Towards International Linear Collider: Experiments...

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Towards International Linear Collider: Experiments At ATF2 (Final Focus Test) K. Kubo (KEK) ATF2 Collaboration 2014.06.18 IPAC14

Transcript of Towards International Linear Collider: Experiments...

Page 1: Towards International Linear Collider: Experiments …research.kek.jp/group/nkocbeam/report/2014/IPAC14_Kubo.pdfTowards International Linear Collider: Experiments At ATF2 (Final Focus

Towards International Linear Collider:

Experiments At ATF2 (Final Focus Test)

K. Kubo (KEK) ATF2 Collaboration 2014.06.18 IPAC14

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ILC beam at Interaction Point (Focal point) For High Luminosity

σy ~6 nm

σx ~470 nm (σz ~300000 nm)

Horizontal beam size is limited by beam-beam force (beamstrahlung)

Luminosity limited by vertical beam size

Flat beam

For small beam we need • Low vertical emittance • Small aberrations in Final Focus System

Being tested at ATF(Accelerator Test Facility at KEK)

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ATF, Accelerator Test Facility at KEK

1.3 GeV

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ATF2 International Collaboration Design, Construction, Operation

CERN Germany DESY France IN2P3; LAL, LAPP, LLR UK Univ. of Oxford Royal Holloway Univ. of London STFC, Daresbury Univ. of Manchester Univ. of Liverpool Univ. College London Italy INFN, Frascati Spain IFIC-CSIC/UV Russia Tomsk Polytechnic Univ.

Japan KEK Tohoku Univ. Univ. of Tokyo Waseda Univ. Nagoya Univ. Kyoto Univ. Hiroshima Univ. China IHEP Korea PAL KNU India RRCAT

USA SLAC LBNL FNAL LLNL BNL Cornell Univ. Notre Dome Univ.

ATF Main Institutes

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Chromatic Aberration in Final Focus Different focal points for different energy particles in beam. Energy spread causes large beam size.

Low energy High energy

Correction: Sextupole magnets located at Horizontal Dispersion Focal strength proportional to particle energy cancels chromatic aberrations.

NOT SO SIMPLE Other aberrations (geometrical aberration) created by the sextupole field Because of energy-independent position spread (beam size)

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Global chromatic correction Final Quads

Chromatic aberration Collision

Chromaticity 2 Chromaticity 1

Mainly for y Mainly for x

1 2

β1/2

ηx

2,1,

Matrix Transfer

xx

Iηη =

−=

6-poles 6-poles

Put two 6-pole magnets symmetrically. Geometrical aberrations automatically cancelled.

Chromaticity

Add 2 regions dedicated to Chromaticity

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Local chromatic correction

Final Quadrupole magnets

Chromaticity

collision

Chromatic Correction 6-pole magnets

Geometric Correction 6-pole magnets

Geometrical Aberration

~nπ ~nπ

Chromaticity Cancel

Horizontal Dispersion

Put 6-pole magnet next to each of Final Quads for Chromatic correction Correct geometrical aberration in upstream

(P.Raimondi and A.Seryi, Phys. Rev. Lett. 86, 3779 (2001))

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Comparison of Chromaticity Correction Methods

• Shorter beam line

• Better designed performance (Large energy acceptance.

Small halo at final quads.)

Advantages of “Local” correction

Disadvantage?

Chosen for ILC

• Complicated design

• Difficulties in tuning (operation)

Chromaticity 2

Chromaticity 1 Global

Local

(P.Raimondi and A.Seryi, Phys. Rev. Lett. 86, 3779 (2001))

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Expected Difficulties/Complications In Local Chromatic Correction

• For Designing – No obvious (simple) symmetries for cancelling aberrations

• In Operation – Interleaved sextupoles Interference between horizontal and

vertical parameters – Nonzero angle dispersion at focal point – Many aberrations can be coupled

Expected difficulties: Motivation of ATF2 Project. Test of Local Correction

Global Correction was successfully tested in 1994 at SLAC: FFTB(Final Focus Test Beam) (V. Balakin et al., Phys. Rev. Lett. 74, 2479 (1995))

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Two Goals of ATF2

• Small beam size (Goal 1) : This Report – Demonstration of a compact final focus system based

on local chromaticity correction – Designed beam size: 37 nm – (Without chromatic correction, beam size is ~450 nm.)

