Electro-Optic Beam Diagnostic at BNL DUV-FELElectro-Optic Beam Diagnostic at BNL DUV-FEL

Henrik Loos

for

National Synchrotron Light SourceBrookhaven National Laboratory

Presented at ICFA Mini-Workshop XFEL 2004

2

OutlineOutline

DUV-FEL accelerator facility

Coulomb field measurement

THz CTR pulse characterization

Issues for ultrafast electro-optic measurement

Summary and outlook

3

DUV-FEL FacilityDUV-FEL Facility

Radiator (NISUS) Wiggler:L = 10 m, w = 3.89 cmB = 0.31 T, K = 1.126

HGHG: 100 µJ @266 nm3rd harm. 1 µJ @89 nm

AdjustableChicane

177MeV

RF zero PhasingPhotoinjectorCTR Monitor

Normal incidence

77 MeV

FEL seedat 800 nm

Modulator Undulator

NISUS pop-inmonitors

FEL MeasurementsEnergy, Spectrum, Synchronizationand Pulse Length Measurementsat 266 nm

Ion Pair ImagingExperimentat 88 nm

Nisus Wiggler

30 mJ Ti:SapphireAmplifier

DispersionMagnet Trim Chicane

Energy 170 MeV

Charge 300 pC

Normalized emittance 4 mm mrad

Compressed bunch length

0.3-0.6 ps rms

Energy spread 0.01 % rms

50 m

4

Detect change in laser polarizationwith /4 waveplate and analyzer.

Signal asymmetry A between linear polarization states gives phase change.

Electro-Optic Bunch DiagnosticElectro-Optic Bunch Diagnostic

DelayMulti-Shot

Single-Shot

ZnTe

Laser

Laser

e-Beam

E-Field

sinRT

RT

SS

SSA

2

sin10R,T

SS

Uses Pockels-effect to detect electric field E of Coulomb field or THz radiation with fs laser.

1

2 vac4130 lErn

Birefringence in <110> cut ZnTe with E-field and laser polarization to [001] axis

5

Experimental SetupExperimental Setup

Z n TeA cce le ra to r

TrimM o d u la to r

D ip o le

M o n ito r

P o la rize r

A n a ly ze r

/4 P la te

toN IS U S

S p ec tro -m e te r

F ib e r

Trim

S eed L ase rD e lay

Electrons

Laser

Constants:

= 800 nm n0 = 2.83r41= 4 pm/V = 10l = 0.5 mm

= 90o at 170 kV/cm

6

-5 0 5 10

0204060

Time (ps)

E (

kV/c

m)

-5 0 5 10

0204060

Time (ps)E

(kV

/cm

)

-5 0 5 10

0204060

Time (ps)

E (

kV/c

m)

-5 0 5 10

0204060

Time (ps)

E (

kV/c

m)

Single Shot Time Calibration Single Shot Time Calibration

Ti:Sa chirped to 6 ps. e-beam ~1ps FWHM. Laser delay changed

from 0.5 to –1.0 mm Strong modulation in

spectrum from uncoated ZnTe crystal.

Average of 50 single shot spectra.

Charge from Coulomb field lower than ‘real’ charge of 250 pC.

s = 0.5 mm, Q = 130 pC

s = 0 mm, Q = 80 pC

s = -0.5 mm, Q = 125 pC

s = -1.0 mm, Q = 130 pC

7

Time resolutionTime resolution

Minimum THz pulse length with 6 ps, 6 nm chirped laser

Distance e-beam/laser (850 µm)

Monochromator (1800/mm) grating is 30 fs. Coherence length in ZnTe (500 µm thick) is 200 fs.

Measured length of 1.6 ps dominated by spectral distortion and confirmed by simulation.

fs20

c

rr

fs8000BW T

8

Jitter MeasurementJitter Measurement

Single shot enables jitter measurement.

Spectral distortions do not affect centroid position.

50 shots = 25 s. Jitter e-beam/seed laser 170

fs. Jitter low-level RF/Ti:Sa 200

fs. Energy jitter after bend

magnet equals 1 ps rf phase jitter mostly rf amplitude jitter.

Use for feedback on laser phase.

-600 -400 -200 0 200 400 6000

5

10

Delay (fs)

= 167 fs

-5 0 5 10

5

10

15

20

25

Time (ps)

Pul

se #

(s)

Head Tail

9

THz Pulse Field CharacterizationTHz Pulse Field Characterization

80 µJ CTR pulse observed at DUV-FEL. E-beam 700 pC, 100 MeV, 150 (???) fs rms.

Measure spatial-temporal electric field distribution with EO sampling.

Understand relay and focusing of CTR. Compare with CTR simulation code. Compare with bolometer measurement.

10

Electro-Optic THz Radiation SetupElectro-Optic THz Radiation Setup

CCD

ZnTe AnalyzerPolarizer

Lens

Electron BeamVacuum Window

Paraboloid

Ti:Sa LaserCoupling Hole, 2 mm /4

Delay

f = 7.5”

f = 1.5”

11

Signal and ReferenceSignal and ReferenceOAPOAP ZnTeZnTe

BSBS

/4/4 Pol.Pol.RefRef

SignalSignal

CameraCamera

12

Image Processing for Field MeasurementImage Processing for Field Measurement

Pixels

Pix

els

100 200 300 400 500 600

100

200

300

400

Horizontal (mm)

Ver

tical

(m

m)

-2 -1 0 1 2

-2

-1

0

1

2

Use compensator waveplate to detect sign of polarization change.

Reference IR (left) and Signal IS (right) obtained simultaneous.

