COTR Diagnostics and Sources of Microbunching › events_folder › uBi12 › ubi12-Talks ›...

COTR Diagnostics and Sources of Microbunching

Alex Lumpkin, Fermilab Presented at Microbunching Instability Workshop

College Park, MD April 13, 2012

OUTLINE

I. Introduction II. Diagnostics with optical transition radiation (OTR) - Characterization of microbunching using Coherent OTR (COTR) III. Early microbunching interplay in a FEL IV. Mitigation options of COTR in imaging V. LCLS X-ray FEL effects VI. Summary

A.H. Lumpkin uBI4 Workshop April 13, 2012 2

Microbunching Mechanisms

• Microbunching of an electron beam, or a z-dependent density modulation with a period λ, can be generated by several mechanisms: – The LSC-induced microbunching (LSCIM) starts from noise

fluctuations in the charge distribution which causes an energy modulation that converts to density modulation following Chicane compression. This is a broadband case. (our topic).

– The laser-induced microbunching (LIM) occurs at the laser resonant wavelength (and harmonics) as the e-beam co-propagates through the wiggler with the laser beam followed by Chicane compression. This is narrow-band. (Oct. 2011 WS)

– In self-amplified spontaneous emission or (SASE) induced microbunching (SIM) the electron beam is also bunched at resonant wavelength and harmonics. This is narrow band.

• A microbunched beam will radiate coherently.(COTR) A.H. Lumpkin uBI4 Workshop April 13, 2012 3

4

Coherent Optical Transition Radiation

Calculations Coherent Spectral-Angular Distribution from a Macropulse, • Number of Photons per Unit Frequency and Solid Angle

E = 220 MeV σx’, y’ = 0.2 mrad

Single Particle OTR Spectral-Angular Distribution

From D. Rule and A. Lumpkin, PAC’01

( ) ( )kk ℑΩ

=Ω ⊥ I

ddNdr

ddNd

ωω1

22//,

2

( )( )2222

22

2

21

2 1

yx

yx

ce

ddNd

θθγ

θθωπω ++

+=

Ω −

Angle (radians)-0.010 -0.005 0.000 0.005 0.010

Rel

ativ

e In

tens

ity (a

rb. u

nits

)0.0

0.2

0.4

0.6

0.8

1.0

A.H. Lumpkin uBI4 Workshop April 13, 2012

5

COTRI Calculations (cont.)

Coherence Function ( ) ( ) ( ) 21 kk HNNN BB −+=ℑ

Fourier Transform of Charge Form Factors

( ) ( ) ( ) ( ) ( )zzyyxx kFkgkgQ

H ==kk ρ

Q = total charge of macropulse

Bunching fraction = fB=NB/N Note: The coherence function reduces to just the number of particles, N , when the number of microbunched particles, NB is zero.

From D. Rule and A. Lumpkin, PAC’01


6

Schematic of APS SASE FEL Experiment


*FEL decommissioned 2004

Sta-5

300-A VLD1 Data

• OTR/COTR data from a) LSCIM and b) LSCIM plus SIM. Peaks about 5x brighter in b). (Dec. 2001)


X (ch)

Y(ch

)

800 µm 800 µm

ND 1.0 500 nm SPF

X (ch) Y

(ch)

(a) (b)

Lumpkin, Dejus, Sereno: PRST-AB April 2009

300-A VLD2 Data

• OTR/COTR data from a) LSCIM and b) SIM. The right hand peak in b) is 100 times brighter. FEL at 530 nm.


500-nm SPF ND 2.0

(a)

(b)

Inte

nsity

X (ch)

Inte

nsity

X (ch)

320 µm (FWHM)

800 µm

(a)

Lumpkin,Dejus,Sereno: PRST-AB April 2009

Spectra in SASE and COTR Observed At and After Saturation

• Spectral evolution shown after undulators 5,7,9.

9 A.H. Lumpkin uBI4 Workshop April 13, 2012

Experimental Gain Curves Compared to GENESIS Results

• Experimental Gain curves for COTR (L) and UR (R) compared to GENESIS (Y. Chae). Data of 12-20-01.


