kekb instrumentation and tuning for crab cavity tuning

22
KEKB instrumentation and tuning for crab cavity tuning Y. Funakoshi KEK

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KEKB instrumentation and tuning for crab cavity tuning. Y. Funakoshi KEK. Evidence of crabbing motion (1): Streak camera. HER crabbing angle measurement and calculation. calculation @ streak camera. Tilt angle [rad]. calculation @ IP. HER crab phase. Crab OFF. - PowerPoint PPT Presentation

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Page 1: KEKB instrumentation and tuning for crab cavity tuning

KEKB instrumentation and tuning for crab cavity tuning

Y. FunakoshiKEK

Page 2: KEKB instrumentation and tuning for crab cavity tuning

Evidence of crabbing motion (1): Streak camera

Page 3: KEKB instrumentation and tuning for crab cavity tuning

HER crabbing anglemeasurement and calculation

HER crab phase

Tilt

ang

le [

rad]

calculation @ streak camera

calculation @ IP

Page 4: KEKB instrumentation and tuning for crab cavity tuning

4

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

-100

-50

0

50

100

-200 -100 0 100 200

BBK2_03031101Crab OFF

BBK:DX1_cBBK:DX2_c

BB Kick (

)m

_ (H OFFSET )m

Δk

Δx x'= 0

Δk

Δx x'=11

= 2.14

-150

-100

-50

0

50

100

150

200

-300 -200 -100 0 100 200 300

BBK2_03011942Crab ON

BBK:DX1_cBBK:DX2_c

BB Kick (

)m

_ (H OFFSET )m

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

T. Ieiri

Page 5: KEKB instrumentation and tuning for crab cavity tuning

5

Method of measurement of beam-beam kick

Beam-beam Kick (Rigid Gaussian)

Beam-Position Change measured at a detector

Effective Horizontal Size at IP

-10 -5 0 5 10 15

-2

0

2

Beam Position, A.U.

Ring Position, A.U.

X

BPM

10

12

14

16

18

20

0 0.02 0.04 0.06 0.08 0.1

Beta_QV1PBeta(IPxP.2)

β*x βdet

ξx

βdet .β x* ≈ 16

By Fukuma & Funakoshi

<COD>

<Dynamic Beta>

Δx: Horizontal Offset€

ΔXdet . =β det .β x

*

2sin(πν )θb−b cos(πν − Δϕ d )

ΔXdet . ∝θb−b

θb−b =−2reNb

γΔx

exp(−Δx

2

(t +2Σx2 )

)

(t +2Σx2 )3/2 (t +2Σy

2 )1/2dt

0

Δ y = 0

θbb+ ≈

−1.94 ⋅reN −

γ +

Δx

Σx2

Δ x < Σx

Σx = (σ x+ )2 +(σ x

− )2

θbb+

Δx

∝N −

Σx2

Missing partner method:Orbit difference of collision and non-collision bunchesT. Ieiri

Page 6: KEKB instrumentation and tuning for crab cavity tuning

6

Calculation of Beam-Beam Kick with and without Crossing Angle

x

z

Use code “BBWS” by Ohmi

How to calculate the kick with an angle of 22 mrad

-0.001

-0.0005

0

0.0005

0.001

-400 -200 0 200 400

BBK_e=24_X'=11

SUM(BBK)_X'=11SUM(BBK)_X'=0

SUM(BBK)

X_Pos ( )m

x=24 nm

Head-on

Crossing

0

5

10

15

100 120 140 160 180 200

Dk/Dx_sizeDk/Dx_X'=0Dk/Dx_x'=11

Slope, Abs (

Δ/kΔ) 10x x

-6

(Horizontal Size at IP )m

< Slope in Linear Region >

Result: The measurement is consistent with the calculation.

A particle is horizontally moved and the kick data are summed up, including the longitudinal profile.