• Control of beam position (Goal 2) – Demonstration of beam orbit stabilization with a few

nanometer precision at the IP • Establishment of beam jitter controlling techniques at the

nanometer level with an ILC-like beam

ATF2 Collaboration, “ATF2 Proposal,” (2005)

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Parameter ILC ATF2 Beam Energy [GeV] 250 1.3 Energy Spread (e+/e-) [%] 0.07/0.12 0.06~0.08 Final quad – IP distance (L*) (SiD/ILD detector) [m]

3.5/4.5 1.0

Vertical beta function at IP (β*y) [mm] 0.48 0.1 Vertical emittance [pm] 0.07 12 Vertical beam size at IP (σ*y) [nm] 5.9 37 L*/β*y (~natural vertical chromaticity, SiD/ILD detector)

7300/9400 10000

Design parameters of ILC and ATF2 Final Focus

ATF2 Collaboration, “ATF2 Proposal,” (2005)

ILC Technical Design Report, https://www.linearcollider.org/ILC/Publications/Technical-Design-Report

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Final Focus Optics, ILC and ATF2

ILC

ATF2

yx,*β

yx,*β

Same magnet configuration, Almost identical optics

Up to 500 GeV ~700 m

1.3 GeV ~30 m

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Tolerances for Final Focus System Magnet Errors Comparison of ILC and ATF2

Field strength

Rotation

Jitter position

Static position

Same magnet names, similar tolerances.

ILC ATF2

ATF2 Collaboration, “ATF2 Proposal,” (2005)

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Figure from: Y. Yamaguchi, Master thesis at Graduate School of Science, The University of Tokyo, 2010

Beam Size Monitor at Focal Point (IPBSM) Shintake Monitor, using interference of laser beam

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Scan interference fringe phase. Fit modulation M:

Fringe phase

Gam

ma-

ray

sign

al G

Example

( ))cos(1)( 00 φφφ ++= MGG

−= 2

222expcosh

M σπθ

Evaluate beam size, σ, from this expression.

Beam size measurement

For Gaussian beam profile Possible errors reduce M and make apparent beam size larger. Measured size: Upper limit

Small M

)2/sin(2 θλ

=h

Large beam

Large M

Small beam

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Measureable Beam Size Range of IPBSM

0

0.2

0.4

0.6

0.8

1

10 100 1000 104

174O

30O

8O

2 O

Mod

ulat

ion

σy (nm)

)2/sin(2 θλ

=hPitch of interference fringe:

Sensitive beam size range depends on crossing angle (θ) of two laser beams.

There are 3 different crossing angle modes for covering wide range.

Crossing angle mode 174 deg. 30 deg.

2-8 deg. continuously adjustable

Covers 25 nm – 6000 nm

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Vertical Beam Size Tuning (Final stage of beam tuning)

Changing parameters Corrected coupling Linear knobs (Linear Optics adjustment)

6-poles horizontal moves yy’ (Focal Position)

6-poles vertical moves yE (Dispersion)

x’y (x-y coupling) Non-linear knobs (2nd order optics adjustment)

6-poles strength x’yy’ yy’E (chromaticity)

Skew 6-poles strength xxy xyE

yEE (2nd order dispersion) yy’y’

Each knob changes one coupling term.

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After each scan, “knob” was set at the peak of the fitted line.

Examples of Tuning knob Scans yy’ coupling knob yy’E coupling knob

M vs. knob strength IPBSM 30 deg. mode

M vs. knob strength IPBSM 174 deg. mode

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History of measured minimum beam size

Still being improved.

0

50

100

150

200

250

300

350

400

Mea

sure

d M

inim

umBe

am S

ize

(nm

)

Dec 2010

Dec 2012

Feb-Jun 2012

Mar 2013Apr 2014

Eart

hqua

ke (M

ar 2

011)

May 2014Jun 2014

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What Contributed to Improvement? • Cures for Higher Order Magnetic Field Errors.

– Multi-pole field components of Quadrupole magnets • Adopt optics with 10 times larger β*x than nominal, smaller beam

size at magnets reduce x-y coupling effects • Replace final QF magnet

(Small aperture, strong multi-pole fields Large aperture, weak multi-pole fields) • Found one coil of strongest 6-pole magnet was shorted

• Exchange with weakest one (January 2013) • Turned off, by changing 2nd order optics (April 2014)

• Suppress Orbit Drifts in Final Focus Beam Line – Improvement of orbit feedback

• Improvement of Beam Size Monitor • Wakefield reduction (?)

