Femtosecond Resolution Bunch Profile
Diagnostics

Bernhard Schmidt
IBIC12, Tsukuba, Japandriving forcesIntro

fs bunches driving fs x-ray pulses :below ps & nC

plasma wakefield experiments :the fs & fC challenge

few fs & few pC

100 fC, 10 fs compressed to 1.5 fs- status of established technologies- progress towards online monitoring

J. Grebenyuk et al., in
preparation- exciting experiments- new and demanding ideas

Bernhard Schmidt| 2.10.2012 |IBIC-2012

Transverse Deflecting Structures (TDS)

TDS

well established - versatile - high resolution - bulky - expensive
RF - transverse deflector at zero crossing

  




 

  
Ry y‘

imprint y‘(s)

Rt =

r

Eb
0 eV !RF sin( 


measure y(s)time resolution✏)

limited by beam emittancestreak power and RF-frequency
beam optics
Bernhard Schmidt| 2.10.2012 |IBIC-2012

longitudinal phase space

TDS

combine TDS with orthogonal dispersion

energyFLASH, LCLS, FERMI: TDS + Dipol spectrometer© C. Behrens

FERMI@Elettra

time

SACLA: TDS + „dispersion-leakage“from compressor
see H. Maesaka: MOIC02

Bernhard Schmidt| 2.10.2012 |IBIC-2012

imaging problems

TDS

TDS imaging suffers form COTR due to microbunching

replace metallic screen by scintillator➝ still COTR from surface

delayed image using gated camera
promptdelayed 200 ns

mask central OTR coneS. Matsubara
MOCC04
C. Behrens et al. , PRSTAB 15, 062801 (2012)use dispersive arm (FLASH, LCLS..)➝ but no operation parallel to FEL- disregard imaging, use wire scanners(LCLS)

Bernhard Schmidt| 2.10.2012 |IBIC-2012

resolution & complications

TDS- induces energy spreadminor effects for high energies- wakefields

non-vanishing problem : intrinsic (y‘-z) correlations
retrieval from 2 streak polarities ( ref. H. Loos)(substantially different bunch profiles)
3.0

current HkAL

2.5
2.0

—

+ 90 deg—- 90 deg—

retrievalbias free measurements
need at least two
opposite streaks

1.5
1.0

att.: TDS is not longer single shot

0.5
0.0
-100

-50
0
50
longitudinal coordinate HµmL

100

data from FLASH © S. Wesch

Bernhard Schmidt| 2.10.2012 |IBIC-2012

way to the future

TDS

machine

f-RF

Vmax

Ebeam

resolution

FLASH

2.8 GHz (S)25 MV

1.2 GeV~10 fsFERMI

3.0 GHz (S)20 MV

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5.7 GHz (C)60 MV

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11.4 GHz (X)

46 MV< 13.6 GeV~ 10 fs~1 fs exp.

30 MeV3 ps
60 A

0.4 GeV
300 fs
0.6 kA

1.4 GeV
30 fs
3 kA

C-band acc. tubes

RF deflectors

BC3

C-band acc. tubes

BC2

S-band acc. tubes

1st bunch compressor(BC1)

C-correction cavity

L-APS cavities

CeB6 E-gun
beam chopper
238MHz buncher
476MHz booster
L-correction cavity

Bunch length measurements with RF deflector

SACLA

H. Maesaka: MOIC02

time resolution ~10 fs limited bybeam size at screen

Bernhard Schmidt| 2.10.2012 |IBIC-2012

the XTCAV project at LCLSTDSXTCAV + spectrometer behind undulator

P. Krejcik, TUPA41

derive photon pulse profile
from long. phase space measurement
expected performancestatus : structures installed
RF-power next spring© P. Krejcik et al.

Bernhard Schmidt| 2.10.2012 |IBIC-2012

beating the limits : new ideas

TDS

D. Xiang, Y. Ding
Phys. Rev. ST Accel. Beams 13, 094001 (2010)„the ideal z-mapper“TDS+ free from correlation problems+ resolution limited by second order terms

by careful adjusting the chicane parametersand streaking strength of TDS

y(s) = f k z

simulation result- resolution scales with δE2- bulky, complex, needs tight balancing
ex: 0.2 fs for δE=0.1%

Bernhard Schmidt| 2.10.2012 |IBIC-2012

optical - streaking

TDS

laser + TDS : high resolution + large dynamic range

TDS
streakenergylaser
streakdivergence
x0 ⇠

2

p
PL

high beam energy -> GW (TW) laser power

CO2 laser
proposed proof of concept
experiment at UCLA
Bernhard Schmidt| 2.10.2012 |IBIC-2012