T. Ieiri

Page 7: KEKB instrumentation and tuning for crab cavity tuning

7

Effect of Transient Beam Loading

Revolution period

Bunch train Gap

=Rωrf

2QVc

I bΔTg

t

Phase

Δϕ max

ΔTg

: length of bunch train: Gap length

t0

D. Boussard

T0

T. Ieiri

Page 8: KEKB instrumentation and tuning for crab cavity tuning

8

Phase and orbit shift along train with 3.06 spacing: Crab ON

Z

0

0.5

1

1.5

2

2.5

3

3.5

0 1000 2000 3000 4000 5000

LER_071031_3.06_1530mA

P_c

Phase_c (deg)

Bucket

0

0.5

1

1.5

2

2.5

3

0 1000 2000 3000 4000 5000

HER_071031_3.06_800mAP_c

Phase_c (deg)

Bucket

LER / 1530mA HER / 800mA

N=1585

X

-0.8

-0.7

-0.6

-0.5

-0.4

0 1000 2000 3000 4000 5000

HER_071031_3.06_800mA

X_c

X_c_Pos (mm)

Bucket

2.5

2.55

2.6

2.65

2.7

0 1000 2000 3000 4000 5000

LER_071031_3.06_1530mA

X_c

X_Position_c (mm)

Bucket

T. Ieiri

~0.2x

Page 9: KEKB instrumentation and tuning for crab cavity tuning

9

Z

LER/1640mA HER/1260mA

2.5

2.55

2.6

2.65

0 1000 2000 3000 4000 5000

LER_060605X_LER_c

X_LER (mm)

Bucket

-0.9

-0.85

-0.8

-0.75

-0.7

-0.65

-0.6

0 1000 2000 3000 4000 5000

HER_060605X_HER_c

X_HER_c (mm)

Bucket

X

-1

0

1

2

3

4

0 1000 2000 3000 4000 5000

HER_060605P_HER-c

P_HER-c (deg)

Bucket

-1

0

1

2

3

4

5

0 1000 2000 3000 4000 5000

LER_060605P_LER_c

P_LER (degree)

Bucket

Phase and orbit shift along train with 3.06 spacing: Crab OFF

N=1585T. Ieiri

Page 10: KEKB instrumentation and tuning for crab cavity tuning

Measurements or experiences on impedance

related issues• The following measurements were done before and after the

installation of crab cavities (by T. Ieiri).– Bunch current dependence of coherent tune shift– Loss factor– Bunch lengthening as function of the bunch current

• No significant difference was observed on these measurements between before and after crab.

• We once observed a coupled bunch instability due to the crabbing mode. However, this was due to wrong setting of the detuning frequency.

• As for the HOMs or LOMs, we have no experience that they induced any coupled bunch instability.

Page 11: KEKB instrumentation and tuning for crab cavity tuning

11

Crab system Adjustment

• Crabbing voltage– Calibrated from klystron output power and the loaded Q value.– The measurement to adjust the phase using beam (below) also gives an

independent calibration of the voltage.– Both are in good agreement within a few percent.

• Crabbing phase– The reference phase is searched to minimize the beam orbit difference between the

crabbing on and off, so that the bunch center is not kicked by the crabbing voltage.– In the high-current operation, the phase is shifted by 10 degrees from the reference

phase to cure the oscillation observed at high-current crab collision (see yesterday’s talk).

• Field center in the cavity– Searched by measuring the amplitude of the crabbing mode excited by a beam

when the cavity was detuned. A local bump orbit was set to adjust the beam orbit on the cavity axis.

K. Akai

Page 12: KEKB instrumentation and tuning for crab cavity tuning

Crab Phase Scan (LER)

0

36

72108

144

180

216

252288

324

360

Phase 0 : 174.8 deg.Vcrab set:1.0MV, estimated: 0.987MV agree very well   

crabHorizontal orbit by crab kick

Horizontal kick by crab cavity (rad)(Estimated from orbits around the ring)

crab

H. Koiso, A. Morita

Page 13: KEKB instrumentation and tuning for crab cavity tuning

An example of crab Vc scan

Page 14: KEKB instrumentation and tuning for crab cavity tuning

Balance of crab kick strength between HER and LER can be tested by measuring the horizontal offset with the same amount of the crab phase shifts.

