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T. Quast, Helmholtz-Zentrum Berlin

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T. Quast, Helmholtz-Zentrum Berlin

## T. Quast, Helmholtz-Zentrum Berlin

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1. T. Quast, Helmholtz-Zentrum Berlin Available and Future Laser Pulse Shaping Technology (Reality and Future Directions for Spatio-Temporal Laser Pulse Shaping) Bild von 3d Ellipsoid courtesy of T. Rao courtesy of L. Hein Future Light Sources Workshop 2012, JLab

2. Laser Pulse Shaping – why do we need it ? Space Charge.. best solution: Space charge forces are linear for 3d ellipsoid: Space charge force of a Gaussian distribution 2nd best solution: „laser world is gaussian“ Transversal flattop + Longitudinal flattop „beercan“

3. „very ambitious“ „advanced“ „easy“ Introduction into Laser Shaping • Three „stages“ of Laser Pulse Shaping + Transversal Longitudinal ≠ Spatio-Temporal (3d)

4. Spatial Shaping • transversal Shaping – every one does it.. • Gauss to flattop – the simple way: • Transport laser to vincinity of gun • Overilluminated iris cuts out only the inner „flat“ part • Iris is imaged onto the cathode principle • High position Stability (iris located near cathode) • Robust and simple setup • Spot size and laser power can be varied easily (but not independent) • Any pattern can be imaged onto cathode pro • what a waste of laser power ! • Problems from laser transmission beamline (spot size / shape stability) are transfomed into intensity fluctuations con Has to be done carefully

5. Transversal Flattop – Pi Shaper • asheric optics – Pi shaper • - aspheric refractive (reflective) elements • High transmission (90%) • very sensitive to input laser parameters • tilt, decenter, size (few mrad, 10ths of µm) • TEM00 mode required (difficult with UV) Use both:„mild“ shaper and pinhole T=70% w=1.1w0 w=w0 w=0.9w0 fromG.Klemz, I.Will, Proc. FEL06 alternative: deformable mirror and genetic algorithm

6. Longitudinal shaping – different methods 1.Direct space to time (with grating and mask) Works well but transmission 10E-2 2.DAZZLER - (Acousto Optic Programmable Dispersive Filter) - only up to 100kHz Rep.rate (this rules out many of the existing bunch patterns) - shaping up to a few ps (restricion from possible crystal length)

7. Longitudinal shaping II • Pulse Stacking … (large variety) 3.Spatial light modulator (SLM) Practically only for fs pulses 4. Pulse stacking with polarizer (pol. beam splitters) Courtesy of S. Schreiber, DESY • difficult geometric alignment • intensity variations due to imperfect polarizers

8. Longitudinal shaping III Birefringent crystals – reduced complexity with Linear setup from H. Tomizawa, RadPhysChem 80, 10 (2010)

9. Longitudinal shaping IV • 3 stage stacker w. birefringent crystals 3x YVO4 d=(24,12,6mm) T= 62%; 532 nm from: A.K. Sharma et al. PRSTAB 12, 033501 (2009)

10. Longitudinal shaping V • High precision pulse shaper (MBI) Taken from: Will, Klemz, Optics Express 16 (2008) , 4922-14935 Theory for N = 10 crystals: 1024 components aranged in 11 groups

11. temperature controlled birefringent crystal motorized rotationstage Shaped ouputpulses Gaussianinputpulses Laser pulse shape measured: OSS signal (UV) FWHM = 25 ps Will, Klemz, Optics Express 16 (2008) , 4922-14935 edge10-90~ 2 ps edge10-90~ 2.2 ps birefringent shaper, 13 crystals shaper for high resolution • 13 crystal pulse shaper for high resolution

12. Pulse shaping • different possible pulse shapes Gaussian: Flat-top: FWHM ~7 ps FWHM ~ 2 ps FWHM ~ 24 ps FWHM ~ 19 ps FWHM ~ 11 ps FWHM ~ 17 ps • Feedback with optical sampling system (OSS): • dynamic range of streak camera not sufficient • scanning of 100 subsequent pulses (~0.2ps res.) • shaping is done after oscillator in IR • sampling for feedback signal in the UV Simulated pulse-stacker without feedback FWHM ~ 24 ps FWHM ~ 24 ps courtesy of I. Will, MBI

13. Self evolving • Self evolving beam - space charge force • start with a parabolic (or half sphere) Laser intensity profile • automatic evolution into a uniform ellipsoidal (3D) beam • Easy - no longitudinal laser shaping - only a short (100fs) pulse (clipped gaussian) needed pro: • cannot put high charge in it • short pulse may damage cathode • only fast response photocathode material => metal • requires high accelerating gradient con: O. J. Luiten et al., Phys. Rev. Lett. 93, 094802 (2004)

14. spatial shaper ZnSe lens achromatic lens camera DAZZLER 3d pulse shaping • Use chromatic aberration of a dispersive lens • Refractive index n is a function of frequency (dispersion) • Focal length of focussing lens changes with frequency • Parabolic frequency change by giving the pulse a cubic phase Courtesy of Yuelin Li

15. 3d pulse shaping • A first proof of principle experiment • In principle it is working • quality suffers from AOPDF limitations • No pinholes in transport ! (changing size) • no dispersion in transport ! • only IR so far (conversion ??) From Y. Li et al. PRSTAB 12, 020702 (2009)

16. 3d pulse shaping (alternative) • Stacking a 3d-ellipsoid… con • Alignment ? • Coherence and diffraction ? • Slice number limited pro • No dependance on nonlinear effects Not been demonstrated yet Z.He et al. Proc of PAC2011, TUP200

17. …back to Reality… • Survey on longitudinal shaping

18. …reality II… • Survey (cont.)

19. pulse shaping • Summary • transversal pulse shaping has to be done carefully • with advanced pulse shaping the beam transport becomes an issue (dont mess it up..) • advanced and controlled pulse stacking setup for flat top laser pulses work nicely • first steps into 3d ellipsoidal shaping are done – further exploration needed • Remarks • Overall performance of a gun largely depends on careful technical implementation of the Ph. Cath Laser • Stable laser parameters improve the overall performance • Put more emphasis on laser diagnostic and feedback • More complicated shaping schemes, utilizing nonlinear effects causes unwanted coupling of parameters • Less degrees of freedom • Potential source of instability • Is it worth it ?

20. Z-polarization gun - laser induced shottky effect z-polarization (interesting concept) • workfunction is lowered of the intense laser field • Requires very moderate laser parameters: 2.6µJ, 100fs, ~800nm • Focussing down to 20µm results in 21 pC „If the Schottky-effect-induced Z-field is large enough, we expect that electrons will make oscillations with the Z-field frequency on the outermost surface of the metal cathode and will be extracted with the external electric field of the RF cavity“ H. Tomizawa, Proc FEL2010