Z Position (VLD #)0 2 4 6 8 10

OTR

Imag

e In

tens

ity (1

05 c

ts)

0.01

0.1

1

10

100

1000

300-A COTR DS6 GEN Norm. @ VLD1 GEN-260A-Norm.@VLD1

Z Position (VLD #)0 2 4 6 8 10

UR

Imag

e In

tens

ity (1

05 c

ts)

1e+0

1e+1

1e+2

1e+3

1e+4

1e+5

1e+6

1e+7

300-A UR HM-DS5 GEN-II-Norm.@VLD1GEN-260A-N1-@VLD1

Lumpkin,Dejus,Sereno: PRST-AB April 2009

Schematic of the APS Injector Area

• Both rf PC gun and rf TC gun beams available for acceleration, compression, and diagnosis.


L5

150 MeV

Sta-5 200-500 MeV

FS3 FS4 FS5

(Decom-missioned)

Enhanced OTR Images Seen

• Examples of images and profiles for L2 phase at 12 deg. (L-uncompr.) and L2 phase at 14.9 deg.(R-compr.)


X

X

X X

Y Y

Inte

nsity

Inte

nsity

Mitigation Techniques

• Diagnostics COTR mitigation options include: – Spectral differences between OTR and COTR can be used to

sort photons. (APS/ANL) – Scintillators can be used to enhance S/B ratio in combination

with a bandpass filter. (APS/ANL) – Temporal gating can sort prompt COTR and delayed scintillator

light, both MCP gate and CCD shutter options. (DESY, ANL, and FNAL examples.)

– Angular distributions of COTR and IOTR are displaced in some cases. Proposed at µBI-08 by R. Fiorito.

– Specific COTR angles can be spatially filtered versus 4π scintillator light. (DESY and SACLA)

• Instability suppression by laser beam heating, reversible beam heating, noise suppression,dispersion, drive laser shorter pulse. A.H. Lumpkin uBI4 Workshop April 13, 2012 13

OTR vs. COTR

Wavelength (nm)0 200 400 600 800 1000 1200

Rel

ativ

e In

tens

ity

0

2

4

6

8

10

12

14

16OTR Rel. Intensity Model CCD response (x10)COTR or OTR gained up (3 keV)

LCLS COTR Case: BC1

• Estimate OTR spectral effect in LCLS OTR12 case.


OTR COTR* (3 keV)

CCD Resp. (x10)

-UV-

*3-keV curve based on Ratner et al., FEL08

COTR Mitigation Test at 325 MeV

• 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

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

X(ch)

Y

I

Spectral Tests on COTR

• ICCD with GaAs PC data; Conditions such that un-filtered COTR needed ND2 to avoid saturation of ICCD.


LSO:Ce,400x40nm, ND1.0 OTR,450x40 nm, ND0.0

Spectrometer results (10-30-08)

• Grating offset adjusted to probe the red/NIR end of the COTR spectra using ICCD readout with GaAs PC.


X(ch

)

Wavelength(ch)

650 nm

900 nm

1250 nm offset, 10 µm Slit, ND1.0, ICCD

880 nm

880 nm In

tens

ity

880 nm

ITT web site

Converter Options

• Reconsider possible scintillators based on Ce doping of yttrium aluminum garnet (YAG), lutetium oxyorthosilicate (LSO), and yttrium aluminum perovskite (YAP).


Converter Spectrum (FWHM)*, Peak

Efficiency Response Time (FWHM)

Comment

YAG:Ce 487-587, 526 nm

1.0* 89 ns* 460 µm T

LS0:Ce 380-450, 415 nm

0.46* 40 ns* 530 µm T

YAP:Ce 350-400, 369 nm

~0.5 28 ns 460 µm T

OTR Broadband 0.0013* ~10 fs Surface

*Berg,Lumpkin,Yang: Linac 2000

Spectral Modulation Effect Seen

• Increased L2 gradient and achieved more COTR with compression. (Dec. 14, 2008.)


X

posi

tion

550 nm

a)

26 nm

X Po

sitio

n

26 nm b)

600 750 900 600 750 900 Wavelength (nm) Wavelength (nm)

Spectral Intensity Modulation

• Table I: Summary of observations on the COTR spectral modulation with the two gratings (12-08).


