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Ellipsoidal Shaping for XFEL Photo Injector

Explore the potential benefits of ellipsoidal shaping for the European XFEL photo injector through selected simulations. Optimization strategies, thermal emittance, and the influence of laser shape are analyzed.

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Ellipsoidal Shaping for XFEL Photo Injector

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  1. Selected simulations for XFEL photo injector Could ellipsoidal shaping be useful for the European XFEL photo injector? M. Krasilnikov, DESY Mini - Workshop on Laser Pulse Shaping DESY Zeuthen 30.11-01.12.2006

  2. Outline • Optimization of the XFEL photo injector • Specifications and layout • Optimization strategy • Flat-top laser (cylindrical laser pulse shape) • Influence of the thermal emittance • Ellipsoidal cathode laser shape • Flat-top imperfection influence • Conclusions M.Krasilnikov, DESY

  3. XFEL Photo Injector Specifications • 1 nC charge • Uniform transverse distribution • Longitudinal flat-top 20 ps with 2 ps rise time • Emittance 0.9 mm mrad incl. thermal emittance • 60 MV/m at photo cathode M.Krasilnikov, DESY

  4. XFEL photo injector layout Superconducting TESLA module (ACC1) RF gun M.Krasilnikov, DESY

  5. Optimization of the XFEL photo injector Cathode laser (XYrms, [Trms]) + gun parameters (RF Phase, Imain, [sol.pos]) [ ] Booster optimization (booster cavity z-position, gradient and RF phase) Initial guess: booster matching conditions Emittance (+slope) 0 step. “tune” the bunch charge: Qbunch->Q=1nC(@z=5cm) Emission effects (SC, Schottky) Xrms,Xemit 1 step. Run ASTRA till z=5m M.Krasilnikov, DESY

  6. Thermal emittance Thermal emittance measured at PITZ is higher than expected from theoretical predictions (Schottky effect, cath. roughness) Ek=0.55 eV ph=4.75 eV M.Krasilnikov, DESY

  7. Simulation Cases 2 3 1 6 4 7 5 Thermal emittance Ek=0.55 eV Ek=1.0 eV Cylindrical laser shape emittance Elliptical laser shape M.Krasilnikov, DESY

  8. Optimization of the XFEL photo injector Case 1: Cylindrical laser shape, flat-top intensity profile 20 ps FWHM 2 ps rise/fall time • Optimized parameters: • Cathode laser XYrms • RF gun launch phase • Main solenoid peak field* • Booster z-position • Booster gradient • Booster RF phase • *Bucking solenoid - always compensated M.Krasilnikov, DESY

  9. Optimization of the XFEL photo injector Bunch slice parameters @ z=15m Case 1: Cylindrical laser shape, flat-top intensity profile M.Krasilnikov, DESY

  10. Optimization of the XFEL photo injector Bunch slice emittance @z=15m Case 2: Cylindrical laser shape, flat-top intensity profile. Increased thermal kinetic energy 0.55 eV → 1.0eV M.Krasilnikov, DESY

  11. XFEL photo injector: Influence of the thermal emittance • ?Further optimization: • Cathode laser XYrms • RF gun launch phase • Main solenoid peak field • Booster z-position • Booster gradient • Booster RF phase M.Krasilnikov, DESY

  12. Optimization of the XFEL photo injector Bunch slice emittance @z=15m Case 3: Cylindrical laser shape, flat-top intensity profile. Increased thermal kinetic energy 1.0eV, optimized Only tiny emittance improvement! M.Krasilnikov, DESY

  13. Laser Shape: Ellipsoid instead of Cylindrical Trms=5.8 ps XYrms=0.44 mm 20 ps FWHM 2 ps rise/fall time M.Krasilnikov, DESY

  14. XFEL photo injector: Ellipsoid Vs. Cylinder Bunch slice emittance @z=15m Case 4: 3D-ellipsoidal laser shape. Ek=0.55eV M.Krasilnikov, DESY

  15. XFEL photo injector: Ellipsoid Vs. Cylinder Case 5: 3D-ellipsoidal laser shape. Ek=0.55eV, optimized • Optimized parameters: • Cathode laser XYrms • Cathode laser Trms • RF gun launch phase • Main solenoid peak field* • Booster gradient • Booster RF phase • *Bucking solenoid - always compensated M.Krasilnikov, DESY

  16. XFEL photo injector: Ellipsoid Vs. Cylinder Charge density and slice emittance M.Krasilnikov, DESY

  17. XFEL photo injector: Ellipsoid with Ek=1.0 eV Case 6: 3D-ellipsoidal laser shape. (=case 5 + Ek=1.0eV) M.Krasilnikov, DESY

  18. XFEL photo injector: Ellipsoid Vs. Cylinder Case 6: 3D-ellipsoidal laser shape. (=case 5 + Ek=1.0eV) M.Krasilnikov, DESY

  19. XFEL photo injector: Ellipsoid with Ek=1.0 eV Case 7: 3D-ellipsoidal laser shape, Ek=1.0eV . optimized M.Krasilnikov, DESY

  20. XFEL photo injector: Ellipsoid Vs. Cylinder Case 7: 3D-ellipsoidal laser shape, Ek=1.0eV . optimized M.Krasilnikov, DESY

  21. XFEL photo injector: Ellipsoid Vs. Cylinder Charge density and slice emittance. Ek=1.0eV M.Krasilnikov, DESY

  22. Ellipsoid Vs. Cylinder: Emittance Summary Projected Slice (centre) M.Krasilnikov, DESY

  23. XFEL photo injector: Ellipsoid Vs. Cylinder Electron bunch (z,x) @z=15m, cylindrical cathode laser shape Electron bunch (z,x) @z=15m, ellipsoidal cathode laser shape M.Krasilnikov, DESY

  24. Laser shape fine adjustment cathode laser shape electron bunch @z=1cm M.Krasilnikov, DESY

  25. Cathode laser imperfections cath.laser profile cath.laser profile Emittance growth due to flat-top modulation M.Krasilnikov, DESY

  26. Cathode laser imperfections: FT modulation Cathode laser intensity and electron bunch density M.Krasilnikov, DESY

  27. Cathode laser imperfections: FT modulation Microbunching instability gain (courtesy M.Dohlus) Electron bunch density modulation vs. cathode laser intensity modulation 30 n=5l=1.1mm n=3l=1.7mm n=1l=4mm M.Krasilnikov, DESY

  28. By optimization the main parameters of the XFEL photo injector one can simulate rather small projected normalized emittance: Slice emittance of the bunch centre reduced in ~10% by applying an ellipsoid laser pulse. Main reduction of the projected emittance is due to significant decrease in head and tail slice emittance of the ellipsoid But practical realization can face problems of shape imperfections (tight tolerances) Conclusions M.Krasilnikov, DESY

  29. M.Krasilnikov, DESY

  30. Optimization of the XFEL photo injector • Based on ASTRA simulations • Actual solenoid position • Strategy: 2-staged* optimization • 1. Only gun (Ecath=60MV/m) • 2. Booster (first ½ of ACC1) Scan of the second ½ of ACC1 gradient • Goal function: emittance (incl. slope) • Penalty function: charge, momentum spread, variable parameters range M.Krasilnikov, DESY

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