Coherent Radiation

CR

Spectral distribution of „bunch radiation“ with Ne >> 1
For all direct radiation processes : incoherent and coherent contribution
Synchrotron Edge Transition - Radiation
Diffraction ..

dU
=
d d⌦

✓

dU
d d⌦

◆

single electron
spectrum

wavelength space2

1(N + N 2 |F3d ( , ⌦)| )Form factor
reveals charge distribution

configuration space

Bernhard Schmidt| 2.10.2012 |IBIC-2012

Coherent Radiation: Basics

CR

IF no long. - trans. correlation : factorize
FT for fixed θ , only weak λ dependencecomplex function

attn.: fails for

r

/(2⇡)

longitudinal form factor

coherent radiation intensity

F ( ) = |F ( )|e( )notice : the phase is missing !
Bernhard Schmidt| 2.10.2012 |IBIC-2012

measuring techniquesCR
Lai, Happek, Sievers (1994)

interferometer : scanning device, simple (one channel readout)..

temporal autocorrelation function

S( ⌧ )F -TransformP (f )
K-K phase
F -Transform

I(t)© B. Leißner et al., PAC1999- not single shot- diffraction limits resolution© R. Thurman-Keup et al., FERMILAB-PUP-08_115-AD
Bernhard Schmidt| 2.10.2012 |IBIC-2012

single shot spatial auto correlator

CR

combined a.c.
tracesG. Andonian
TUPA39resolution determined by transverse beam size
data taken at A0 photoinjector (Fermilab) and mixing angle
Bernhard Schmidt| 2.10.2012 |IBIC-2012

CRISP4 : four stage grating spectrometer

CRD

diffractive grating + parallel read out : single shot spectrometerbut : higher order problem ! λ/2 in 2. order and λ in 1. order have same diffraction angle- pre-filter short wavelengths- only factor 2 in λ with one grating possible , needs cascaded grating stages







4 stages : 1 decade in λ(parallel)
2 x 4 stages : 2 decades in λ(sequentially)

monitoring : relative intensities ok









form factor :absolute intensity as function of λrequired ! Needs detailed
understanding of all components

Bernhard Schmidt| 2.10.2012 |IBIC-2012

installation & calibration

CRD

absolute |F| measurement needs
full understanding of all details

CTR screen

tunnel0.6

rel. absorption

0.5
0.4

normal layout

0.3
0.2

new scheme

0.1
10

40
100
wavelength HµmL

20

200

0.4

400

120 channels pyroelectricdetectors

sensitivity HVênJL

0.0

wavelength calibrated..

0.3

0.2

0.1

0.0

4

10

20
40
100
wavelength HµmL

200

20 m in vacuum
beam-line &spectrometer

400

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1.0

Ê

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0.8

transmissionÊÊÊdiamond window tomachine vacuum

Ê

0.6

Ê

Ê

— LDPE: fit Æ d = 42 µm
Ê SP4 short
Ê SP4 long

0.4
Ê

0.2
0.0

Ê

4

10

20
40
100
wavelength HµmL

Bernhard Schmidt
200

400

| 2.10.2012 |IBIC-2012

CRISP4 now an online-toolCRD

regularly used
as online
tool for monitoring
and compression
control

Bernhard Schmidt| 2.10.2012 |IBIC-2012

compare TDS and CRISP4 : |form factors|
1

2.5

Á3

bunch shape 1

TDS profiles» F{ »

10-1

1.5

10-2

1.0

Á2

10-3

Á1

0.5

l HµmL1bunch shape 2

-300

-200

-100
0
100
200
longitudinal coordinate HµmL

300

10-1

» F{ »

current HkAL

2.0

0.0

Chapter10-2- very good agreement for long wavelengths
10-3- TDS profiles show less short wavelength content

l HµmL1bunch shape 3- smoothed and averaged by tomography- finite resolution- TDS and CRISP4 are 60 m apart !

» F{ »

10-1

10-2

10-3

4

10

20

40
l HµmL

Bernhard Schmidt

100

200

400

| 2.10.2012 |IBIC-2012

reconstructed shapes (KK-phase)

CRD

Q = 0.137 nC
E = 500 MeV- single bunch with 47 pC- very sharp leading spike (8.5 fs rms) on long tail1
2012-02-21T060930-28

FLASH-CRISP4û141m

20.fs Hpeak-fwhmL 285.7fs Hfull-rmsL

250

Q=0.047nC

2012-02-21T060930-19

E=700.MeV
in total 1 bunches

200

»F»

current @AD

0.1
150

100
0.01
50

0

reconstructed profiles along
bunch train

5

10

50
l @mmD

100

500

-200

0

200

400
t @fsD

600(using Krames-Kronig relation, not unique, „a possible bunch shape“)
Bernhard Schmidt| 2.10.2012 |IBIC-2012

