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S. ZHANG, S. BENSON, D. DOUGLAS, G. NEIL, AND M. SHINN FEL’ 2009, LIVERPOOL, UK

STUDY OF OPTICAL FREQUENCY CHIRPING AND PULSE COMPRESSION IN A HIGH-GAIN ENERGY-RECOVERY-LINAC-BASED FREE-ELECTRON-LASER . S. ZHANG, S. BENSON, D. DOUGLAS, G. NEIL, AND M. SHINN FEL’ 2009, LIVERPOOL, UK. Outline Of Talk. Introduction Basic Principle Experiment Analysis and Discussion

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S. ZHANG, S. BENSON, D. DOUGLAS, G. NEIL, AND M. SHINN FEL’ 2009, LIVERPOOL, UK

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  1. STUDY OF OPTICAL FREQUENCY CHIRPING AND PULSE COMPRESSION IN A HIGH-GAIN ENERGY-RECOVERY-LINAC-BASED FREE-ELECTRON-LASER S. ZHANG, S. BENSON, D. DOUGLAS, G. NEIL, AND M. SHINN FEL’ 2009, LIVERPOOL, UK

  2. Outline Of Talk • Introduction • Basic Principle • Experiment • Analysis and Discussion • Summary and Acknowledgement

  3. Introduction: What Is It All About? • Motivation • Chirp effect has been studied in SASEs, what are the characteristics of the chirped pulses from FEL oscillator? • Add linear chirp to FEL pulse in an oscillator by chirping E-beam. • Is it possible to compress the pulses further down? • Do we have necessary tools to proceed? • The JLAB ERL machine can provide linear chirp on electron bunch. • Using sextupoles in the Bates bend to cancel RF curvature effects, set second order dispersion and reduce chromatic aberration. • Addition of four trim quadrupoles to set the momentum compaction and linear dispersion. • Available diagnostic technique allow a complete characterization of ultrashort optical pulses. • Once the chirp is known, pulse compression can be performed.

  4. JLAB ERL-FEL Facility 14.2 kW @ 1.6 microns 10/30/06

  5. Longitudinal Match Scenario E E E E f f f f E E f f Requirements on phase space: • high peak current (short bunch) at FEL • bunch length compression at wiggler using quads and sextupoles to adjust compactions • “small” energy spread at dump • energy compress while energy recovering • “short” RF wavelength/long bunch, large exhaust dp/p (~12%) • get slope, curvature, and torsion right (quads, sextupoles, octupoles) Courtesy D. Douglas

  6. How to Chirp Electron Bunch • Setting Up LPS from Injector to Wiggler Transport. Launch f Input phase to linac vs. phase at the wiggler Arrival f The upper left:trim quadrupoles too strong. Upper right: trim quads too weak. Lower left: mispoweredsextupoles, Lower right: properly set to produce maximum compression.

  7. Longitudinal Phase Space • Streak Camera with SR

  8. Longitudinal Phase Space • No phase distortion observed at high current! S.Zhang et al., FEL06,JACowW / eCon C0508213, THPPH066 (2006)

  9. Bunch Length Diagnostics Simplified optical layout of a polarization autocorrelator (Martin-Puplett interferometer). The optical pulses come from the optical transitional radiation generated by the short electron bunches interacting with a thin aluminum foil.

  10. Bunch Length Diagnostics-THz Schematic of the optical setup for THz spectral measurement.Michelson interferometer (FTIR, rapid scan device) • Comparison between the measured THz spectra and calculations with different bunch lengths. Exp1 and Exp2 correspond to the spectra measured with FTIR.

  11. Electron Beam Parameters • Electron beam parameters Coherent OTR interferometer autocorrelation scans with different quadrupole magnetic field settings (G=-200~-240).

  12. An Important Tool: FROG • FROG (Frequency-Resolved-Optical-Gating) • An proven, Indispensable, the state of art diagnostic for ultrashort laser pulses • FROG completely reveals intensity and phase of laser pulse Example: Linearly chirped Gaussian pulse The spectrogram essentially uniquely determines both the waveform intensity, I(t), and phase, (t). Algorithms exist to retrieve E(t) from its spectrogram.* Baltuska,J. Quant. Electron.,35, 459 (1999). R. Trebino et al., Rev. Sci. Instrum. 68, 3277 (1997).

  13. FROG Setup

  14. FROG Setup Single shot noncoliner SHG configuration. A 0.3mm type-I BBO was used. Imaging spectrometer plus CCD camera to record the traces.

  15. FROG Test • Single-shot FORG was tested with a commercial Ti:Sapphire Laser

  16. JLAB FEL Pulses • FEL Cavity • Oscillator consisting Concave OC and HR, ROC~16m,Nearly concentric cavity. • Rayleigh range about 1.5m, variable by changing ROC of the HR. A typical 1.6um FEL spectrum and autocorrelation trace (CW lasing/2kW)

  17. JLAB FEL FROG • A Typical FEL FROG traces and analysis Original Retrieved FROG error is 0.0016

  18. Analysis of FEL Pulses • Measurement accuracy check: the retrieved spectrum agrees well with the measured E-beam: 115MeV/2.3mA/18MHz FEL: 1.6um/2.5kW Solid curves are FROG analysis. Exp1 and exp2 are measured spectra.

  19. Measurement • Trim Quadrupole setting was scanned to take FROG data • FROG traces changes significantly G= -200 -205 -210 -215 -220 -225 -230 Freq Time • Measured FROG traces under different trim quadrupole settings, • Top row, original. Bottom row, retrieved. FEL cavity, 11% OC/ 1.6um

  20. Analysis • .Asymmetry seen on both temporal and spectral distribution. • Parabolic-like phase, 2nd order + higher orders of dispersion.

  21. Correlation • . Trend of optical pulse and e-bunch width agree. • The spectral width changes significantly. • Pulse duration (FWHM) between e-beam bunch and FEL pulses at different trim-quadrupole settings. Solid triangle stands for pulse spectral width (FWHM).

  22. Pulse Compression • . Large linear chirp leads to bigger compression • G=-215, initial 122fs pulse compressed to 54fs, a factor of over 2. • G=-200, from 126fs to 107fs, compression factor <1.2 • Original and compressed pulses at different trim quadrupole settings.

  23. Measurement with Different OC • . Use a different 5%, transmission output coupler, yielding different results. G= -210 -220 -230 Top row: original Bottom: retrieved • Analysis graphs from top to bottom are for G=-210, -220 and -230, respectively.

  24. Stability Makes Measurement Accurate • Unlike the SASE experiment, our ERL Oscillator showed very good stability during the measurement , with repeatable and consistent result • . T= 0 T=2 T=4 • FROG traces sampled at different time with the same electron beam setup. Top row is the raw and bottom is the retrieved. • Time sequence: 1stcol, 0min. 2nd col., 2 min. 3rdcol, 4min.

  25. Summary • What we’ve learnt • Chirped electron bunch passes chirp onto FEL pulses. • Original output pulses can be compressed to much shorter pulses. • Nonlinear chirp accompany linear chirp. • What we are trying to understand • How does FEL gain affect the chirp on FEL pulses, what is the optimal OC for this application? • What is the effective way to generate large linear chirp on FEL pulses. What is the limit? • Comparison with Simulation Acknowledgement Authors would like to thanks L. Giannessi for discussion and help with simulation. C. Liu provided assistance in part of data processing. This work was supported by the Commonwealth of Virginia, and by DOE Contract DE-AC05-060R23171

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