Imaging Techniques of Relativistic Beams: Issues & Limitations

Imaging Techniques of Relativistic Beams:

Issues & Limitations

Alex Lumpkin and Manfred Wendt,

Fermilab,

Presented at the LCWS11

September 27, 2011

Granada, Spain

Outline

I. Introduction

II. Beam profiling with YAG:Ce* scintillation• Scintillator resolution• Depth-of-focus issue

III. Optical Transition Radiation (OTR)• OTR basics• OTR point-spread-function (PSF) aspects• Microbunching instability and coherent OTR

IV. Future tests

V. Summary

A. Lumpkin and M. Wendt LCWS September 26, 2011 2

*Yttrium Aluminum Garnet, Cerium doped

Fermilab A0 Photo Injector

Beamline and diagnostics support for EEX applications


3.9 GHz TM110 Cavity

Parameter Units Value

Energy MeV 15

Energy spread keV 10 ‒ 15

transverse emittance mm mrad 2.6±0.3

Bunch length psec 3.1±0.3

Bunch intensity nC ~0.1 ‒ 5

Intro to Beam-Size Imaging

• The charged-particle beam transverse size and profiles are part of the basic characterizations needed in accelerators to determine beam quality, e.g. transverse emittance.

• A basic beam imaging system includes:– conversion mechanism (scintillator,

optical or x-ray synchrotron radiation (OSR or XSR), Cherenkov radiation (CR), optical transition radiation (OTR), undulator radiation (UR), and optical diffraction radiation (ODR).

– optical transport (lenses, mirrors, filters, polarizers).

– imaging sensor such as CCD,CID, CMOS camera, with or without intensifier and/or cooling

– video digitizer– image processing software


video digitizer

optical transport

imagingcamera

conversionscreen

beam

image processing& controls software

Identify Corrections to Consider

• System related– YAG:Ce powder and crystal screen spatial resolution.– Camera resolution and depth of focus.– OTR polarization effects and OTR point spread function.– Camera calibration factor.– Finite slit size (if applicable) .

• Accelerator / beam related– Beta star term in spectrometers.– Macropulse blurring effects on energy spread , beam size,

and beam divergence in OTR images.


Observed vs Actual Slit Image Size

• Uncorrelated terms are treated as a quadrature sum to actual image size Act (see Lyons’ textbook a).– Observed image size Obs– YAG screen effects YAG– Camera resolution Cam– Finite slit width Slit

• In addition there can be macropulse effects and OTR polarization effects.


and solving for the actual beam size we have,

aLouis Lyons, Statistics for Nuclear and Particle Physicists (1986)

𝑂𝑏𝑠2=𝐴𝑐𝑡 2+𝑌𝐴𝐺2+𝐶𝑎𝑚2+𝑆𝑙𝑖𝑡 2

𝐴𝑐𝑡=√𝑂𝑏𝑠2−𝑌𝐴𝐺2−𝐶𝑎𝑚2−𝑆𝑙𝑖𝑡2

Converter Screen Properties

YAG:Ce (Cerium doped)powder or single crystal

OTR screen,e.g. Al or aluminized Si

Efficiency ~100x 1x

Spatial resolution Volume effect, grain size EM surface phenomena

Spectral content Narrow band (~20 nm) Broad band

Saturation, non-linearities at high beam intensities no

Response time ~50 – 100 nsec ~10 fsec (skin depth)

Screen geometry: normal / angular (450)

depth of focus, scattering, effective thickness, system simplicity, etc.

Screen thickness,energy deposition,beam scattering

100 μm rangeminimum: 1 μm (fragile!)maximum: some 100 μm

Light scatteringHalo effects through scintillating volume

None


YAG:Ce Powder Scintillator Screens

• YAG:Ce screens, used at the A0 Photoinjector:– The screens have nominally a 5 µm grain size and are coated at

50-µm thickness on various metal substrates.– Substrates are Al or SS and 1 mm thick.– In the A0PI arrangement the scintillator was on the front surface

of the substrate, and oriented at 450 to the beam direction.– Powder screens are kindly provided by Klaus Floettmann (DESY).

