1. Experience with diagnostics
at FLASH
Holger Schlarb
DESY
22607 Hamburg
Current compression scheme
Slice measurements
Electro-optic techniques
Summary
Outlook
LCLS ICW /10
Holger Schlarb, DESY

2. Longitudinal phase space injector - present design -
MeV
Superconducting TESLA module
bunch compressor
bunch compressor
RF gun
12/20 MV/m
< 60 fs
Laser
4 - 5 MeV
Small space charge on cathode
sL= 4.4 ± 0.1 ps 10 20 30 40 50
Time (ps)
LCLS ICW /10
Holger Schlarb, DESY
Courtesy: M. Dohlus

3. Diagnostics for long. phase space
Laser
THz
Laser
EO-container
EO/CDR/CTR/TR
Streak camera
Laser phase
CDR
ACC 1
BC2
ACC 23
BC3
ACC 45
ISR
Diag.
Undulators
TEO
PP-laser
Tosylab
CSR/SR
CDR
LOLA
ISR
Diag.
RF gun
THz
LOLA: transverse deflecting structure
bunch profile, slice emittance & energy spread
EO: electro-optic
 bunch profile, timing (TEO)
ISR: incoherent synchrotron radiation
 energy spread & beam energy
CRD: coherent radiation diagnostics (see Oliver Grimm)
 longitudinal spectrum of e-beam (THz radiation, 10GHz-30THz)
CTR : coherent transition radiation
CSR: coherent synchrotron radiation
CDR: coherent diffraction radiation
LCLS ICW /10
Holger Schlarb, DESY

4. Transverse deflecting structure
collaboration between DESY and SLAC
vertical deflecting RF structure (2.856 GHz) operated at zero crossing
vertical size of beam at imaging screen  depends on bunch length
40 MW klystron power to “streak” the 0.5 GeV at TTF2
‘Parasitical’ measurement using hor. kicker and off-axis screens
Resolution: TTF2 ~ fs (depending on vertical beam size)
Fast hor.
kicker
~ z
Vy(t)
S-band 2
~ z e - z 2 . 4 m
3.66 m b c p D y 6
~20°
Vertical streak
LCLS ICW /10
Holger Schlarb, DESY
TTF2: M. Ross et.al. + MIN DESY

5. Waveguide Beam Direction Load RF Input LOLA IV
LCLS ICW /10
Holger Schlarb, DESY
Courtesy: M. Nagl, M. Ross et al.

6. Examples for bunch images
LOLA off:
LOLA on:
Typical streak strength: mm / ps
Head
Time resolution: vertical rms beam size (LOLA off) / streak
time
Tail
LCLS ICW /10
Holger Schlarb, DESY

7. First attempts to compare TCAV & simulations
Image with LOLA x
Simulation with CSR
LCLS ICW /10
Holger Schlarb, DESY
Courtesy: M. Dohlus (DESY)

8. Projected and slice emittance measurements
- early results -
Slicing used for meas.
Emittance (100%)
time
Mismatch factor B
Longitudinal Slices of 250um or 154fs
(100%) ~ 7.5um head … 4 um tail
(90%) ~ 6.3 um head … 1.5 um tail
Mismatch phases indicate gradual rotation of the slice rms –ellipses in hor. phase space along the bunch. Most likely caused by chromaticity.
Mismatch phase
Normal coordinates:
LCLS ICW /10
Holger Schlarb, DESY
Courtesy: M. Röhrs

9. LOLA in the FLASH beamline
Beam direction
GUN ACC1 BC2 ACC2/ BC ACC4/ LOLA Dogleg UND1 … UND6
Q9ACC7
Q9/10ACC6
Q9/10ACC5
Q9/10ACC4
Horizontal Kicker
Off-axis screen
ACC5
ACC4
LOLA
Slice emittance and centroid shifts
Keep constant and small y at OTR
-> six quads are scanned simultaneous
-> check upstream optics (matching!)
LCLS ICW /10
Holger Schlarb, DESY

10. Optics for slice emittance measurements
Scan of horizontal phase advance (~210 deg range) using the 6 quadrupoles Q9ACC4 – Q10ACC6 upstream of LOLA
Streak at the screen is held constant (y = const)
Values of the beta functions at the screen: ~5m - 10m
Q9ACC4
LOLA
OTR
small
large
Courtesy: M. Röhrs
LCLS ICW /10
Holger Schlarb, DESY

