(Talk)

16th December 2003 G. Blair, RHUL 1

The Laser System at PETRA WireThe Laser System at PETRA WireG. A. Blair, Royal Holloway Univ. LondonG. A. Blair, Royal Holloway Univ. London

ACFA Workshop, MumbaiACFA Workshop, Mumbai

1616thth December 2003 December 2003

Accelerator-Related Session Accelerator-Related Session

• Motivation for the projectMotivation for the project• Laserwire at PETRALaserwire at PETRA

- Environment at PETRAEnvironment at PETRA- Installation of HardwareInstallation of Hardware- First measurementsFirst measurements

• Conclusions and Outlook Conclusions and Outlook


MotivationMotivation

• Maximise Luminosity performance of Linear ColliderMaximise Luminosity performance of Linear Collider• Control of transverse beam size and emittance in the Beam Control of transverse beam size and emittance in the Beam

Delivery System (BDS) and at the Interaction Point (IP)Delivery System (BDS) and at the Interaction Point (IP)• Conventional techniques (wirescanner) at their operational limitConventional techniques (wirescanner) at their operational limit• Development of standard diagnostic tool for LC and LC Test Development of standard diagnostic tool for LC and LC Test

Facility operation based on optical scattering structures Facility operation based on optical scattering structures Laserwire, Laser-InterferometerLaserwire, Laser-Interferometer

• FeaturesFeatures- Resolution error smaller than 10%Resolution error smaller than 10%- Fast (intra-train) scanningFast (intra-train) scanning- Non-destructive for electron beamNon-destructive for electron beam- Resistant to high power electron beamResistant to high power electron beam

Dyx

repeb HfNn

L **

2

4


Trans.+Long. ProfilesTrans.+Long. Profiles

Trans: 10-100 mLong: ~200 m

Trans: 10-100 nm


LC Layout and ParametersLC Layout and Parameters

CLICCLIC NLC/GLCNLC/GLC TESLATESLA

BDSBDSxx/ / mm

yy/ / mm3.4 to 153.4 to 15

0.35 to 2.60.35 to 2.67 to 507 to 50

1 to 51 to 520 to 15020 to 150

1 to 251 to 25

IPIPxx/ n/ nmm

yy/ n/ nmm196196

4.54.5335335

4.54.5535535

55


Optical Scattering StructuresOptical Scattering Structures

• Scanning of finely focused laser beam through electron beamScanning of finely focused laser beam through electron beam• Detection of Compton photons (or degraded electrons) as function Detection of Compton photons (or degraded electrons) as function

of relative laser beam positionof relative laser beam position• ChallengesChallenges

- Produce scattering structure smaller than beam sizeProduce scattering structure smaller than beam size- Provide fast scanning mechanismProvide fast scanning mechanism- Achieve efficient signal detection / background suppressionAchieve efficient signal detection / background suppression


Laserwire for PETRALaserwire for PETRA

EnergyEnergy

Bunch LengthBunch Length

Charge/bunchCharge/bunch

Hor. beam sizeHor. beam size

Ver. beam sizeVer. beam size

E/GeVE/GeV

zz/ps/ps

nCnC

xx//mm

yy//mm

4.5 to 124.5 to 12

~100~100

1 to 31 to 3

500 to 100500 to 100

~100~100

• Positron Electron Tandem Positron Electron Tandem Ring AcceleratorRing Accelerator

• Injector for HERA, upgrade to Injector for HERA, upgrade to synchrotron light sourcesynchrotron light source

Long free straight sectionLong free straight section Easy installation of hardware Easy installation of hardware

due to existing access pipe due to existing access pipe and hut outside tunnel areaand hut outside tunnel area

• Q-switch Nd:YAG with SHGQ-switch Nd:YAG with SHG• From CERN LEP polarimeterFrom CERN LEP polarimeter Trans Mode: large MTrans Mode: large M22 ~9 ~9 Long Mode: stability Long Mode: stability ± 20%, ± 20%,

beating beating ps substructure ps substructure Homegrown timing unit for Homegrown timing unit for

external triggering external triggering

WavelengthWavelength

EnergyEnergy

PulselengthPulselength

ReprateReprate

Beam sizeBeam size

DivergenceDivergence

l/nml/nm

E/mJE/mJ

dt/nsdt/ns

ffreprep/Hz/Hz

x,yx,y/mm/mm

/mrad/mrad

1064/5321064/532

250/90250/90

1010

3030

~7~7

0.70.7

Laser parameter

PETRA parameter


Laserwire for PETRALaserwire for PETRA


Signal and BackgroundsSignal and Backgrounds

• Signal: Compton scattering Signal: Compton scattering • Background sources:Background sources:

