ATF Damping RingBeam Development

2010.02.12K. Kubo

ATF Damping Ring の主な目標• Final Focus Test (ATF2) からの要請

– Low vertical emittance, y < 12 pm

– Stable beam, jitter << beam size• Fast Ion instability の研究からの要請

– Low vertical emittance, y ~ 5 pm

– Stable beam, jitter << beam size

（現状：　 single bunch ではほぼＯＫだが、 multibunch での不安定性などの系統的なデータがまだない。）

• ILC DR design と同程度の低エミッタンス– y = 2 pm

　　　（現状： 5 pm 程度は確認できているが、 2 pm は未確認。）

Low Emittance Tuning

Monitor: • 補正のための情報

– BPM (Beam Position Monitors) 、 total 96– Turn by turn BPM (tune measurement)

• エミッタンスの確認– Beam size monitors

• SR-interferometer• X-ray SR profile monitor• Laser Wire monitor

Low Emittance Tuning

Corrector:

• Steering magnets – 47 horizontal and 51 vertical

• Skew Qauds – Trim coils of sextupole magnets, total 68

Skew correctors - trim coils of sextupole magnets

+ I

+ I

- I /2

- I /2- I /2

- I /2

The trim windings of all 68 sextupole magnets have been arranged to produce skew quadrupole fields, used as correctors.

Currents of the top and the bottom poles are the same. Currents of other poles are one half.

Skew Quad Field by Sextupole Magnet

(Suggested by T. Raubenheimer)

Figure by Okugi

6

Layout of the SR-interferometer

Synchrtron light Polarizer

Band pass filter

Double slit

f=600mmLens

Interferogram 7m

CCD

1.6.1997 T.Naito

a D

L L

1.3m

0

y

I aJ0{1exp[ (2DaL

0

)2] cos(2DyL

)}

SR interference beam size monitor

Interference pattern

7

Beam image (x:39µm, y:7.3µm)X-ray SR beam size monitor(Tokyo Univ., KEK)

Routine of Low Emittance Tuning

Three consecutive corrections

• COD correction

• Vertical COD-dispersion correction– minimize dispersion and orbit simultaneou

sly with weights

• Coupling correction– minimize vertical orbit change by horizont

al steerings

Simulated vertical emittance

Corrections Average Ratio of target (11pm)

COD 23 pm 20%

+ Dispersion 16 pm 51%

+ Coupling 5.8 pm 91%

COD+Dispersion+coupling

COD+Dispersion

COD

y (pm)

Num

ber

of e

ntrie

s

Distribution from 500 random seeds

Magnet offset: measured + 30 micronBPM-magnet offset: 200 micronMagnet rotation: 0.3 mrad BPM rotation: 20 mrad

Example of Low emittance data in ATF DR

By 2004, we confirmed very low emittance beam, around 4 pm.

Vertical emittance measured by Laser Wire (April 16, 2003)

by Y. Honda

10

8 6

4

2

0

vert

ical

em

ittan

ce (

pm)

0 1 2 3 4

bunch current (mA)

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 2 109 4 109 6 109 8 109 1 1010

Horizontal Emittance

x emittance (run B)x emittance (run D)simulation (0.4% coupling)

x e

mit

tanc

e [1

0-9]

bunch intensity [electrons/bunch]

15

20

25

30

35

40

0 2 109 4 109 6 109 8 109 1 1010 1.2 1010

bunch length(runD') [psec]bunch length(runE') [psec]bunch length(runF') [psec]simulation (0.4% coupling)simulation (6% coupling)simulation (3% coupling)

Bu

nch

Len

gth

(rm

s) [

pse

c]

Bunch Intensity [electrons/bunch]

4.5 10-4

5.0 10-4

5.5 10-4

6.0 10-4

6.5 10-4

7.0 10-4

7.5 10-4

8.0 10-4

8.5 10-4

0 2 109 4 109 6 109 8 109 1 1010 1.2 1010

Energy Spread (runD)Energy Spread (run E)simulation (0.4% coupling)simulation (6% coupling)

