recent advances in incoherent and coherent otr diagnostics · 2010. 12. 5. · 2. 1 1 4. 2 2 by...

Recent Advances in Incoherent and Coherent OTR Diagnostics

R. B. Fiorito

Institute for Research in Electronics and Applied PhysicsUniversity of Maryland, College Park , MD

PAC09, May 3-8,2009, Vancouver, BC

Outline of Talk*

1) Explanation of TR and OTR2) Incoherent OTR Properties and Applications3) Coherent TR, OTR Properties 4) COTR Observations for Long pulses (ct and Interpretation5) Mitigation of COTR 6) Simulation Code predictions using ELEGANT

*Special thanks for assistance in preparing this talk to

A. Shkvarunets (UMD)H. Loos, Z. Huang (SLAC)F. Sannibale (LBNL)

A. Lumpkin (FNAL)C. Welsche (U Liverpool)

AND ALL MY COLLEAGUES!

Transition Radiation in a NutshellDefinition: Radiation which occurs when charge moving at constant velocity

crosses a boundary between media with different dielectric constants

Weissacker-Williams(virtual photon picture) Reflection and refraction of virtual photons of all frequencies

(up to plasma frequency) at the interface

What is really radiating? the image charge current (Important to remember)

q v

BTR FTR

1/

ve x

y

zb

Fourier Spectrum of Virtual Photon Pulse

Fourier components are transverse plane waves with spectrum:

1ˆ ( ) q rE K

c c

b is the radiation impact parameter which represents the effective spatial extent and source size, b = of the virtual photon with wave number c.

K1 ()

1

1

Properties of Incoherent Transition Radiation (ctb ) useful for beam diagnostics

1. Prompt : fs response time

2. Broad band: flat frequency spectrum up to plasma freq. ofradiator material

3. Intensity linear with charge I ~ N

4. Angular distribution peaks at 1/

5. Polarized in direction of radiation lobes (radial polarization)

6. Beamed in direction of velocity of charge( relativistic)

4. OTR: High spatial resolution for imaging (independent of particle energy)ultimate resolution: point spread function determined by field of charge andangular acceptance of optics

5. Low photon yield per electron ~ 0.001- 0.001 in visible band

Diagnostics of beam parameters using IOTR

1-Near Field Imaging (spatial distribution)size (x, y)position (x, y) (offset)

2-Far Field Imaging (angular distribution)divergence (x’, y’) [angular resolution < 0.01/]trajectory angle (X’,Y’) [

/ 1/d l

Point Spread Function of OTR for Imaging determined field of charge and angular acceptance of Lens

1 11 0( ) ( /( )) ( / )PSFE K r r J r

For

*D. Xiang,et. al. PRSTAB (2007)

*>>

~

Resolution: Beam images taken with six different diagnostic screens under the stable experimental conditions (Q ~ 500 pC) at the ATF/BNL 40 MeV linac byYakimenko, et. al.

Linearity : Electron beam horizontal spot size as a function of charge, measured with scintillating diagnostics and OTR ( sub micron resolution independent of energy ).

Comparison of Imaging Screens YAG, Phospor, OTR

4

max = 1/2 (S) 2 2

2

, 2 2 2 2

1

d I er ,d d c ( )

1

Ve

()

IOTR Angular Distribution: Beam Trajectory Angle and Divergence

{ independent of frequency and beam size (or position )}

Full AD Horizontally Polarized AD

RadiallyPolarized

AD PatternCentered onDirection of

Ve

E

θθ

>> >>

Effect of Divergence on OTR Angular Distribution and Horizontal Scans

( 48 MeV CLIC2 Test Facility Beam )

IOTR Interferometry provides greater sensitivity to beam parameters

2i

V

2 2 1V

2 2 2TOT

2 2 2 2 1 e

L / L

L (

d I e ,d d c ( )where :

( )

,

/ )

• Center of pattern measures trajectory angle of particle

• Visibility of OTRI measures beam divergence and/or E/E)

• Radial Polarization of OTRI can be used to separately measure x’ and y’