- Synchrotron radiationSynchrotron radiation- Cosmic raysCosmic rays- BremsstrahlungBremsstrahlung

• Simulation with Geant4 plusSimulation with Geant4 plus

tool kits with realistic setuptool kits with realistic setup


Setup at PETRASetup at PETRA

HALL EAST

Q-SWITCH

TUNNEL AREA

EMBL HUT

OUTIN

OUT IN

INOUT

LASER

PD TRIGGER BOX

SCOPE

PIT.Y

PIT.E

CAL

BPM

IP

PD@IP

PETRATIMING

e+

COMPTONPHOTONS


Installation at PETRAInstallation at PETRA


Lab Measurements at RHULLab Measurements at RHUL


DetectorDetector• Requirements for detector materialRequirements for detector material

- short decay time (avoid pile up)short decay time (avoid pile up)- short radiation lengthshort radiation length- small Moliere radiussmall Moliere radius

• Cuboid detector crystals made of PbWO4Cuboid detector crystals made of PbWO4• 3x3 matrix of 18x18x150 mm crystals3x3 matrix of 18x18x150 mm crystals• Energy resolution Energy resolution

better than 5% better than 5%


Detector CalibrationDetector Calibration

• Detector studies with DESY II testbeamDetector studies with DESY II testbeam• Beamline with electrons with energy Beamline with electrons with energy

from 450 MeV to 6 GeVfrom 450 MeV to 6 GeV• Ten detector crystals were calibrated Ten detector crystals were calibrated

using a single PMTusing a single PMT• Combination of nine crystals in matrixCombination of nine crystals in matrix• ResolutionResolution

- High intrinsic resolutionHigh intrinsic resolution- Full matrix less good Full matrix less good


First Photons 31.07.03First Photons 31.07.03

Laser on Laser off

Photodiode at IP

Q-switch

Calorimeter


First Beam Profile ScansFirst Beam Profile Scans

• Positron beam in PETRAPositron beam in PETRA• Beam energy: 7 GeVBeam energy: 7 GeV• Bunch pattern: 14 x 1 bunch evenly filledBunch pattern: 14 x 1 bunch evenly filled• Average current: 12 mAAverage current: 12 mA

- Bunch charge = avg. current / (reprate * Nbunches) = 6.5 nCBunch charge = avg. current / (reprate * Nbunches) = 6.5 nC

• Laser energy measured: 40 mJ (specs 90 mJ), PLaser energy measured: 40 mJ (specs 90 mJ), PLL= 4 MW= 4 MW

• Optimization: qswitch delay, timing of ADC sample pointOptimization: qswitch delay, timing of ADC sample point• Vertical and horizontal orbit bumps to steer positron beamVertical and horizontal orbit bumps to steer positron beam

- Closed symmetric bumps using four steerersClosed symmetric bumps using four steerers- Bump length: 50 m, max offset: 10 mmBump length: 50 m, max offset: 10 mm

• Operation of fast piezo scannerOperation of fast piezo scanner


The LaserThe Laser

• The laser has been given to us by B. Dehning from CERN. It The laser has been given to us by B. Dehning from CERN. It has been used at LEP to measure beam polarizationhas been used at LEP to measure beam polarization

• It’s a Nd:YAG Q-switched system, running with 30 HzIt’s a Nd:YAG Q-switched system, running with 30 Hz• pulse energy measured: 40 mJ, power: 4 MWpulse energy measured: 40 mJ, power: 4 MW• synchronization to PETRA beam by triggering the Q-switch synchronization to PETRA beam by triggering the Q-switch

Pockels-cellPockels-cell• transverse beam quality is modest (multimode)transverse beam quality is modest (multimode)

• measured spot size at IP: measured spot size at IP: σσLL = (80 = (80 ± 10)± 10) μμmm


