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

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

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

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

Overview of the experimental setups TDS diagnostic @ 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

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”)

Results at 3 compression settings Compression setting A SDUMP SMATCH 232±4 fs 278±7 fs Energy 700MeV Charge 0.33nC Resolution SDUMP ~15fs SMATCH ~30fs Good agreement of the reconstructed profiles +90 deg 201±9 fs 209±5 fs -90 deg

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

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

Results at 3 compression settings Compression setting B

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

Results at 3 compression settings Compression setting C

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

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)

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 710MeV Charge 0.3nC 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

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

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

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.

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