• Fringe position measures beam energy (E)

x

y

(vacuum coherence length)

Two Foil OTRI

eL

Optical Diffraction-Transition Radiation Interferometry Extends OTRI diagnostics to low energy and/or low emittance beams

Beam

backward OTRODR(u) +OTR(s)

ODR(u) + OTR(s) + BOTR

Micro-mesh

Micro-mesh front foil overcomes scattering limit of conventional OTRI

Comparison of OTR and ODTR InterferogramsVertical (y) beam waist, E= 95 MeV (NPS linac) = 650 x 70 nm

vertical ( y ) scans

y

x

OTRI

ODTRI

ANL/AWA: E =13.8 MeV, L =3.7 mm, 632 x10nm, Kapton foil: t = 9.15μ, index = 1.8

ODR-DTR Transmission Interferometer

horiz

= 0.6 mrad

fitdata

horizontal

0 2.5/γ

vertical

0

0

Transmission interferometer needed at E

Near field image of: beam

Far field image of angular distribution

observ

Beam Splitter

Lens focused to Infinity

mirror

Bandpass Filter

Electron Beam

OTRI

Quad

At or near a beam waist, (e.g. x waist): 0xx

2 2 2 2x x x xx

RMS Emittance Measurement using Near and Farfield Incoherent OTR

2 2 2x x x

xrms

’

xrms

Quadrupole Scan General Case: the minimum of the spot size orminimum of the divergence is not close to a zero of the correlation term

( Emittance = 4.5micron, Energy =7.4 MeV )

(see Poster TH6REP053)

New theoretical analysis allows more precise emittance measurement using OTR observables (x, x’) *

Idea: The beam size and the beam divergence have different minima w.r.t. to s, f . For each minimium we get a different value for the beam size-divergence correlation but the same rms emittance:

1) true beam waist: difficult to achieve,

i.e. need to move foil along sat a constant focusing strength f

2) beam spot minimum:achievable with a usual quad

scan, i.e. vary f and minimize beam image observed with OTR

3) divergence minimum:achievable by varying f to

maximizing OTRI fringe visibility(angular quad scan)

2 2 2 2x x x xx

( , )x s f( , )x s f

0

( ( , )) 0 0f f

wx f s x

sx

2min min'

, )0 /

(

s L

xsf

Lx

xf

x

2min min

'( , )0

s L

xf

xx

x Ls f

*( see Poster TH6REP053 for details )

Optical Phase Space Mapping: localized divergence and trajectory angle measured within beam image

Electron beam

Image Camera

AD Camera

MaskPolarizer

BandpassFilter

Optical Radiation

(e.g. OTRI or OSRI )

Localized (inter beam) OTRI divergence and trajectory Angle measurements using a 1mm pinhole (optical mask)

1 2

4

3

5X horizontal

2

Carsten P. Welsch – Halo Measurements

Proposed Study of Halo’s using Dynamic Masking using Digital Micromirror Array (DMA)

(1) Aquire profile (2) Define core

(3) Generate mask(4) Re-Measure

2009 Particle Accelerator Conference, Vancouver, Canada, May 2009

Bunch Length Diagnostic usingBunch Length Diagnostic usingRadiation Fluctuation AnalysisRadiation Fluctuation Analysis

Note 1:can be used Note 1:can be used with any kind of with any kind of radiating process (OSR, radiating process (OSR, OTR,etcOTR,etc.).)

Note 2: that coherent component alsoNote 2: that coherent component alsohas fluctuations which typically are not has fluctuations which typically are not measurable (rel. amplitudes orders of measurable (rel. amplitudes orders of magnitude smaller incoherent case) but magnitude smaller incoherent case) but in case of COTR due to chaotic in case of COTR due to chaotic microbunchingmicrobunching they are visible.they are visible.