11. LOLA in the FLASH beamline
Beam direction
GUN ACC1 BC2 ACC2/ BC ACC4/ LOLA Dogleg UND1 … UND6
Q9ACC7
Q9/10ACC6
Q9/10ACC5
Q9/10ACC4
Horizontal Kicker
Off-axis screen
ACC5
ACC4
LOLA
Slice emittance and centroid shifts C1 C2 C3 C4
Slice energy spread & energy correlation
Keep constant and small y at OTR
-> six quads are scanned simultaneous
-> check upstream optics (matching!)
Large dispersion + small spot at OTR
-> change of optics, open collimators
-> check dispersion & streak calibration
LCLS ICW /10
Holger Schlarb, DESY

12. Optics for energy-time correlation measurements
Objectives: small beta function values (~ 3m) , maximum streak, large dispersion at the screen (~290mm),
Standard optics`:
Optics for the measurements:
LOLA
OTR
LCLS ICW /10
Holger Schlarb, DESY
Courtesy: M. Röhrs

13. Screen Calibration
Time axis (vertical): Measurement of the vertical beam position for different phases
Energy axis (OTR 5ECOL):
Measurement of the horizontal dispersion by variation of the current in the dipole
LCLS ICW /10
Holger Schlarb, DESY
Courtesy: M. Röhrs

14. On-crest operation: Longitudinal density profile
4.8 ps (BCs off)
Head
Tail
rms-lengths: 3.8 ps (BCs on)
Charge: 1 nC, Energy: 650 MeV
LCLS ICW /10
Holger Schlarb, DESY
Courtesy: M. Röhrs

15. On-crest operation: Longitudinal phase space
Bunch compressors on, Charge: 1 nC, Energy: 650 MeV
130 keV rms
Dispersion at the screen: 290 mm
Total rms energy spread: 0.09% (585 keV)
rms slice spread < 0.02% (130 keV) limited by transverse beam size
LCLS ICW /10
Holger Schlarb, DESY
Courtesy: M. Röhrs

16. On-crest operation: Slice emittance
1σ-emittance, 100% of particles
Systematic rms error of absolute values ~ 30% due to quadrupole gradient end energy errors But: ratios not affected!
Projected emittance upstream of BC3: mm mrad
Bunch compressors on, Charge: 1 nC, Energy: 650 MeV
LCLS ICW /10
Holger Schlarb, DESY
Courtesy: M. Röhrs

17. SASE at 13.7 nm (5µJ): Longitudinal profile
Parameter: Charge: 0.5 nC Energy: 677 MeV ACC1-phase: -9˚ ACC23-phase: -25˚ ACC45-phase: 0˚
Spike width: ~75 fs (FWHM) Resolution: ~20 fs Charge in spike: ~0.12 nC (23%) spike current: ~1.7 kA
LCLS ICW /10
Holger Schlarb, DESY
Courtesy: M. Röhrs

18. SASE at 13.7 nm: Longitudinal phase space
Energy spread in the spike: ~0.23% (1.6 MeV)
Result for SASE operation at 31.4 nm (450 MeV): 0.4% peak energy spread, similar shape of energy-time correlation
Dispersion: 233 mm; Time resolution: ~ 50 fs; Energy spread resolution: ~ 0.06% (380 keV)
LCLS ICW /10
Holger Schlarb, DESY
Courtesy: M. Röhrs

19. SASE at 13.7 nm: Slice emittance
Vertical rms width during the scan: < 220 µm (60 fs resolution)
Projected emittance: mm mrad
Similar result for SASE operation at 31.4nm
LCLS ICW /10
Holger Schlarb, DESY
Courtesy: M. Röhrs

20. Tomography: one slice Region used for tomography (duration: 50 fs)
LCLS ICW /10
Holger Schlarb, DESY
Courtesy: M. Röhrs

21. Tomography: one slice Courtesy: M. Röhrs LCLS ICW2006 10/10
Holger Schlarb, DESY
Courtesy: M. Röhrs

22. Overview on EO-techniques
Electro-optic Sampling :
+ simple (laser) system
+ arbitrary time window
+ high resolution
- no single bunch
Spectral Decoding:
+ simple (laser) system
+ high repetition rate
- limited resolution (500fs)
- distorted signal for
e-bunches < 200fs
Temporal Decoding:
+ large time window
+ high resolution (120fs, GaP)
- mJ laser pulse energy
- low repetition rate
Spatial Decoding:
+ simple laser system
+ high repetition rate
+ high resolution (170fs, ZnTe)
- more complex imaging optics
Courtesy: B. Steffen et al