En

ergy

Sp

read

Bunch Intensity [electrons/bunch]

Energy spread Bunch length

Measurement compared withsimulation withIntra-beam scattering

Intensity dependence

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

0 2 109 4 109 6 109 8 109 1 1010

Vertical Emittance (Inc. Multibunch)

y emittance (single, runB)y emittance (single, runD)simulation (0.4% coupling)y emittance (20 bunch projected)

y e

mit

tanc

e [1

0-12 ]

bunch intensity [electrons/bunch]

Multi bunch emittanceMeasured using Laser wire

0.0 100

5.0 10-12

1.0 10-11

1.5 10-11

2.0 10-11

2.5 10-11

0 5 10 15 20

Multibunch Y emittance by LW (23April2004)

Y emittance (1.5E9 bunch intensity)

Y emittance (4.6E9 bunch intensity)

Y e

mit

tanc

e of

eac

h bu

nch

[rad

.m]

bunch number

4.6x109 bunch intensity

1.5x109 bunch intensity

Recent efforts for low emittance

　 y ~ 5pm 確認後、低エミッタンスの追及は ATF の主要な課題でなくなり、 2007-2008 年には通常の補正では 20 pm 程度のエミッタンスにしかならない状態であった。

Main motivation: ATF2 needs low emittance beam (12 pm at N~1E10 in design)Study of Fast Ion Instability

• Re-alignment of magnets• Reset optics• BBA (Beam Based Alignment) measurement• Optics correction (Beta-beat correction) • Improvement of BPM• Improvement of beam size monitors

（各項目がどの程度有効であったか、定量的評価はできていない。）

Re-alignment of DR magnets

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 20 40 60 80 100 120 140

2008 March

y (m

m)

s (m)

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 20 40 60 80 100 120 140

2009 October

y (m

m)

s (m)

RMS deviation from smooth curve: 80 micron 50 micron

Spec: 60 micron from “smooth curve”

3

0/2cos

nnn LsnC

by M. Takano

Simulated emittance vs. Magnet misalignment

0

2 10-12

4 10-12

6 10-12

8 10-12

1 10-11

1.2 10-11

1.4 10-11

0 50 100 150 200

BPM offset error 0BPM offset error 200 micron

< y>

(m

)

RMS of random misalignment of magnets (micron)

Beam Based AlignmentSimulation of the tuning showed importance of BPM - magnet center offset error

Emittance vs. BPM offset error with respect to the nearest magnet

Beam Based Alignment - Method

1. Make vertical local bump at the Magnet-BPM.2. Change Magnet strength.3. Measure the orbit difference for all BPMs.

Normal Quads: Vertical, Skew Quads: Horizontal4. Estimate the minimum orbit difference point.

BPMBPMs BPMs

Magnet (quad or skew quad)

Trim windings of sextupole magnets

Fig. by Okugi

Beam Based Alignment - results

0

1

2

3

4

5

6

7

-400 -200 0 200 400

Num

ber

of E

ntri

es

Difference of offset (m)

Typical statistical error of offset evaluated from pulse to pulse jitter:~ 30 m for Quad

~ 80 m for Skew Quad

Long term StabilityDifference between April 2008 and April 2009. Significant change in one year for some BPM-magnet pairs. Probably came from BPM electronics.

Optics matching (Beta-beat Correction)

0

5

10

15

20

25

0 20 40 60 80 100 120 140

y (m

)

arc arcarc

0

5

10

15

20

0 20 40 60 80 100 120 140

y (m

)

s (m)

Calculated vertical beta functions of two different optics matching conditions.

December 1999, when we observed small vertical emittance (about 5pm).

May 2008, when we could not achieve low emittance.