In real beams, due to presence of random modulation in the partiIn real beams, due to presence of random modulation in the particle cle distribution, and to the variation of this modulation passage todistribution, and to the variation of this modulation passage to passage, passage, incoherent radiation is emitted with fluctuating intensityincoherent radiation is emitted with fluctuating intensity

StupakovStupakov, , ZolotorevZolotorev SLACSLAC--PUB 7132, PUB 7132, 1996) showed that the variance of the 1996) showed that the variance of the fluctuating radiation intensity fluctuating radiation intensity incohincoh. . part of spectrum can be used to part of spectrum can be used to measure bunch length.measure bunch length.

Log

Brig

htne

ssLog Frequency

Single passagespectrum

Coherentcomponent

AverageSpectrum

tb

~1/

22

2009 Particle Accelerator Conference, Vancouver, Canada, May 2009

Absolute Bunch Length byAbsolute Bunch Length byIOSR Fluctuation Analysis at ALSIOSR Fluctuation Analysis at ALS

222 411

By using a By using a bandpassbandpass filter with bandwidth filter with bandwidth

and measuring the passage to passage and measuring the passage to passage relative variance relative variance 22 of the radiation of the radiation

intensity, the bunch length intensity, the bunch length can be can be

measured from:measured from:

SannibaleSannibale et al. PRSTet al. PRST--AB AB 12, 032801 (2009).

COTR recently observed in e beams whoseand bunch lengths are much longer than optical wavelengths

(e.g. LCLS and FLASH at DESY)

Questions: • Why is COTR observed in this situation? • What are the spatial and angular distributions of the COTR?• What is the spectrum of the radiation?

a. Is there enhanced COTR at given wavelengths?b. Is the COTR resonant (i.e. the signature of periodic microbunching)

or purely random in time (chaotic)• Where in the beam profile is the COTR produced?

is it localized or random in space?• COTR is a nonlinear function of N: bad for beam profiling;• Can COTR it be mitigated? how? spectral filtering? spatial filtering• Can COTR be used as a diagnostic for underlying physics?

Coherent TR Radiation diagnostics

22

2

, ,

( 1) ( , ) ( ,{ }

(

)

)

T z z ze

z z

N N f kd Id I Nd d d d

f

f k

F

1/ ?

2 2 2

2 22

?

exp[ ( ) ] exp[ ( / ) ]( )

( ) exp[ ( ) ] exp[ ( / ) ] 1

1r r

z z z z

f F

f F k

k

zk k and k k^ q; ;If transverse and longitudinal bunch distributions are Gaussianand 1,

,z

Degree of Coherency or Coherent Gain Coh Icoh zG I / I Nf f^= =

>>

>>

25 Henrik [email protected]

25LCLS COTR Results3/27/2009

COTR image produce by a bunch of charges

image formed by the intensity of OTR fields from an entire bunchof N electrons is obtained from the superposition of thefields from each particle at locations rj

and ,

zj

IOTR intensity: convol. of with single particle INTENSITY on the foilCOTR Image: convolution ofthe particle distribution withsingle particle FIELD on the foil

Two regimes for COTR Imaging:1) Beam size

COTR Image is convolution of PSF with beam distribution, or in extreme case , just the PSF

2) Beam size COTR Image is gradient of beam distribution

Loos, et. al. SLAC Pub 13395 and Proc. of FEL08

0.4

Far field (Angular) Intensitiesof COTR and IOTR

02/γ

radius (mm)

2 21( ) exp( / )e r r

Calculations of Near Field and Far field Intensity of COTR 250 MeV Gaussian beam (σ = 0.2mm > )

2( )eE

0 0.2 0.6 0.8

0 1/Observation angle

2

2

( )

( ) exp ( sin / )

COTR IOTRI I f k

f k

( )e r

COTRIOTR

Near Field Intensity of COTR

CTR from periodically micro-bunched beams• Produces periodic modulations in longitudinal charge and form factor leading toappearance of lines in the CTR spectrum at the harmonics ( nkr

) of the inter

micro-bunch modulation frequency. • Theory well developed by Rosenzweig, Travish and Remaine NIMA1995• Experimentally studied in a number of experiments in the THz and optical regimes.

