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

2. 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

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
28 February, 2019
M.Krasilnikov, DESY

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

5. 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:
Emission effects (SC, Schottky)
Xrms,Xemit
1 step. Run ASTRA till z=5m
28 February, 2019
M.Krasilnikov, DESY

6. 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

7. 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

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
28 February, 2019
M.Krasilnikov, DESY

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

10. 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
28 February, 2019
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
28 February, 2019
M.Krasilnikov, DESY

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

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

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

17. 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

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

19. 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

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

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

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

23. XFEL photo injector: Ellipsoid Vs. Cylinder
Electron bunch cylindrical cathode laser shape
Electron bunch ellipsoidal cathode laser shape
28 February, 2019
M.Krasilnikov, DESY

24. Laser shape fine adjustment
cathode laser shape
electron
28 February, 2019
M.Krasilnikov, DESY

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

26. Cathode laser imperfections: FT modulation
Cathode laser intensity and electron bunch density
28 February, 2019
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=5 l=1.1mm
n=3 l=1.7mm
n=1 l=4mm
28 February, 2019
M.Krasilnikov, DESY

28. 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
mm mrad
28 February, 2019
M.Krasilnikov, DESY

29. 28 February, 2019
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
28 February, 2019
M.Krasilnikov, DESY