800

1000

outlook CR methods

CRD

shorter bunches, less charge... (few fs - pC bunches)
wavelength range 1 µm - 20 µm

can be covered by prisms ! (KRS-5 or ZnSe)

more sensitive detectors (ex. HgCdTe arrays)
T. Maxwell
TUPA47© T. Maxwell

Bernhard Schmidt| 2.10.2012 |IBIC-2012

Optical Replica Synthesizer

ORS

proposed by Saldin, Schneidmiller and Yurkov in 2004

„Operating range of proposed diagnostic technique nicely includes thatof most ultrashort XFEL injector linacs, so it should be ideal for mosteveryday diagnostics as well as many more exotic ones“.

• overlap beam with laser pulse in resonant undulator (modulator)• generates energy modulation• transform into density modulation by small chicane (R56)• produce coherent radiation pulse at laser wavelength in second undulator• measure temporal profile of „optical replica pulse“ with established optical methods (FROG,...)© S. Khan

Bernhard Schmidt| 2.10.2012 |IBIC-2012

ORS limitations

ORS

ultimate resolution limit : phase slippage
t

Nu /c

5 periods, 800nm ➝ 13 fs

beam constraints :beam size in undulators small (diffraction limit)x ⌧pLu /(2⇡)5 periods, 1 m ➝ σx << 350 µm
longitudinally „smooth“ beam (no microbunching)technical contraints :spatial and temporal overlap of laser and beamtime jitter laser to beam << laser pulse length
sufficient laser power to overcome intrinsic energy spreadsufficient radiated power for FROG operation

Bernhard Schmidt| 2.10.2012 |IBIC-2012

ORS installation at FLASHORS
G. Angelova et al, PRSTAB 11, 070702 (2008)

Ebeam
q
x

Nu u
L

PL
L

R56

700 M eV
0.5 1.2 nC
0.3 mm5 · 0.2 m
800 nm
0.5 mJ
400 f s170 µmtwo orthogonal
undulators

Bernhard Schmidt| 2.10.2012 |IBIC-2012

ORS@FLASH : first run 2007-2008

ORS
Proc. FEL-2009

temporal and spatial overlap establishedmoderate signal for ~150 fs FWHM bunches

slightly more compression:'30(USBDF

SFDPOTUSVDUFEQSPöMF

g
sin

le

o
sh

t

dominated by COTR from intrinsic
microbunching

Bernhard Schmidt☹

| 2.10.2012 |IBIC-2012

ORS@FLASH : new plans

ORS

avoid COTR by using 270 nm seed beam (below COTR „cut-off“)

needs „transient grating (TG) FROG
with reflective optics
from T. Nagy and P. Simon, Optics Express 17, 8144 (2009)nonlinear medium

two beams create „transient grating“
third beam probes, creates forth beam
forth beam intensity reveals info

beam splitterconcerns :TG-FROG as is needs > 50 µJ

50 fs (rms)spectrometer

100 fs (rms)Proc. FEL-2012~2 kA

Bernhard Schmidt| 2.10.2012 |IBIC-2012

ORS ideas, plans, status

ORS

interesting technique of non-vanishing complexity
requires considerable technical infrastructure
still in experimental phase

Dao Xiang : IPAC 2012 : the „simplified ORS“
no second undulator, no FROGOTR screen + spectrometer

OTR spectrum reveals bunch shape„up-converted“ THz spectrum

simulation © D. Xiang, SLAC

including phase problem !

Bernhard Schmidt| 2.10.2012 |IBIC-2012

Electro Optic Methods : Basics

EO

Intra-beamline measurement of thebunch Coulomb field
EO crystal
probing laser pulse• Field induced refractive index change• Polarization-modulation of probing laser• Temporal structure of Coulomb field i.e. (beam current)è impressed to ellipticity of optical pulse
Coulomb fieldparticle
beam
1

I(t)
signal ⇠ Er (t) =2⇡b✏0 cI(t) = current

Bernhard Schmidt| 2.10.2012 |IBIC-2012

EO decoding schemes

EO
classical✦ needs very short laser pulse✦ time jitter << bunch length✦ multi-shot (scanning)✦

sampling
linearly chirped laser pulse✦ converts time ➝ wavelength✦ technically simple✦ systematic distortions and bias✦

spectral (SD)spatial (SPD)cross-correlation technique in vaccum
detection system (imaging) close to beam
✦ bias free up to „crystal limits“✦✦probe + sample pulse
SHG needs considerable power✦ more demanding✦ bias free up to „crystal limits“✦✦

temporal (TD)Bernhard Schmidt| 2.10.2012 |IBIC-2012

Electro optic : resolution limits

EO

Limits from anorganic EO material : phonon resonances & phase slipping
the EO „response function“, λ=800 nm
0.25