• Observed Characteristics– The response time is about 80 ns FWHM.– There have been reports of saturation of the mechanism for

incident electron beam areal charge densities ~10 fC/µm2.• This effect can cause a charge dependence of the observed image

size in addition to the low-charge, screen resolution limit.


YAG:Ce Powder vs OTR Screen

# of bunches X5 linear polarization Fit σ (pixel) Size (μm)

YAG:Ce 1 none 5.67 ± 0.05 128.7

vertical 5.71 ± 0.04 129.6

OTR 10 none 5.49 ± 0.05 124.5

vertical 4.47 ± 0.09 101.0


• Both screen surfaces at 450 to the beam direction.• Gaussian fits to the projected beam profiles

of 10 images.• Deduced YAG resolution term (page 6): 80 ± 20 μm• YAG resolution,

averaged using three measurement sets: 60 ± 20 μm

Screen Resolution vs. Thickness


Scintillator Thickness (m)

0 200 400 600 800

Res

olu

tio

n

( m

)

0

50

100

150

200

250

300

350

Powder Data Crystal Data

2

Chromox,APS/ANL,450

YAG:Ce,A0PI,450

YAG:Ce, single crystalSCSS and Mainz,00

YAG:Tb,BNL,00

Chromox,Elettra,450

YAG 5 μm grain size

• Scintillator screen resolution vs. thickness after applying corrections discussed on page 6.






Depth-of-Focus Issues

• 1 mm (X5) / 4 mm (X24) spaced slits, 50 μm wide– Camera calibration ~30 μm / pixel.

• Depth-of-focus issues in extended field of view for 450 arrangement of the YAG:Ce scintillator screen


X5 X24

MATLAB Emittance Code

• Application tool provides online emittance and C-S parameter calculations to facilitate operations.


courtesy R. Thurman-Keup






Optical Transition Radiation (OTR)


• OTR can be used for beam– profile / size – position – divergence

• Charged particle passing a media boundary (EM dipole).

– energy, – relative intensity – bunch length

OTR angular intensity distribution of a single charged particle

OTR Calculation


2222

22

2

21

2 1

yx

yx

c

e

dd

Nd

Angle (radians)

-0.010 -0.005 0.000 0.005 0.010

Rel

ativ

e In

tens

ity

(arb

. uni

ts)

0.0

0.2

0.4

0.6

0.8

1.0

• OTR single particle spectral-angular distribution:

– Ω spatial angle– ω angular frequency– N1 # of photons

– Θx,y radiation angle

– E, ħ, c, π constants

• Coherent spectral-angulardistribution from a macropulse

– N # of photons from per unit frequency and solid angle (typ. 1 e -> 0.001 photons)

– r reflection coefficient– I interference function (double foil)– F coherence function (can be non-linear)

kk

I

dd

Ndr

dd

Nd

1

22//,

2

E = 220 MeVx’, y’ = 0.2 mrad

Prototype Imaging Station

• New developed imaging station in collaboration with RadiaBeam, Inc.


OTR / YAG Configurations


• Switchable assemblies to compare options. – Still tweaking!

Impedance Screen

YAG:Ce, plus Al on Si mirror

OTR, normal to beam, plus Al on Si mirror

Impedance Screen

OTR foil, 1µm plus 1µm foil at 450

Option 1 Option 2

OTR,100 µm Al plus Al on Si mirror

screen

mirror

beam

light

Test of OTR Normal to Beam

• Optics focused on crystal location: – gives superposition of focused OTR and

defocused OTR source from mirror.


Single-Gaussian Fitσ1 = 16.2 ± 0.2 pix

Double-Gaussian Fitσ1= 9.2 ± 0.1

σ2 = 28.1± 0.2

OTR Polarization and PSF History

• 1996: Lebedev evaluated OTR resolution.• 1998: Castellano, et.al. points out the OTR PSF has a

polarization feature.– Calculated a 12λ (FWHM) effect for the total width

and 0.1 rad collection angle.