Beta-beat increase emittance sensitivity to errors. Reset optics to “design” and correct optics looking betafunction (Not completely successful but some improvement.)

Damping Ring BPM electronics upgrade

• Closed orbit 測定精度の向上• Turn-by-turn データの取り込みが可能

– 現在、９６個中２０個の BPM で交換済– 残り７６個は今年中に交換予定

BBA, Orbit, Dispersion, Coupling cor. 等の精度向上Turn by turn data の解析による Coupling CorrectionOptics 診断の精度向上

ビームサイズモニターの性能向上

ビームサイズモニターの主要な役割• 補正の各ステップの有効性の評価 (Tuning by try and error.)• 低エミッタンスの確認

問題点• SR interferometer, X-ray profile ： 　測定限界が６ミクロン程度　（エミッタンスにして 10 pm 　程

度）。• Laser Wire モニター：　　　測定に時間がかかる（調整終了後５～１０分程度）ため、補正

の各ステップでの評価に使いずらい。

ビームサイズモニターの性能向上 (2009 年 )

• SR interferometer– 光路の再アラインメント – スリット間隔 40 mm 60 mm ~4 micron vertical beam size

• X-ray SR profile monitor– 機械的振動の低減 ~4 micron vertical beam size

• Laser Wire – Storage mode だけでなく通常の運転（ビーム取り出し）モー

ドでも測定できるように変更• ビームタイムを占有することなく測定可能　（測定に要する時間

は長い）今後、レーザーの交換（大出力）の計画あり

• 測定時間の短縮

Recent Vertical Emittance history

(preliminary)

For smaller emittance (~2 pm)

• BPM electronics upgrade

• BBA

• ORM analysis

• New correction method – Normal mode analysis

• Beam size monitor improvement

ORM (Orbit Response Matrix) Analysis• 各 steering magnet を変えて orbit を測定• モデルとの比較により、エラーを推定できる

– Quadrupole strengths, – BPM gains and couplings,– Corrector magnet strengths and tilts.

Can be used forOptics correction (Quad strength)Coupling correction (minimize HV response)

過去 Optics correction に使われ、ある程度成功していた。しかし、最近あまり有効でない（ BPM electronics, 又は o

ptics の再現性などの問題か）。　 要検討

• To “calibrate” a BPM, we can observe turn-by-turn normal mode motion on the beam, e.g. by “shaking” the beam first at one betatron frequency, then at the other.

• This allows us to determine the components of beam motion along the normal mode axes, regardless of any gain errors.

• If the dispersion is parallel to the “horizontal” normal mode, then quantum excitation will not excite any motion in the “vertical” normal mode: this is the optimum condition for minimising the vertical emittance.

• The effects of dispersion and coupling are fixed simultaneously.

lab x

lab y

normalmode x

normalmode y

Tuning methodUsing turn-by-turn BPM Data

by Andy Wolski

• BPM “Calibration”– ビームをキックして turn-by-turn データをとる。– 各 BPM について、周波数成分の解析によ

り、 Normal mode の方向を決める。• Correction

– 各 BPM で vertical like normal mode の dispersion が小さくなるように skew quad corrector をセットする。

• 上の２つを繰り返す（ skew quad corrector 　を変えると normal mode の方向も変化する。）

lab x

lab y

normalmode x

normalmode y

Tuning Simulation Results

2% BPM gain errors.No BPM calibration.

Distribution peaks at ~ 12 pm.

10% BPM gain errors.With BPM calibration.