Measurement of the Measurement of the longitudinal Profilelongitudinal Profile

• The longitudinal profile has been measured with a streak The longitudinal profile has been measured with a streak camera: FESCA 200 from Hamamatsucamera: FESCA 200 from Hamamatsu

• largest window of the camera: 500 ps with a resolution of 5 largest window of the camera: 500 ps with a resolution of 5 ps (fwhh)ps (fwhh)

• The camera was triggered with the laser via a fast photo The camera was triggered with the laser via a fast photo diodediode

• Problem: Problem: stability of the trigger probably not better than 0.5 nsstability of the trigger probably not better than 0.5 ns


Averaged ProfileAveraged Profile• Measured averaged profile: Measured averaged profile:

fits to gaussian with a width of 12.5 ns (as expected) fits to gaussian with a width of 12.5 ns (as expected)


Structure in the Longitudinal Structure in the Longitudinal ProfileProfile

• Example of a single shot measurement of the profileExample of a single shot measurement of the profile500 ps window, resolution 5 ps 500 ps window, resolution 5 ps

60 ps

66 ps


Unfortunately, the structure is Unfortunately, the structure is not stablenot stable

• The longitudinal structure is due to longitudinal mode The longitudinal structure is due to longitudinal mode beating – this was expectedbeating – this was expected

• The beating changes from shot to shotThe beating changes from shot to shot

30 ps

79 ps


Laser Laser Transverse Transverse

ProfileProfile

Units – number of CCD pixels


Laser SummaryLaser Summary• As expected for a this type of laser, the longitudinal As expected for a this type of laser, the longitudinal

profile shows substructure due to mode beatingprofile shows substructure due to mode beating• The spikes have a width of 30 to 60 ps and a distance of The spikes have a width of 30 to 60 ps and a distance of

60 to 80 ps60 to 80 ps• Unfortunately, the structure is not stable and changes Unfortunately, the structure is not stable and changes

from shot to shotfrom shot to shot• To overcome this, the laser has to be equipped with a To overcome this, the laser has to be equipped with a

frequency stabilized seed laser or eventually with an frequency stabilized seed laser or eventually with an EtalonEtalon

• Hot spots a problemHot spots a problem


Orbit ScanOrbit Scan

• First scan with signal on scopeFirst scan with signal on scope• Then sampling of peak using ADCThen sampling of peak using ADC• Moving beam orbit up and down Moving beam orbit up and down

with vertical orbit bumpwith vertical orbit bump• 5k counts at each orbit position5k counts at each orbit position• 3 min for each spectrum3 min for each spectrum• 40 min for complete scan40 min for complete scan• Background with 20k countsBackground with 20k counts

- Mainly synchrotron radiation Mainly synchrotron radiation and bremsstrahlungand bremsstrahlung

- Rate changed by factor 10Rate changed by factor 10• Signal rate expected at peakSignal rate expected at peak

- 200 200 γγs x 380 MeV avg Energys x 380 MeV avg Energy


Result Orbit ScanResult Orbit Scan

• Gaussian approximation of beam shape Gaussian approximation of beam shape

σσm m = (0.175 = (0.175 ± 0.020± 0.020statstat ± 0.038 ± 0.038syssys) mm) mm

• Vertical beam sizeVertical beam size

σσee = sqrt( = sqrt(σσmm - - σσLL ) )

laser laser σσLL = (40 = (40 ± ± 10)10) μμmm

σσee = (170 = (170 ± 23 ± 37) ± 23 ± 37) μμmm• Result of fit sensitive to Result of fit sensitive to

background modellingbackground modelling• Systematic error dominated Systematic error dominated

by vertical orbit jitterby vertical orbit jitter• More measurements and More measurements and

understaning of bkg sources understaning of bkg sources necessarynecessary


Fast Scanner OperationFast Scanner Operation

• Next scan with remote Next scan with remote controlled fast scannercontrolled fast scanner

• Orbit position stableOrbit position stable• Scan range: Scan range: ± 2.5 mrad± 2.5 mrad

- Scan line = range * fScan line = range * flenslens==

0.625 mm (± 20%)0.625 mm (± 20%)• Change amplitude of scanner Change amplitude of scanner

power supply (1-100V)power supply (1-100V)• Take 5k countsTake 5k counts• Record laser IP image with Record laser IP image with

CCDCCD• Move laser beamMove laser beam• Take 5k counts ...Take 5k counts ...