Rescale and normalize both. Calculate asymmetry A of Signal. Subtract asymmetry pattern w/o THz.

A = 2IS/IR - 1

13

Time Dependent MeasurementTime Dependent Measurement

Use ‘mildly’ compressed bunch of 500 fs rms and 300 pC to get both 0-phasing and electro-optic measurement.

Temporal scan by varying phase of accelerator RF to both sample and cathode laser.

Approximately equivalent to varying delay between both lasers but much faster and computer controlled.

Measured to be 1.2 ps/degree.

14

Measured THz Field MovieMeasured THz Field Movie

15

Transverse-Temporal DistributionTransverse-Temporal Distribution

Time (ps)

Hor

izo

nta

l pos

. (m

m)

-1 0 1 2

-1

0

1

-0.5

0

0.5Image asymmetry Take horizontal slice

through images.

Asymmetry of 1 equals 170 kV/cm electric field strength.

Charge 300 pC.

Saturation and ‘over-rotation’ at higher compression.

Needs crystal « 500 m.

16

Simulation of CTR PropagationSimulation of CTR Propagation

20 ps

30 m

m

Decompose radiating part of coulomb field in Gauss-Laguerre modes.

Calculate transmission amplitude and phase through experiment for THz spectral range.

Use bunch form factor to reconstruct radiation field in time and space.

Example: 300 pC, 300 fs

17

Focus Distribution of THzFocus Distribution of THz Focus spot size

3 mm diameter.

Single cycle oscillation.

300 fs rms length.

Electric field strength more than 300 kV/cm at 300 pC charge.

Pulse Energy 4 J.70 J (700 pC, 150 fs)

18

Simulation vs. ExperimentSimulation vs. Experiment

Simulation gives 2 times more field.

Tighter focus in simulation.

Up to 50 kV/cm measured.

Simulation/2

Experiment

19

Single Cycle THz PulsesSingle Cycle THz Pulses

Pulse energy from field ~60 nJ.

Pulse energy with Joule-meter 170 nJ.

Pulse energy from simulation 800 nJ.

Good match of temporal and spectral properties.

Factor 2 and 4 difference in field and energy.

Measured 80 J to have 1 MV/cm field in focus.

-25

0

25 -1 mm

-25

0

25 0 mm

-25

0

25

Fie

ld (

kV/c

m)

1 mm

-1.5 -1 -0.5 0 0.5 1 1.5

-25

0

25 2 mm

Time (ps)

20

THz SpectrumTHz Spectrum

Present intensity limited by geometric apertures.

Low frequency cutoff at 15 cm-1 or 0.5 THz.

0 10 20 30 40 500

0.2

0.4

0.6

0.8

1

Wavenumber (cm-1 )

Effi

cie

ncy THz exp.

THz theor.FormfactorEfficiency

21

Potential Ultrafast EO-DetectionPotential Ultrafast EO-Detection

Intense ultrafast THz source. Modulated electron beam (@DUV-FEL). High pulse energy CTR (C...R).

Broadband, uniform response EO-material. EO-Polymer Composites.

Time domain laser pulse measurement.

Amplified fs-laser (injector drive laser). Spectral phase measurement. FROG, SPIDER. Not limited by laser pulse length.

)(1)()( 0 tEtEtE THz

22

Modulated Beam StudiesModulated Beam Studies

-3 -1.5 0 1.5 3

-20

0

20

E

(ke

V)

-3 -1.5 0 1.5 30

200

Cu

rre

nt

(A

)

Time (ps)

-3 -1.5 0 1.5 3

-50

-25

0

25

50

Time (ps)

Ene

rgy

(ke

V)

70 MeV180 pC

~100 fs e-beam structures from modulated drive laser. Measured with longitudinal tomography. Use to test electro-optic resolution, can be further

compressed.

23

Broadband Electro-Optic MaterialsBroadband Electro-Optic Materials EO-polymers* have 20x larger EO-coefficient than

ZnTe. No phonon resonances in far-IR.

Phase mismatch. Lifetime ~weeks.

10 µm sufficient. Cooling?

A.M. Sinyukov, L.M. Hayden, to be published* 20% DCDHF-6-V/20% DCDHF-MOE-V/60% APC

24

Spectral Phase Interferometry for Direct Electric-Field Reconstruction(Walmsley group, Oxford)

800 nm

800 nm

400 nm

Measuring the Spectral Phase: SPIDERMeasuring the Spectral Phase: SPIDER

})()(cos{)()(2)()()( cccccccc IIIIS

Mix 2 replicas from EO-modified pulsewith original streched pulse.

25

Summary and OutlookSummary and Outlook Simple single shot chirped EO setup sufficient for jitter

measurement. Jitter of 170 fs equal to low-level rf/laser jitter and estimates

from HGHG. Enables noninvasive laser/e-beam synchronization-feedback. Ultrafast EO measurement requires time-domain method.

High intensity THz pulses up to 1 MV/cm field strength from CTR.

CTR simulation, pulse energy and electro-optic measurement in resonable agreement.

Extract THz to accessible user station for various applications. Use time-domain single-shot EO method and apply to THz

from modulated electron beam.

26

AcknowledgementsAcknowledgements

SDL/DUV-FEL Team

G.L. CarrJ. GrecoH. Loos†

J.B. Murphy

J. RoseT.V. ShaftanB. Sheehy

Y. Shen

B. SinghX.J. Wang

Z. WuL.H. Yu

† In future at SLAC

This work was supported by DOE Contracts DEAC No. DE-AC02-98CH10886 and AFOSR/ONR MFEL Program No. NMIPR01520375.