Distribution peaks at ~ 1.2 pm.

by Andy Wolski

（ 4 個の電極毎のエラー）

Stability目標：• pulse to pulse jitter < beam size

– (At IP of ATF2: < 0.3 　　in vertical)• bunch to bunch jitter << beam size (multibunch 運転 )

– (At IP of ATF2: < 2 nm, 0.05 　　in vertical)

現状：• Single bunch 運転では問題なし• Multibunch

十分なデータがまだない。

Single bunch ビームの安定性Synchrotron and betatron oscillation estimated fro

m orbit data of many pulses (single bunch)

0

50

100

150

200

250

-0.0004 -0.0002 0 0.0002 0.0004

Cou

nt

E/E

Longitudinal: 0.2 Horizontal: ~0.08 Vertical: 0 ~ 1 (large error due to BPM resolution)

Energy jitter

Multibunch ビームの安定性なぜ必要か• Final focus line （ＡＴＦ２）での bunch-by-bunch feedback によるビー

ムの安定化の実証のため。 (< 0.05 )• Fast Kicker による安定したビーム取り出しの実証のため。• Ring での Fast Ion Instability の研究のため。（他の原因による不安定

性は抑えなければならない。）

現状• Single bunch に比べて jitter が大きいが、系統的なデータがほとんど

ない。– 真空度に関係がある。– バンチ数、 fill パターンに依存。– intensity に依存。– Tune のわずかな変更には反応しない。– Simulation では、 Cavity Wakefield による instability はないはず。– . . . . . . .

まとめ低エミッタンス (single bunch)• 補正直後 y 10 pm 　以下 (about 5 pm) は確認できる。

– Better than design assumption of ATF2– Good enough for Fast Ion Instability test.

• 長期間保持できるかどうかの系統的データが必要• BPM の calibration, BBA, Optics 診断などの方法、有効性につい

て、引き続き研究が必要。• For smaller emittance (~ 2pm 、ＩＬＣの設計値 )

– Upgrade of BPM electronics• New correction method will be tried

– Beam size monitor improvement (e.g. laser wire upgrade)

ビームの安定性• Single bunch ではＯＫ• Multibunch では今後の研究が必要。

Extra slides

Routine of Low Emittance Tuning(a) COD correction: using steering magnets,

minimize BPM reading

and , :x(y): horizontal (vertical) BPM reading.

(b) V-COD-dispersion correction: using steering magnets,

minimize dispersion and orbit y: measured vertical dispersion.

r : weight factor = 0.05

(c) Coupling correction: using skew quads,

minimize vertical response to horizontal steering

x(y): horizontal (vertical) position change at BPM due to excitation of a horizontal steering magnet.

Two horizontal steering magnets were used (Nsteer=2). About (n+1/2)p phase advance between the two.

　

x2

BPM

BPM

2y

　

y2

BPM

r2 y2

BPM

　

Cxy y2

BPM

x2

BPM

　

　　

　

　　

H steers

Nsteer

Reset optics and 　 Beta-beat Correction

0

5

10

15

20

0 5 10 15 20 25 30

Before correctionCorrected

y (m

)

Quad QF1R

West arc

East arc

Vertical beta-function at all quadrupole magnet of one family in the arc sections. (Should be flat for matched optics.)

Before and after a beat correction. (Example)

Condition was improved but not completely satisfactory.Need more study for better modeling.

Stored Beam – 10 minute time scale; ATF lifetime ~ few minutes

DR BPM resolution improvement by digital read-out system (SLAC, FNAL, KEK)

beam position read-out vs. beam intensity:

scattered plot : existing analog circuit.

line plot : digital read-out introduced for test.

aiming εy ~ 1 pm

Digital read-out

Analogue read-out

Plans• If turn-by-turn data are available, BPM calibration against normal

mode motion is fast and quite straightforward.– Good quality calibration data have been obtained at CesrTA; but

correction is difficult in CesrTA because there are few skew quadrupoles.

• ATF BPMs should be upgraded with Echotek electronics (for turn-by-turn measurements) during the course of 2010.– initial tests with some Echoteks already installed indicate

impressive performance (resolution, stability...)• It is hoped to test the calibration and tuning method later this year,

when the new BPMs are available.• The proposed technique is a development of the “gain mapping”

that has been used successfully to calibrate the BPMs and improve the optics tuning in KEKB.

by Andy Wolski