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Beam Size Tuning after 3 weeks shutdown Small beam (~60 nm) observed ~32 hours from operation start

0

200

400

600

800

1000

10 20 30 40 50 60

2-8 deg. mode30 deg. mode174 deg. mode

σ y (n

m)

Time (hours) from Operation Start after 3 Weeks Shutdown Week 2014 April 7

Different colors: different crossing angle modes

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Beam Size Tuning after 3 days shutdown Small beam (~60 nm) observed ~16 hours from operation start

0

200

400

600

800

1000

10 20 30 40 50 60 70

2-8 deg. mode30 deg. mode174 deg. mode

σy (n

m)

Time (hours) from Operation Start after 3 days shutdown

Week from April 14, 2014

Week 2014 April 14

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Beam is stable for 30 – 60 min. without tuning. Examples of consecutive beam size measurements

April 17, 2014 Mean: 0.42 Standard dev.: 0.04 IPBSM

Modulation (174 degree Crossing angle)

Mean: 0.50 Standard dev.: 0.04

May 22, 2014

Beam size Evaluated from Modulation (no systematic error assumed)

Mean: 55 nm Standard dev.: 3 nm

Mean: 50 nm Standard dev.: 3 nm

Bunch charge ~ 0.16 nC Bunch charge ~ 0.09 nC

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Example of vertical beam size measurement (Fringe Phase Scan)

There must be some systematic errors, drift of beam position or monitor’ laser, etc., which tend to reduce modulation and make apparent beam size larger.

Data on April 17, 2014. IPBSM with a crossing angle of 174 degrees. Bunch charge ~0.16 nC. Fitted modulation is 0.45, evaluated beam size 53 nm

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Data of Last Week (June 12)

0

1

2

3

4

34 36 38 40 42 44 46 48 50

20140612

Ent

ries

Evaluated σy (nm)

Mean: 44 nm Standard dev.: 3 nm

Beam Size Evaluated from Modulation (no systematic error assumed)

0

1

2

3

4

0.5 0.55 0.6 0.65 0.7 0.75

20140612

Ent

ries

Modulation (Crossing Angle 174 degrees)

Mean: 0.58 Standard dev.: 0.05

IPBSM Modulation (174 degree Crossing angle)

Bunch charge ~ 0.16 nC

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IPBSM modulation as function of bunch population. Measured with crossing angle 174 degrees (left) and 30 degrees (right).

Beam Size Depends on Bunch Intensity

nm. 3-2by size beamintensity -zero than larger bemay nC) 0.16-0.1(at size beam minimum Measured

nm/nC. 100 as fitted is ,)0()( Assuming 2222

+= wqwq yy σσ

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Intensity Dependence • Beam size strongly depends on bunch intensity.

– Most probably, due to transverse wakefield. – Compare with ILC, much stronger effects: Low beam energy (1.3 GeV/250 GeV) and Long bunch (7 mm/0.3 mm)

• Estimation of effects of Wakefield – Wakefield of Cavity BPMS, Bellows, Steps are calculated. – Experimental studies by introducing wakefield source on mover.

• Reduction of wakefield – Shield discontinuities in beam line – Remove possible strong wakefield sources – Move possible sources from high beta region to low beta region

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Remaining Issues for Goal 1 (small beam)

Beam Size Still Slightly Larger Than Designed 37 nm • Confirm emittance of incoming beam • Confirm optics matching (β*y) • Further Improvement of beam size monitor • Detect/correct beam position drift/jitters

– High resolution BPMs at IP region will solve the question. (related to Goal 2, stabilization of beam orbit)

Study of intensity dependence (Wakefield)

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Status of Goal 2 (Stable beam)

• Intra-pulse, bunch to bunch feedback successfully demonstrated – Sub-micron to micron level stability. Limited by BPM resolution

and bunch to bunch uncorrelated jitters.

• For nanometer level stabilization – High resolution BPMs installed around focal point. – Basic BPM performance studies on going. – Feedback is being prepared.

Other reports in this conference. ID: 2811, TUPME009, P. Burrows, et.al. ID: 2781, THOAA02, N. Blaskovic, et.al.

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SUMMARY Small beam • Performance of Final Focus System of ILC, Local

Chromatic Correction, Has Been Demonstrated. – Vertical beam size ~45 nm was confirmed at low

intensity. • Beam size tuning method for this beam size level

established – Small beam routinely observed.

Stable beam • Feedback system successfully tested. • Nanometer level stabilization being prepared.