23. Results on temporal decoding - cross-check of theory -
Typical measurement at medium compression
< 200 fs
EO signals seen: typical 150 fs-200 fs (FWHM)
with GaP, corresponds to fs for e-bunch
due to crossed polarizer setup.
Courtesy: B. Steffen et al (DESY)
G. Berden (FELIX)
S. Jamison et al (Dundee)
LCLS ICW /10
Holger Schlarb, DESY

24. Time jitter measured by EO-SD
here 270 fs (rms) over 5 min incl. slow drifts
without slow drifts typically <200 fs (rms)
Courtesy: B. Steffen et al (DESY)
LCLS ICW /10
Holger Schlarb, DESY

25. EO measurement and LOLA
Longitudinal profile for two different compression scenario
LOLA EO
Shortest pulse observed
red: temporal decoding
blue: squared signal from a transverse deflecting cavity
Reasonable good agreement, cross-check for resolution!
needs further analysis …
Courtesy: B. Steffen et al (DESY)
G. Berden (FELIX)
S. Jamison et al (Dundee)
LCLS ICW /10
Holger Schlarb, DESY

26. Timing between pump-probe laser & FEL
Transport of laser pulse critical
TEO
100 shots
Time [sec]
t= 130 fs
Time
in Tunnel
Pulse in fiber will be broadened (50 fs to 0,4 ns)
and distorted due to high order dispersion (~100 pulses seen)
150m
Courtesy: Armin Azima DESY
D. Fritz/A. Cavalieri
Michigan
LCLS ICW /10
Holger Schlarb, DESY

27. Summary
Operation with highly non-uniform compression causes significant difficulties for the standard diagnostic tool such as
BPM
Beam imaging screens
Wire scanner
Only access to relevant phase space volume possible with transverse deflecting structure
but current setup requires tomographic quadrupole scans
incompatible with SASE operation (on-line diagnostics)
still time consuming
Electro-Optical techniques
come to there theoretical limits for GaP (~120fs FWHM)
need still to be improved to provide operations tool (e.g. FB)
TEO ready for Pump-probe experiments (timing!)
LCLS ICW /10
Holger Schlarb, DESY

28. Outlook and future developments
2007 installation of optical replica synthesizer (< 5fs resolution)
in cooperation with Uppsala & Uni. Stockholm
preparation of longitudinal feedback system (mainly new monitor systems)
allow for laser based beam manipulation and external seeding option:
requires ~ fs rms arrival time stability
incoming orbit
energy
exiting orbit
time
compression
Fast FB
A/ ACC1
Tarrial, A/ ACC1
LCLS ICW /10
Holger Schlarb, DESY

29. Principle of the Arrival Time Detection
sampling time of ADC
The timing information of the electron bunch is transferred into an amplitude modulation. This modulation is measured with a photo detector and sampled by a fast ADC.
MHz
(54 MHz)
LCLS ICW /10
Holger Schlarb, DESY
Courtesy: F. Löhl

30. Beam Pick-up Output signal measured in EOS hutch
Isolated impedance-matched ring electrode installed in a „thick Flange“
Broadband signal with more than 5 GHz bandwidth
Sampled at zero-crossing with laser pulse
LCLS ICW /10
Holger Schlarb, DESY

31. Electro-Optical-Modulator (EOM)RF
signal
bias
voltage
Commercially available
with bandwidths up to 40 GHz
(we use a 12 GHz version)
Lithium
Niobate
LCLS ICW /10
Holger Schlarb, DESY

32. Test Bench for the Arrival-time Monitor System
LCLS ICW /10
Holger Schlarb, DESY
Courtesy: F. Löhl

33. Measurement of Bunch Arrival Time over Bunch Train
Beam loading compensation off
~ 3 ps difference over bunch train
~ 3 ps difference over bunch train
Beam loading compensation on (not optimized)
dA/A ~ 0.2%
ACC1
Bunch to bunch time jitter
rms(tn – t(n+1)) ~ 30fs
~ 1 ps difference over bunch train
LCLS ICW /10
Holger Schlarb, DESY
Courtesy: F. Löhl

34. Same method used for chicane bpm
Large transverse dynamic range (5-15cm), but high resolution ~ <10 um
Strip-line
Left pickup
Flat chicane chamber
Right pickup
beam
ADC
EOM1 -
ADC
EOM2
compare time of flight across stipline
centering of measurement is perform by optical delay line
LCLS ICW /10
Holger Schlarb, DESY
Courtesy: K. Hacker

35. End