1. Micro-bunching studies & Comparison of results from TDS and THz spectroscopy.
Minjie Yan
Ch. Gerth, E. Hass
Deutsches Elektronen-Synchrotron (DESY)
Mini-Workshop on Longitudinal Diagnostics for FELs
PSI, 11.Mar.2012

2. Comparison of results from TDS and THz spectroscopy
Overview of the experimental setups
Results at 3 compression settings
TDS profiles, TDS profiles converted to spectra
THz spectra, THz spectra reconstructed to profiles
Microbunching studies
Observation of microbunching in longitudinal phase space at different solenoid currents

3. Overview of the experimental setups
Longitudinal measurements on 12.Feb.2013
THz spectroscopy (see Eugen’s talk)
Transverse Deflecting Structure (TDS, see Christopher’s talk)
Nomenclature:
THz spectroscopy
TDS diagnostic
CRISP 141m
CRISP 202m
SMATCH
SDUMP

4. Overview of the experimental setups
TDS FLASH
CCD
Dispersive section: SDUMP
In combination with a dipole magnet Longitudinal phase space measurements
Record resolution: 5fs
CCD camera, YAG screen at 45deg

5. Overview of the experimental setups
TDS FLASH
CCD
Non-Dispersive section: SMATCH
Longitudinal profile measurements
In combination with a fast kicker Online-Monitor during FEL operation
CCD camera, CRY19 screen at 35deg (angle between screen-normal and beam axis)
Exactly the same location as the CRISP4 202m

6. 1D density distribution reconstruction from TDS measurements
Due to the intrinsic density distribution in the transverse plane y(z) and the intrinsic tilt y’(z), the measured profile may be incorrect. y y´ z z
By measuring the profiles at both the positive (+90deg) and negative (-90deg) zero-crossings of the TDS, the correct density distribution can be reconstructed. (H. Loos, “Reconstruction of a filamentary phase space from two projections”)

7. Results at 3 compression settings
Compression setting A
SDUMP
SMATCH
232±4 fs
278±7 fs
Energy MeV Charge nC
Resolution SDUMP ~15fs SMATCH ~30fs
Good agreement of the reconstructed profiles
+90 deg
201±9 fs
209±5 fs
-90 deg

8. Results at 3 compression settings
Compression setting A
Profile comparison CRISP4 data: average over 100 single shot. TDS data: average over 30 single spectrum from reconstructed profile.
Spectrum comparison CRISP4 data: reconstructed from averaged spectrum TDS data: average over 30 reconstructed profiles

9. Results at 3 compression settings
Compression setting B
SDUMP
SMATCH
122±4 fs
140±5 fs
Resolution SDUMP ~15fs SMATCH ~45fs
Very different profiles measured at SMATCH for different zero-crossings. But the reconstructed profile is in good agreement with SDUMP.
+90 deg
103±6 fs
86±4 fs
-90 deg

10. Results at 3 compression settings
Compression setting B

11. Results at 3 compression settings
Compression setting C
SDUMP
SMATCH
74±4 fs
84±4 fs
Resolution SDUMP ~10fs(at +90deg) ~20fs (at -90deg) SMATCH ~36fs
Large energy spread: >2% (beam was cut off at the SDUMP screen edge)
SMATCH station limited due to resolution
+90 deg
64±5 fs
58±5 fs
-90 deg

12. Results at 3 compression settings
Compression setting C

13. Comparison of results from TDS and THz spectroscopy
Overview of the experimental setups
Results at 3 compression settings
TDS profiles, TDS profiles converted to spectra
THz spectra, THz spectra reconstructed to profiles
Microbunching studies
Observation of microbunching in longitudinal phase space at different solenoid currents

14. Motivation for microbunching studies
SDUMP
SMATCH
We have observed microbunching at the SDUMP and SMATCH station. Some people don’t believe it. Is it real?
Since SDUMP station has better time resolution and emission of COTR is suppressed, we decided to do measurements there.
We loaded an old magnet-file, for which microbunching was observed.
It has been shown that solenoid current has a strong influence on the microbunching. (B. Schmidt and S. Wesch, talks at the Microbunching Instability workshop, 2008, Berkeley)

15. Influence of solenoid on the microbunching
-315A 167±4 fs
The machine was in SASE condition with ~20uJ
Then we turned on the dipole magnet for SDUMP section.
Energy MeV Charge nC
Change of the solenoid current by 5A washes out the microbunching structure, without touching anything else.
At -320A
washed out microbunching in longitudinal phase space
but not visible in the current profile.
At -315A
pronounced microbunching in longitudinal phase space
and in the current profile
The distribution of microbunching fluctuates from shot to shot.
Microbunching modulation period ~ [16fs, 30fs] ( ~[5um, 10um])
Resolution 15fs (~5um)
-320A 168±3 fs

16. Fourier Transform of the TDS profiles
Fourier transform of 30 single-shot profiles at each solenoid setting.
Spectrum shows increase by a factor of 10 at ~8um.
Does this part come from the front peak?
Time resolution

17. Fourier Transform of the TDS profiles
See what the spectrum looks like if the front peak is cut off.
The increase around ~8um stays the same.
Time resolution

18. Summary
First comparison of the longitudinal diagnosis with the TDS (SDUMP & SMATCH) and CRISP4 (before & after dogleg)
TDS measurements at positive and negative RF phase zero-crossing are necessary.
Influence of solenoid current on the microbunching visible in the longitudinal phase space measurements. The same effect has been observed with the spectrometer in 2008.

19. Acknowledgement
C. Behrens
B. Schmidt
S. Wesch
Thank You