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

Outline Optimization of the XFEL photo injector Conclusions 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 28 February, 2019 M.Krasilnikov, DESY

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 28 February, 2019 M.Krasilnikov, DESY

XFEL photo injector layout Superconducting TESLA module (ACC1) RF gun 28 February, 2019 M.Krasilnikov, DESY

Optimization of the XFEL photo injector Cathode laser (XYrms, [Trms]) + gun parameters (RF Phase, Imain, [sol.pos]) [ ] Emittance (+slope) Booster optimization (booster cavity z-position, gradient and RF phase) Initial guess: booster matching conditions 0 step. “tune” the bunch charge: Qbunch->Q=1nC(@z=5cm) Emission effects (SC, Schottky) Xrms,Xemit 1 step. Run ASTRA till z=5m 28 February, 2019 M.Krasilnikov, DESY

Thermal emittance Ek=0.55 eV Thermal emittance measured at PITZ is higher than expected from theoretical predictions (Schottky effect, cath. roughness) Ek=0.55 eV ph=4.75 eV 28 February, 2019 M.Krasilnikov, DESY

Simulation Cases Thermal emittance Ek=0.55 eV Ek=1.0 eV 2 Cylindrical laser shape 2 1 3 emittance 4 Elliptical laser shape 6 7 5 28 February, 2019 M.Krasilnikov, DESY

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 28 February, 2019 M.Krasilnikov, DESY

Optimization of the XFEL photo injector Case 1: Cylindrical laser shape, flat-top intensity profile Bunch slice parameters @ z=15m 28 February, 2019 M.Krasilnikov, DESY

Optimization of the XFEL photo injector Case 2: Cylindrical laser shape, flat-top intensity profile. Increased thermal kinetic energy 0.55 eV → 1.0eV Bunch slice emittance @z=15m 28 February, 2019 M.Krasilnikov, DESY

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 28 February, 2019 M.Krasilnikov, DESY

Optimization of the XFEL photo injector Case 3: Cylindrical laser shape, flat-top intensity profile. Increased thermal kinetic energy 1.0eV, optimized Bunch slice emittance @z=15m Only tiny emittance improvement! 28 February, 2019 M.Krasilnikov, DESY

Laser Shape: Ellipsoid instead of Cylindrical Trms=5.8 ps XYrms=0.44 mm 20 ps FWHM 2 ps rise/fall time 28 February, 2019 M.Krasilnikov, DESY

XFEL photo injector: Ellipsoid Vs. Cylinder Case 4: 3D-ellipsoidal laser shape. Ek=0.55eV Bunch slice emittance @z=15m 28 February, 2019 M.Krasilnikov, DESY

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 28 February, 2019 M.Krasilnikov, DESY

XFEL photo injector: Ellipsoid Vs. Cylinder Charge density and slice emittance 28 February, 2019 M.Krasilnikov, DESY

XFEL photo injector: Ellipsoid with Ek=1.0 eV Case 6: 3D-ellipsoidal laser shape. (=case 5 + Ek=1.0eV) 28 February, 2019 M.Krasilnikov, DESY

XFEL photo injector: Ellipsoid Vs. Cylinder Case 6: 3D-ellipsoidal laser shape. (=case 5 + Ek=1.0eV) 28 February, 2019 M.Krasilnikov, DESY

XFEL photo injector: Ellipsoid with Ek=1.0 eV Case 7: 3D-ellipsoidal laser shape, Ek=1.0eV . optimized 28 February, 2019 M.Krasilnikov, DESY

XFEL photo injector: Ellipsoid Vs. Cylinder Case 7: 3D-ellipsoidal laser shape, Ek=1.0eV . optimized 28 February, 2019 M.Krasilnikov, DESY

XFEL photo injector: Ellipsoid Vs. Cylinder Charge density and slice emittance. Ek=1.0eV 28 February, 2019 M.Krasilnikov, DESY

Ellipsoid Vs. Cylinder: Emittance Summary Projected Slice (centre) 28 February, 2019 M.Krasilnikov, DESY

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 28 February, 2019 M.Krasilnikov, DESY

Laser shape fine adjustment cathode laser shape electron bunch @z=1cm 28 February, 2019 M.Krasilnikov, DESY

Cathode laser imperfections Emittance growth due to flat-top modulation cath.laser profile cath.laser profile 28 February, 2019 M.Krasilnikov, DESY

Cathode laser imperfections: FT modulation Cathode laser intensity and electron bunch density 28 February, 2019 M.Krasilnikov, DESY

Cathode laser imperfections: FT modulation Microbunching instability gain (courtesy M.Dohlus) Electron bunch density modulation vs. cathode laser intensity modulation 30 n=5 l=1.1mm n=3 l=1.7mm n=1 l=4mm 28 February, 2019 M.Krasilnikov, DESY

Conclusions 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) Ek=0.55 eV 0.68 mm mrad 0.46 mm mrad Ek=1.0 eV 0.77 mm mrad 0.50-0.57 mm mrad 28 February, 2019 M.Krasilnikov, DESY

28 February, 2019 M.Krasilnikov, DESY

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 28 February, 2019 M.Krasilnikov, DESY