3 r1r1 r2 0

r3 r4 0

x

pxy

py

xpxypy

p

2 detR1

normal physical

Example of parameter scan

K. Ohmi

sharply-peaked-optimum-on-a-broad- shoulder(SPOOABS)

An example: the Horizontal Offset and the crossing angle at the IP

•Luminosity degrades by a small error in any one of the collision parameters. The horizontal offset of two beams and the crossing angle at the IP are such an example.

•Horizontal offset must be much less than 25 μm, and the crossing angle less than 1.5 mrad to see the effect of crab crossing.

•There are more than 20 of such parameters. If one of them is largely off, the optima of other parameters cannot be found.


Downhill simplex method

Reflect

Expand

Contract+

Contract-

Shrink

1

2

3

Method of Minimization • {1, 2, 3} 1(best)<2(next-to-the worst)<3(worst)

• Evaluate 3R

• If 3R<1, • If 3E<3R, {1, 2, 3E} : Expand , if not, {1, 2, 3R} : Reflect

• If 1<3R<2, {1, 2, 3R} : Reflect• If 2<3R<3, Reflect 2 proposed by A. Hutton

• If 3C+<3R, {1, 2, 3C+} : Contract+ , if not, {1, 2, 3R} : Reflect• If 3<3R, Reflect 2

• If 3C-<3, {1, 2, 3C-} : Contract- , if not, {1, 2S, 3S} : Shrink/Reflect2

39

Simplex makes an N+1 vertex shape for N parameters. So for a 2-parameter plane, you make a triangle in parameter space, and measure the luminosity at each of the three vertices. Then you try to "walk" uphill, by reflecting the lowest point to the opposite side of the other two. After a while, when there is no improvement, you shrink the scale of the simplex down and continue trying to move up the smaller-scale peak.

Simplex scan (example)

Synchrotron-betatron resonance

• The horizontal tune is set near the half integer resonacne and its synchrotron sidebands.

• At the resonance, the single beam sizes blowup(left).

• This effect can be calculated by “anomalous emittance” effect.

• The blowup depends on the sextupole setting (below).

LER tune

54 sextupole families

Sextupoles A B C D E F

Jumped from x=0.516 to 0.506.x=.512 is the resonace

2x+z=integer

BeamCurrent

Difference between “A” and “D”

• We tested 5 different sextupole configurations.• We checked beam loss by changing tune across the resonance.

Finding better sextupole setting: “bungee jump”

No beam loss with “D” sexts

19

Synchro-betatron resonance

• In many cases, the synchro-betatron resonance (2x + s = integer) limits the KEKB performance.– Beam size blowup, short lifetime, beam loss etc.

• The resonance is stronger in HER where no local chromaticity correction is installed.

• Strength of the resonance is strongly dependent a choice of sextupole setting.

• The resonance has something to do with the low specific luminosity at high bunch currents?

LER x, y tune dependence

x -x =integer

x +s =integer

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 x 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 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

Synchro-betatron resonance in HER

• Positive-– We could NOT operate

under the resonance(2x+s=integer)

• Negative-– We could operate

under the resonance(2x- s=integer)

4. Summary


Crab Crossing Started at KEKB Two crab cavities were installed. Beam commissioning with crab crossing started in Feb.

2007. The effective head-on collision was confirmed by

streak camera crab-phase scan horizontal beam-beam kick, etc.

The highest vertical beam-beam tune-shift parameter is about 0.088 so far, which is higher than the geometrical gain due to head-on colliison by 15%. 16.1/nb/s was obtained yesterday.

There are a few speculated reasons for why the luminosity is lower than the prediction, but not yet confirmed.

We will do more study & machine tuning. Crab crossing is a must for SuperKEKB.


First in the worldFirst in the world


Phase stability (histogram of phase detector signal)

HER LER0.007° 0.046°

Distribution of cavity phase (cavity feedback loops on)

Linear scale

Log scale

The tuner phase unstability was suppressed by low level RF control system.

Luminosity (estimated)

1350/700

1400/725

1500/775

1600/825

Beam currents1700/875 mA

3.5 buckets3x74x7 3.27

3x114x4

3.063x154x1

2.883x152x2

The specific luminosity is assumed to be on the line Green Ratio=1.

Crab Voltage Scan

4.6 mrad

y*

design

Oct. 13 100 bunches Nov. 19 1585 bunches

Lifetime

Luminosity

Lifetime

y*

Luminosity

The ratio of crab voltages was adjusted to give the same kick in both rings. The scan was done, keeping the voltage ratio.


LER HER

RF Phase Stability

• Phase stability of the crab mode was better than the requirement with the rf feedback.

• Slow stability below 1 Hz is shown above.

• Independent measurement by a spectrum analyzer shows better than 0.01 deg for f > 2 kHz, 0.1 deg for 2 Hz < f < 2 kHz.

• Backlash or friction exists in the coaxial tuner for the LER.

86

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

Sign Change in the Crab Angle

Luminosity

Vert. size LER σy blowup

12080

Reversed LER crab angle

Reversed LER & HER

Reversed HER

HER σy blowup

Both correct

20060

H. Koiso


Crossing angle

Transformation from lab. Frame

to head-on frame.

222

2*

**

**

**

*

**

11

tantan

sincos/

cos/

sin

cos/)tan(

]sin1[tan

yxzz

xzz

z

yy

x

xx

x

pppph

hppp

xhzz

pp

xhyy

hpp

xhzx

1000tan0

0cos/10000

00cos/1000

000100

0000cos/10

0tan0001

(: half crossing angle)

Linear part

Oide and Yokoya for storage rings (1989)


Transverse kick by Crab Cavity

10000

010000

001000

000100

000010

00001

x

x

Crab cavity makes z dependent dispersion zx = - at the IP, which cancels the crossing angle effect ( << 1).

1000tan0

0cos/10000

00cos/1000

000100

0000cos/10

0tan0001

.

Crossing-angle term Crab-cavity term


Crab-crossing simulation 0-mrad vs. 11-mrad crossing angle (K.Ohmi)

• Beam-beam limit is ~0.06 for 11 mrad half-crossing angle (both models agree well).

• 0-mrad (head-on) collision gives a higher y.• Beam-beam limit for 0-mrad crossing depends on the

model.

Weak-Strong model Strong-Strong model

Bunch current Bunch current

Experience with Crab cavities

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