Microbunching Studies using IR Spectroscopy and TDS Stephan Wesch, C. Behrens, R. Ischebeck, M. Röhrs, H. Schlarb, B. Schmidt, R. Tarkeshian FLASH seminar 25th November 28
Overview 1 Motivation 2 Instrumentation 3 Measurements 4 Conclusion
IR CTR du dλ 1 µjµm 5 4 3 2 1 3 5 1 2 3 4 5 6 E = 7 MeV, Q =.5 nc Oncrest spectra: coherent radiation in spectral range of λ = [3.3, 2] µm intensity level in same order as from compressed bunches >1% of the electrons inside the bunch radiate coherently strong fluctuations of the oncrest spectrum nearly always present!
NIR CTR Single shot (SASE conditions) Average InGaAs spectrometer: extension of IR spectrum (λ = [.9, 1.7] µm) average intensity decreases down to optical wavelengths single shot spectrum consists of extreme narrow and highly fluctuating spikes intensity [a.u.] 6 4 OTR measurements should confirm coherent signals! 2 1. 1.2 1.4 1.6
COTR (?) Observation in June 8 showed first evidence of coherent signals in the optical regime 5 1 28 6 5T121514 meas profile long. distribution 35 3 25 2 15 1 Kly. Ampl. =.;Pulse = 25; 28 6 5T121514 meas profile data rms = 4.797ps cal = 44.954fs/pixel X [pixel] 15 2 25 3 35 4 45 5 1 15 2 25 time [ps] 5 15 1 5 5 1 15 2 time [ps] courtesy H. Schlarb Questions: reproduceable? special FLASH setting? location (longitudinal) in electron bunch?
Microbunching Substructure inside the electron bunch, which is not related through the correleted energy spread and can radiates coherently via CTR, CRD or CSR. Simple LSC model:? noise LSC energy mod. R56 BC2 density mod. LSC energy mod. R56 BC3 density mod. transformation from energy to density modulation is given by R56 substructure length should be varied by changing R56 spectrum shape and intensity should be influenced Note: both R56 are mandatory for detectable IR radiation see FLASH talk (8-4-8)
IR spectrometer see H. Delsim-Hashemi FLASH talk (8-3-4) Characteristics: staging of 2 dispersive reflective gratings radiation detection by 6 pyroelectric senors (only coherent signals) single shot capability in 3 wavelength ranges i. 3.3 µm - 9 µm ii. 9 µm - 22 µm iii. 25 µm - 65 µm CTR off-axis screen served by TDS kicker port about 4 m upstream of TDS screen beam transport with THz beamline
TDS Port characteristic: deflection of single bunch by fast TDS kicker on screen imaging radiation by Basler monochrome camera 2 color filters (Schott glas BG39 / RG78) in filter wheel to resolve spectral content TDS port + filter wheel + camera system
TDS Spectral response of camera incoherent OTR intensity Intensity variation in respect to camera gain.8 no filter BG39.6 spectral reponse a.u..4 RG78.2. 4 5 6 7 8 9 courtesy H. Schlarb
Plan FLASH settings: E = 78 MeV φ ACC1 = φ ACC23 = φ ACC456 = Q =.75 nc (± 1.3%) Procedure:. set BC currents 1. readjust phases 2. focus on TDS screen 3. take TDS images 4. take IR spectra
IR spectrometer I du dλ 1 µjµm 2.5 2. 1.5 1..5 IBC3 3 A IBC3 5 A IBC3 6 A IBC3 7 A IBC2 64 A I BC2 - variation: no significant intensity variation except for I BC2 =64 A no drift of center wavelength 2.5 2. IBC2 57 A IBC2 6 A IBC2 64 A IBC2 7 A. 3 5 1 2 I BC3 - variation: increase of spectral intensity with R56 no shift of center wavelength du dλ 1 µjµm 1.5 IBC3 6 A 1..5. 3 5 1 2
IR spectrometer II I BC3 - variation: F long 1 3 25 2 15 1 5 Λ 3.4, 13.9 µm IBC2 62.5 A IBC2 64 A 3 4 5 6 7 Formfactor: F 2 = N 2 du 1 dλ du incoh. dλ averaging F over λ = [3.4, 13.9] µm I BC2 - variation: F long 1 3 25 Λ 3.4, 13.9 µm 2 15 1 Results: increasing BC3 current < F > increases up to saturation (factor of 4!) increasing BC2 current < F > no significant effect 5 IBC3 6 A 55 6 65 7
Videos OTR images I
OTR images II intensity (a.u.) intensity (a.u.) 5 4 3 2 1 BG39 filter 2 4 6 8 I BC3 8 6 4 2 no filter I BC2 =64A I BC2 =57A I BC2 =A intensity (a.u.) intensity (a.u.) 1 8 6 4 2 RG78 filter 2 4 6 8 I BC3 RG78 filter, no streak 1 8 6 4 2 Σ ROI intensity: intensity increases with decreasing BC3 current for all filters! low BC2 current offers highest intensity in red regime Note: problems with saturation at oncrest phases! 2 4 6 8 I BC3 2 4 6 8 I BC3
OTR spectra III rel. fluctuation (%) rel. fluctuation (%) 1 8 6 4 2 BG39 filter I BC2 =64A I BC2 =57A I BC2 =A 2 4 6 8 I BC3 1 8 6 4 2 RG78 filter Σ ROI fluctuation: rel. fluctuations 1% to 2% for BG39 about 3% up to 4% for RG78 filter low current setting shows maximum fluctuations Note: charge is stable around 1.3% RG78 filter images at high current show weak intensity 2 4 6 8 I BC3
OTR spectra V rel. ratio 1.8.6.4 BG39 RG78 I =64A BC2 I BC2 =57A I =A BC2 rel. ratio.2 2 4 6 8 I BC3 RG78 LOLA off 1.5 1.5 Intensity ratios: for decreasing BC3 current pronunciation in the red regime is significant comparsion between TDS on - off reasonable 2 4 6 8 I BC3
Conclusions I. Coherent transition radiation signals reaches form the infrared into the optical wavelength regime! II. Spectra and TDS images show rapid fluctuating structure! III. R56 dependency IR: increasing intensity for increasing R56 BC3 no obvious R56 BC3 dependency O: increasing intensity for decreasing R56 BC3 both R56 at minimum value create maximum intensity more intensified signal in the red spectral part IV. Problem of the determination of incoherent OTR! V. Problem with camera saturation occurs!