• 2007: FNAL & JLAB observed OTR / ODR polarization effects on the beam image size.

• 2007: Xiang, et.al. calculates PSF for OTR and ODR.• 2010: OTR polarization on beam images reported by

FNAL A0 photoinjector staff at the BIW10.• 2010: KEK OTR experiment uses a polarizer to analyze

the details of the PSF (IPAC2010).– In 2005 the experiment was performed without polarizer.


Point Spread Function (PSF)


• At the diffraction limit the image of a point source radiates a ring pattern defined by the OTR point spread function (PSF):

– maximum acceptance angle– magnification factor– radius

of the lens

∆ 𝑥 ≈1.22𝜆 /∆ 𝜃

22 2

12 20( , , ) [ ( ) ]

m

mf J d

OTR response Point charge diffraction

PSF: convolution integral of

Z

Source Lens Image

a b

𝜃=𝑅𝑖/𝑎 /ikR M

/M b a𝜃𝑚

𝑅𝑖

2 105

1 105

0 1 105

2 105

0

5 104

0.001

0.0015

0.002

Radius on image plane

f^2

Cross sectionExample:M=1, E=4GeV, λ=500nmcourtesy C. Liu

PSF Properties


• Insensitive to the beam energy• Sensitive to the acceptance angle• Polarizer mitigates PSF imaging errors

– E.g. horizontal polarizer reduces verticalimage size by

– With a mask blocking rays ofangle ≤Θ1, the PSF will be:

𝑦 /√𝑥2+𝑦 2

2 105

1 105

0 1 105

2 105

0

5 104

0.001

0.0015

0.002

¦Èm=0.1rad¦Èm=0.05rad¦Èm=0.1rad¦Èm=0.05rad


f^2

PSF dependence on acceptance angle

2 105

1 105

0 1 105

2 105

0

5 104

0.001

0.0015

0.002

¦Ã=8000¦Ã=100¦Ã=8000¦Ã=100


f^

2

PSF dependence on beam energy

2 105

1 105

0 1 105

2 105

0

5 104

0.001

0.0015

0.002

¦È1=0¦È1=0.025rad¦È1=0.050rad

¦È1=0¦È1=0.025rad¦È1=0.050rad

Radius on image plane (m)

f^2

Lens

O I

1

22 2

12 2( , , ) [ ( ) ]

m

mf J d

courtesy C. Liu

OTR PSF Calculations (MATLAB)

• 14.3 MeV, M=1, λ=500 nm, θmax=0.010, sigma =25 µm

• This version with convolutions implemented at FNAL.


-1 0 1

x 10-4

-1

0

1

x 10-4

-1 0 1

x 10-4

-1

0

1

x 10-4

-1 0 1

x 10-4

-1

0

1

x 10-4

X

Y

Total PSF Hpol PSF

Vpol PSF At Image plane

courtesy R. Thurman-Keup

PSF Polarization Convolved

• 14.3 MeV, M = 1, λ = 500 nm, θmax=0.010, σ = 25 µm


Original Sigma = 25 μmTotal PSF Sigma = 33.18 μmHorPol-HorProj PSF Sigma = 38.01 μmHorPol-VerProj PSF Sigma = 29.39 μm

-150 -100 -50 0 50 100 1500

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

x (m)

Total

HorPol-HorProj

HorPol-VerProj

-150 -100 -50 0 50 100 1500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

x (m)

Convolution with Beam

None

TotalHorPol-HorProj

HorPol-VerProj

Inte

nsity

Inte

nsity

-1 0 1

x 10-4

-1

0

1

x 10-4

-1 0 1

x 10-4

-1

0

1

x 10-4

-1 0 1

x 10-4

-1

0

1

x 10-4

Hpol PSF

PSF Polarization Convolved (cont.)