– Crab phase of LER and HER were changed by10°: The horizontal offset at IP (beam-beam kick) changed by15μm. (1) 。( ideally zero)

– Change LER phase only by1° or HER only by1° brought offsets of 16~19μm.(2) 。( calculated value from crabbing angle is18μm. )

When we changed HER crab Vc from 1.55 to 1.43MV, the horizontal offset did not change with the same 10° phase change.(3) 。

Crab phase vs Horizontal beam-beam kick

(1) (2) (3)

0.94MV (LER)1.43MV (HER)

0.94MV (LER)1.55MV (HER)

Page 15: KEKB instrumentation and tuning for crab cavity tuning

15

Orbit control and feedback near crab cavities

Dependence of RF power on the loaded Q value and a horizontal beam orbit for a beam current of 2 A.

• We have chosen QL=1~2 x105 for a good compromise.

– This value is suitable for operating the system with a possible error of ΔX=1mm, and

– A high power source of 200 kW is sufficient for conditioning the cavity up to 2 MV.

Typical parameters for the crab crossing.

★Orbit feedback around crab cavitiesOnly horizontal orbit feedback is considered.◆4 horizontal steering magnets to make an offset bump for each ring◆4BPM to monitor the offset at the crab cavity. 2 upstream (entrance) BPMs and 2 downstream (exit)

System speed (design) <~1 Hz.◆We found that the orbit is stable enough. Usually, we do not need the orbit feedback.

Page 16: KEKB instrumentation and tuning for crab cavity tuning

16

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

Page 17: KEKB instrumentation and tuning for crab cavity tuning

Horizontal offset scan• The (HER) beam

current seems to be limited by the short life time of the LER beam.

Collision center given by the beam-beam kick

LER lifetime

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

Page 18: KEKB instrumentation and tuning for crab cavity tuning

Beta’s with dynamic beam-

beam effect

before summer βx*= 0.9m

this fall βx*= 0.9m

crab

crab

with/without beam-beam effects

Page 19: KEKB instrumentation and tuning for crab cavity tuning

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.

10

Crab Cavity

Alignment Bump I+ x I- ~1.5mA2

Life peakoriginal height~ 4mm

Page 20: KEKB instrumentation and tuning for crab cavity tuning

Horizontal offset target scan

11

Original crab bumpCould not go to the right direction.

crab bump -5mm in addition to original crab bump

Physical aperture around the crab is responsible to the short beam lifetime and restricted the luminosity.Physical aperture around the crab is responsible to the short beam lifetime and restricted the luminosity.

II+ + x Ix I-- ~1.5mA ~1.5mA22II+ + x Ix I-- ~1.5mA ~1.5mA22

Page 21: KEKB instrumentation and tuning for crab cavity tuning

Summary• The crabbing motion of the beam with a single crab cavity

scheme has been confirmed by the streak camera measurement.

• Effective head-on collision was confirmed by the measurement of the beam-beam deflection.

• The synchronous phase shifts along the train from the transient beam loading have some size at KEKB. However, their impact on the luminosity is very small owing to the cancelation of the effect between the two rings.

• We have observed almost no harmful effects due to the crab cavities or the single crab cavity scheme (global crab cavity scheme) from the viewpoint of the beam dynamics.– Dynamic aperture, synchro-betatron resonance (yesterday’s talk)– Bunch current dependent tune shift, loss parameter, bunch lengthening,

coupled bunch instabiity

Page 22: KEKB instrumentation and tuning for crab cavity tuning

Summary [cont’d]• Crab Vc is determined so as to maximize the luminosity in the

real beam tuning situation at KEKB.• The phase errors of the crab cavities are much less than the

allowed level of the errors obtained from the beam-beam simulations.

• At KEKB, the dynamic beam-beam effects (emittance enlargement and the beta-beating) are very important, since the horizontal tune is very close to the half-integer. Due to this problem, physical aperture around the crab cavity limited the ring acceptance. After solving this problem, we could increase the bunch current (of HER) and the luminosity.

• In autumn this year, we will operate KEKB for about 2 months. There is possibility that it will be the final opportunity to operate the crab cavities at KEKB.