SCANNER

125 mm

IMAGINGLENS

TODUMP

VIEWPORTMIRROR SPLITTER

CCD

BEAM

LENS


Data and AnalysisData and Analysis

• Seven scan points recordedSeven scan points recorded• 5 min / point5 min / point• 40 min for full scan40 min for full scan• Positron beam position stable Positron beam position stable

within within ± 40 ± 40 μμmm• Moving low energy pedestalMoving low energy pedestal• No background modelNo background model• Orbit stable Orbit stable bkg const. bkg const.• Simple pedestal cut insteadSimple pedestal cut instead• Sufficient background Sufficient background

rejectionrejection


New Setting 5.12.03New Setting 5.12.03

• Positron beam in PETRAPositron beam in PETRA• Beam energy: 7 GeVBeam energy: 7 GeV• Posittron beam optics not as in October scans!Posittron beam optics not as in October scans!• Bunch pattern: 14 x 1 bunch evenly filledBunch pattern: 14 x 1 bunch evenly filled• Low current: 7.1 mA, first bunch 0.458 mALow current: 7.1 mA, first bunch 0.458 mA

- Bunch charge = avg. current / (reprate * Nbunches) = 3.9 nCBunch charge = avg. current / (reprate * Nbunches) = 3.9 nC

• High current: 40.5 mA, first bunch 2.686 mAHigh current: 40.5 mA, first bunch 2.686 mA- Bunch charge = 22.3 nCBunch charge = 22.3 nC

• Vertical and horizontal orbit bumps to steer positron beam Vertical and horizontal orbit bumps to steer positron beam into laser beaminto laser beam- Closed symmetric bumps using four steerersClosed symmetric bumps using four steerers

• Scanning of laser beam using the fast piezo scannerScanning of laser beam using the fast piezo scanner


Results 04.12.03 DataResults 04.12.03 Data

• Slopy Gaussian approximation of beam shapeSlopy Gaussian approximation of beam shape

σσm m =(68 =(68 ± ± 3 3 ± ± 20) 20) μμm at low currentm at low current

σσm m =(80 =(80 ± ± 6 6 ± ± 20) 20) μμm at high currentm at high current


Conclusions and OutlookConclusions and Outlook

• Laserwire at PETRA produced first compton photons and Laserwire at PETRA produced first compton photons and measure vertical beam size Next steps:measure vertical beam size Next steps:

• Full characterisation of laser: beam size, divergence, and Full characterisation of laser: beam size, divergence, and power (stability) with slot scans and imaging techniquespower (stability) with slot scans and imaging techniques

• Update all readout software, merge BPM and PMT softwareUpdate all readout software, merge BPM and PMT software• Do more systematic scans with the fast scannerDo more systematic scans with the fast scanner• Go to smaller spot sizes and reduce error barsGo to smaller spot sizes and reduce error bars• Build second dimension scanner.Build second dimension scanner.

• Start designing a complete laser-wire emittance Start designing a complete laser-wire emittance measurement system for the LC BDS. measurement system for the LC BDS.


CollaboratorsCollaborators

• DESYDESY• BESSY (Thanks to T. Kamps for many of these slides)BESSY (Thanks to T. Kamps for many of these slides)• UK: RHUL, UCL, RAL, (Oxford).UK: RHUL, UCL, RAL, (Oxford).• CERN: (Laser, plus collaboration)CERN: (Laser, plus collaboration)

Close contact with:Close contact with:• SLACSLAC• KEK KEK


People People

Thanks to PETRA and BKR shift crews !Thanks to PETRA and BKR shift crews !

K Balewski, G Blair, S Boogert, G Boorman, J K Balewski, G Blair, S Boogert, G Boorman, J Bosser, J Carter, J Frisch, Y Honda, S Bosser, J Carter, J Frisch, Y Honda, S Hutchins, T Kamps, T Lefevre, H C Lewin, F Hutchins, T Kamps, T Lefevre, H C Lewin, F Poirier, I N Ross, M Ross, H Sakai, N Poirier, I N Ross, M Ross, H Sakai, N Sasao, P Schmüser, S Schreiber, J Sasao, P Schmüser, S Schreiber, J Urukawa, M Wendt, K Wittenburg,Urukawa, M Wendt, K Wittenburg,

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