• 14.3 MeV, M = 1, λ = 500nm, θmax = 0.010, σ = 10 & 50 µm


Original Sigma = 50 μmTotal PSF Sigma = 55.63 μmHorPol-HorProj PSF Sigma = 58.49 μmHorPol-VerProj PSF Sigma = 53.05 μ

-300 -200 -100 0 100 200 3000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

x (m)


None

TotalHorPol-HorProj

HorPol-VerProj

Inte

nsity

Original Sigma = 10 μmTotal PSF Sigma = 22.59 μmHorPol-HorProj PSF Sigma = NAHorPol-VerProj PSF Sigma = 16.63 μm

-50 -40 -30 -20 -10 0 10 20 30 40 500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

x (m)


None

TotalHorPol-HorProj

HorPol-VerProj

Inte

nsity

Polarized Beam Images at XUR

• OTR Perpendicular component has 15 % smaller profile.– Beam measurements with a vertical stripe, optics generated.


Total Pol.: LeftSingle-Gaussian Fitσ1 = 66.8 ± 0.3 μm

Vert. Pol.: RightSingle-Gaussian Fitσ1 = 55.1 ± 1.1 μm

10µm effect @ 55 µm

(Cal.: 5.3 µm/pixel)

KEK Experimental OTR PSF


with respect to zero which included a constantbackground; b is the amplitude of the distribution; c is thedistribution width; σ is the smoothing parameterdominantly defined by the beam size; and Δx is thehorizontal offset of the distribution with respect to zero

courtesy A. Aryshev

*Legend reversed

• KEK staff used vertical polarizer and small beam to observe PSF and suggested potential use of structure.– Use PSF valley for profile measurements at the PSF limit.

OTR vs. COTR

Wavelength (nm)

0 200 400 600 800 1000 1200

Rel

ativ

e In

ten

sity

0

2

4

6

8

10

12

14

16

CC

D R

esp

on

se

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

OTR Rel. Intensity ModelCCD response COTR with OTR gained up (3 keV)

COTR Case at 250 MeV

• Estimation of OTR/COTR spectral effect for LCLS case.


OTR~1/λ2

COTR(3 keV)

CCD Resp.

-UV-

COTR Mitigation Test at St-5/ANL

• Reduction of COTR effects with 400x40 nm BPF, but need more sensitive camera than 40dB analog CCD to see remaining OTR.


Y

X(ch)

Y

X(ch)

I I

COTR:ND0.5 COTR:400x40 nm LSO: 400x40 nm

X(ch)

Y

I

40-MeV Injector for ASTA/FNAL


• Injector being installed with First beam expected in 2012.

electron gun

booster cavities

3rd harmonic

cavity

flat beam transform

chicane

deflecting mode cavity

beam dump

1st cryomodule

test beamlines beam

dump

spectrometer magnet

40-MeV Injector

Booster cavity 2 (from DESY and Saclay) installed in NML

First cryomodule (from DESY) installed at NML.courtesy M. Church

Summary

• Scintillator resolution terms should be characterized,– Use normal incidence of beam preferred geometry to minimize

depth-of-focus, effective radiation thickness in beam images.

• OTR polarization effects need to be elucidated– Plan to optimize OTR PSF and optical resolution.– Plan to use linear polarizers with OTR imaging for the

perpendicular profile components at ASTA.

• Mitigate microbunching instability effects for profiling of bright beams.– Plan to use 400x40 nm band pass filters and LYSO:Ce* crystals

after bunch compression at ASTA to suppress expected diagnostics complications due to COTR.

• The future remains bright for imaging techniques!


*Lutetium Yttrium oxyorthosilicate (420 nm, violet), Cerium doped

Imaging Techniques of Relativistic Beams: Issues & Limitations

Documents

actual beam size

wendt lcws

beamsize imagingthe

transverse size

beam scattering100

beam divergence

beam quality

finite slit size