Grating Lines/mm

Wavelength (nm)

Peak Sep. (nm)

Mod. Freq. (THz)

Mod. Period (µm)

95 600 17 14.1 21 95 650 19 13.5 22 95 700 23 14.1 21 95 750 26 13.9 22 95 800 31 14.5 21 135 550 25 24.8 12 135 600 30 25.0 12 135 650 31 22.0 14 135 700 36 21.8 14

dν = -c dλ/λ2

ANL MCP Gate Results

• Recent ANL tests with MCP-CCD (See J. Dooling Talk)


FLASH : Gated ICCD on COTR

• MCP gate used to reject prompt COTR emissions


M.Yan,GSI WS 2011

FNAL Digital CCD Shutter Option

• The present digital CCDs provide a time shutter option at the 10-µs level at the video readout rate. For single pulse or pulses at or less than video rate (1-30 Hz) this could be used to reject the prompt COTR.

• A proof-of-principle test on 10-µs shutter was done with OTR and LYSO:Ce at FNAL photoinjector in 2011. Full rejection ratio not known yet. >50


A.S. Johnson et al. IPAC12

LYSO:Ce

OTR

Temporal Profile Measurement at SACLA under COTR

FEL'11@Shanghai 24 20 fs resolution

< 10 μm resolution 5712 MHz HEM11

4th Microbunching Instability WS, Univ. of Maryland April 11-13, 2012

Courtesy of the SACLA Team 24 A.H. Lumpkin uBI4 Workshop April 13, 2012

Profile Measurement using Spatial Mask and Fluorescence Screen

4th Microbunching Instability WS, Univ. of Maryland April 11-13, 2012

Courtesy of the SACLA Team

Deflector ON

Deflector Off

X-ray FEL Impact of µB Instability

• Laser heater simulations performed for FIR modulations.


Laser heater-LCLS (135 MeV)


Emma,Huang: uBI-II

Microbunching Instability at LCLS

Laser Heater Off

SXR Spectrometer Dump YAG Screen

e- energy


J. Welch et al., FEL10

Microbunching Instability at LCLS

Laser Heater ON – No Instability


J. Welch et al., FEL10

e- energy

Laser-induced energy modulation used to suppress uBI at LCLS in x-ray FEL, but COTR interferences still exist.

SUMMARY


• Interplay of the microbunching mechanisms is possible. – LSCIM can lead to prebunching an FEL at resonance condition. – Studies of COTR strength after chicane: possible 2-D spatial

imaging, 2-D angular distributions, x,y-spectral imaging, longitudinal imaging.

– At some point LSCIM or laser heater might interfere with LIM. – Energy modulation due to LSCIM can be measured directly.

• Benchmark codes on microbunching fractions. • Mitigation techniques for diagnostics have been

demonstrated for moderate to extensive COTR. • Intrinsic suppression of µBI by laser beam heating,

reversible beam heating, and dispersive elements. • Future tests planned at ASTA/Fermilab; need modeling.

Acknowledgements

• Many discussions with D. Dowell, Z. Huang, H. Loos of SLAC and W. Fawley (LBNL), D. Rule (NSWC), and R. Fiorito (UMD).

• Support from K.-J. Kim, R. Gerig, and H. Weerts of the Argonne Accelerator Institute.

• The collaborations on ANL linac tests of N. Sereno, J. Dooling, S. Pasky, Y. Li of ANL.

• The support of M. Wendt of Fermilab. • Two PRST-AB articles available: April and August 2009. Earlier reports in BIW08, FEL08, Linac08, µBunching II.


APS Spectral Filtering on LSO:Ce

• 400x10 nm BPF used to suppress COTR spikes on 500-µm thick LSO:Ce crystal scintillations into CCD camera.


COTR Spikes

COTR Spikes

X Position (ch) X Position (ch)

Inte

nsity

Inte

nsity

40-MeV Injector for ASTA/FNAL

A.H. Lumpkin BIW12 April 17, 2012 33

• 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 of M. Church

Prototype Imaging Station

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

A.H. Lumpkin BIW12 April 17, 2012 34

COTR Diagnostics and Sources of Microbunching › events_folder › uBi12 › ubi12-Talks ›...

Documents

Transcript of COTR Diagnostics and Sources of Microbunching › events_folder › uBi12 › ubi12-Talks ›...