4

microbunch FF

Macrobunch FFn 2CTR nI N bÞ µ

2 2 2exp( ( ) )L n r zn

F b k k

2 2

1

exp( / 2 )( ) 1 cos( )2

zn r

nz

zz b nk z

Model for Charge density

Longitudinal form factor:

Periodically modulated beam

bunching fraction

COTR Observations

CTR Spectral Observations showing Periodic Microbunching

CTTR previously observed in THz laser photo cathode modulation experiments at BNL-SDL

(J.Neumann et. al. J.Appl.Phys. 2009)

LongitudinalForm factorfor differentmicrobunchmodulationfrequencies

Longitudinalprofile of e

beam (Q=20pc)

Development of COTR Sidebands in a SASE FEL: exponential gain regime to saturation

(A.Lumpkin,et. al. PRL 2002)

COTR from energy modulated IFEL passing through a chicane*

n 2COTR nI bµ =

0 l 0sin(k z )g = g + hIFEL energy modulation

*C. Sears, et.al. PRSTAB 2008



COTR Images observed at LCLS

OTR2: Light linear with charge

OTR12, BC1 off: Enhanced by ~4-6, beam size changes

OTR12, BC1 on: Enhanced by



LCLS COTR Observations compared with Wire Scanner at LCLS after 1st Bunch Compressor

400μm

σ = 300μm

400μm

LSC microbunch Instability:1) shot noise: longitudinal density fluctuations produce energy modulations 2) energy modulations convert to increased density modulation by R56

in

DL1 (or in bunch compressor sections)

LSC microbunch Instability:1) shot noise: longitudinal density fluctuations produce energy modulations 2) energy modulations convert to increased density modulation by R56

in

DL1 (or in bunch compressor sections)



COTR: Evidence for LSC microbunch instability

10.2 10.4 10.6 10.8 11 11.2 11.40

2

4

6

8

10

QB Strength (kG)

OT

R In

tens

ity (

10 6

Cou

nts)

= 0.096 kG

10.2 10.4 10.6 10.8 11 11.2 11.4

60

70

80

90

100

Bea

m S

ize

( m

)

HorizontalVertical

10.2 10.4 10.6 10.8 11 11.2 11.40

2

4

6

8

10

QB Strength (kG)

OT

R In

tens

ity (

10 6

Cou

nts)

= 0.096 kG

10.2 10.4 10.6 10.8 11 11.2 11.4

60

70

80

90

100

Bea

m S

ize

( m

)

HorizontalVertical

OTR12, BC1 offOTR12, BC1 off

BC1BC1250 MeV250 MeV

L1SL1S

wirewirescannerscannerL1XL1X

LLDL1DL1135 MeV135 MeV

L0L0gungun


L1SL1S



L0L0gungun

OTR12

109

1010

106

107

108

Number of Electrons

OT

R In

tens

ity (

CC

D C

ount

s)

IncoherentCoherentLinear FitQuadratic FitCoherent Fraction Fit

Dogleg is achromat: preserves microbunchingDogleg is achromat: preserves microbunching

Charge DependencyCharge Dependency



COTR Spectrum of Compressed Beam

400 450 500 550 600 650 700 7500

2000

4000

6000

OT

R In

tens

ity (

Cou

nts)

IncoherentCoherentCoherent

400 450 500 550 600 650 700 75010

0

101

Wavelength (nm)

Gai

n F

acto

r

Measure COTR spectrum with CCD and transmission grating

OTR12 at normal Compression60 μm bunch length

Spike width resolution limited

COTR gain increases exponentially from blue to red

Measure COTR spectrum with CCD and transmission grating

OTR12 at normal Compression60 μm bunch length

Spike width resolution limited

COTR gain increases exponentially from blue to red

Δλ = 25 nm


L1SL1S



L0L0gungun


L1SL1S



L0L0gungun

OTR12

Shot to shot optical fluctuations ( cb = 60 μm ) ; I > I incoherent

D. Ratner, A. Chao, Z. Huang, SLAC 13392; FEL08

Theoretical Microbunching Instability Gain Curves compared to Data

256 0 0 0( , , , , ....) | ( ) |g d s =xG k R E b k

bunching fraction

E= 250 MeV, = 0.2 mm

Gain

250 MeV, Far field Intensities

Angle [1/]0 2 4 6 8 10

Inte

nsity

[arb

. uni

ts]