GaP

0.20

»GEO HfL» @10-12 mêVDavoid „slipping“ by thin crystals
less signal

300 –m200 –m100 –m
50 –m

0.15
0.10
0.05
0.00

0

2

f=wê2p @THzD
4

6

8

10

σb ≥ 50 fs - GaP
σb ≥ 100 fs - ZnTe

short bunches ⇢ high frequencies
excite phonon resonancescomplex response, ringing...

Bernhard Schmidt| 2.10.2012 |IBIC-2012

status spectral decoding EO-SD

EO

Problem :frequency mixing of bunch field an laser distorts the „linear chirp“ (the „time scale“).
➝ SD restricted to „long“ bunches. 100 µm GaP : σb ≥ 350 fs
But : technically less demanding, robust, compact• Yb fibre laser (1030 nm) avoids delicate TiSa
• almost perfect phase matching, thick crystals allowed• read out by commercial InGaAs spectrometer (fibre coupled)

0.6 ps (rms)laser pulse
e-beam

4 ps (rms)drawback: finding the temporal overlap is (still) cumbersome

Bernhard Schmidt| 2.10.2012 |IBIC-2012

FLASH after first compressor

Example : online monitor developed at PSI & DESY (Proc. DIPAC 09, B.Steffen,V. Schlott, F. Müller)

Spatial Decoding

EO

first demonstrated at SLAC, 2005

M. Veronese, BIW12

TUPA43

recent example

single shot technique, resolution not limited by frequency mixing
ideal for arrival time monitoring !
Bernhard Schmidt| 2.10.2012 |IBIC-2012

Temporal decoding

EO

gate pulse

cross correlation in
BBO crystal

needs > µJ laser power (2.order process)complex optics
about crystal limit

B. Steffen et al., PRSTAB 12, 032802 (2009)

rather experiment but monitor

Bernhard Schmidt| 2.10.2012 |IBIC-2012

EO : new ideas

EO
overcome problems
with resonances
simulated FROG images

5 fs

add spectrometer to EO-TD set-up
15 fs
Gaussian

50 fs

200 fsoriginal
retrieved

caveat:- needs very precise knowledge of crystal response- inversion of complex profiles ?
Bernhard Schmidt| 2.10.2012 |IBIC-2012

EO : frequency up-conversion

EO

most simple set-up
needs narrow band (long) laser pulse
data

spectral broadening („side bands“)
by THz spectrum of bunch© G. Berdenno phase information (yet) ! (as with CTR spectra..)Bernhard Schmidt| 2.10.2012 |IBIC-2012

a really ambitious project

EO

talk by Y. Okayasu
MOCC03

codes time & radial
information into spectrum
several new anchallenging concepts !

under development© H. Tomizawa, 2009

Bernhard Schmidt| 2.10.2012 |IBIC-2012

Summary- progress in TDS technology towards ~ 1 fs regime- coherent radiation methods made it to online monitors, perspectives for shorter bunches- electro-optic methods : more experiment than monitors, interesting approaches ahead- ORS : proof of feasibility still missing, complex technology- interesting new ideas and concepts waiting to be tested© CHAVAL

Thank you and all contributors !
there is always room for the discovery of wonderful newtechniques...

Bernhard Schmidt| 2.10.2012 |IBIC-2012

additional stuff

Chapter

Bernhard Schmidt| 2.10.2012 |IBIC-2012

fs resolution achieved: „zero phasing“

dedicated experiment at LCLS
running linac at „zero phase“
chirp maps time ➝ energy
high resolution(10-4) „A-line“ spectrometercompensates
intrinsic energy spread< 2 fsBernhard Schmidt| 2.10.2012 |IBIC-2012

Back to time domain : the phase problem

CRD(Lay, Happek, Sievers 1994)

complex function, S(z) is real
Kramers-Kronig relation: connects real andimaginary part (for a certain class of S !)needs |F(λ)| over a wide range ..
but neither „0“ nor „infinity“..
extrapolation needed (long wavelengths)„minimal“ or „canonical“ bunch profile
measured values
Bernhard Schmidt| 2.10.2012 |IBIC-2012

EOSD: early days

EO

2005
~1ps

Bernhard Schmidt| 2.10.2012 |IBIC-2012