1e-6

1e-5

1e-4

1e-3

1e-2

1e-1

1e+0

COTRIOTR

4.3GeV, Farfield Intensity

Angle [1/]

0 2 4 6 8 10 12 14 16 18 20In

tens

ity [a

rb.u

nits

]1e-6

1e-5

1e-4

1e-3

1e-2

1e-1

1e+0

Coherent OTRIncoherent OTR

Mitigation of COTR* by Fourier Plane Spatial Filtering

* See also PAC09 Poster Papers: TH5RFP043, TH6REP021

λ=600nm λ=600nm

Mask

Optical system for spatial filtering/mitigation of COTR

OTR target Lens1,F1=250mm

Lens2, F2=125mm

Splitter with mask

Sensor focusedon target, 1:1

Sensor focusedon splitter,

angular image, 1:1

2 F1

F1, angularImage plane

2F2

2F2 2F1



LCLS Simulation of COTR using ELEGANTP

ositi

on y

(m

)

-600 -400 -200 0 200 400 600

-300

-200

-100

0

100

200

300

Position x (m)

Pos

ition

y (m

)

-600 -400 -200 0 200 400 600

-300

-200

-100

0

100

200

300

SimulationSimulation

MeasurementMeasurement

Use Elegant tracking data at OTR22 for 2% initial modulation at 40 μm

COTR simulation calculates form factor transversely resolved for 11 λ’s

Indication that the two rings are from current peaks in bunch at head & tail

Use Elegant tracking data at OTR22 for 2% initial modulation at 40 μm

COTR simulation calculates form factor transversely resolved for 11 λ’s

Indication that the two rings are from current peaks in bunch at head & tail

x (m)

z (

m)

-400 -300 -200 -100 0 100 200 300 400 500

-20

-10

0

10

20

Y. Ding

Elegant TrackingElegant Tracking

-1mm X 1mm

coherent Imax=1.34E9 incoherent Imax=1.26E9

Simulation Code predicts IOTR and COTR distributions using ELEGANTFERMI@Trieste, Linac1 (250 MeV), =0.5-1.0m

Y

coherent Imax=3.84E12 incoherent Imax=3.16E12-5 5 -5 5

-1mm X 1mm

Near field(image)

Far field(ang.distrib.)

Particle distribution

E (M

eV)

-2.5mm Z 2.3mm

260

230

Y

X

Energy distribution

x

y

() y

()

-0.75mm X 0.75mmcoherent Imax=6.46E10

(E= 1.2 GeV, = 2-4um, )

E(MeV)

-0.14mm Z 0.15mm

1208

1180

Y

bunch distribution

Incoherent Imax=5.94E9

IIRTR and CIRTR expected from FERMI@Trieste after 2nd

bunch compressor linac 4

Coherent Imax=3.6E15

-4 4

Incoherent Imax=3E13

energy distribution

Slide Number 1Slide Number 2Slide Number 3Slide Number 4Slide Number 5Slide Number 6Slide Number 7Slide Number 8Slide Number 9Slide Number 10Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16Slide Number 17Slide Number 18Slide Number 19Proposed Study of Halo’s using Dynamic Masking using Digital Micromirror Array (DMA)Slide Number 21Slide Number 22Slide Number 23Slide Number 24COTR image produce by a bunch of chargesSlide Number 26Slide Number 27Slide Number 28Slide Number 29Slide Number 30COTR Images observed at LCLSLCLS COTR Observations compared with Wire Scanner at LCLS after 1st Bunch CompressorCOTR: Evidence for LSC microbunch instabilityCOTR Spectrum of Compressed BeamSlide Number 35Slide Number 36Slide Number 37LCLS Simulation of COTR using ELEGANTSlide Number 39Slide Number 40

recent advances in incoherent and coherent otr diagnostics · 2010. 12. 5. · 2. 1 1 4. 2 2 by...

Documents