1. Impact of the Cathode Roughness on the Emittance of an Electron Beam M.Krasilnikov, DESY Zeuthen WSHQE, Milano 4-6.10.2006

2. 2 Outline Motivation Cathode roughness models Cathode roughness effects: –Geometric Normal emission, 2D vs. 3D 2D model with emission distribution –Electric field Schottky effect, electron affinity Single bump model Conclusion

3. 3 Motivation. XFEL Photo Injector L-Band (1.3GHz) 1.5-cell RF gun with Cs2Te photo cathode Ecath=60MV/m Ek = thermal kinetic energy of electrons at photo cathode Normalized transverse emittance after XFEL injector as function of the initial kinetic energy of the photo emitted electrons Slice emittance @z=18m RMS normalized emittance in XFEL injector

4. 4 Motivation. Cathode roughness Roughness measurements by INFN/LASA Profile Paolo Michelato, INFN Milano – LASA “High QE Photocathodes lifetime and dark current investigation”, PITZ collaboration meeting 4-5.02.006

5. 5 Motivation. Cathode roughness Roughness measurements (imaging) by INFN/LASA Paolo Michelato, INFN Milano – LASA “High QE Photocathodes lifetime and dark current investigation”, PITZ collaboration meeting 4-5.02.006

6. 6 Motivation. Thermal emittance Measurements at PITZ Ecath=42MV/m Eemis=24MV/m Ecath=60MV/m Eemis=42MV/m

7. 7 Cathode Roughness Models

8. 8 Periodic Roughness. 2D vs. 3D 0 1 -4-2024 k*x z/h Transverse component of the velocity at the cathode surface Transverse emittance induced by the cathode roughness

9. 9 2D Periodic Model with Emission Distribution Without roughness 2D (x,z) cathode roughness

10. 10 2D Periodic Model with Emission Distribution Thermal emittance growth due to the cathode roughness Vs. roughness parameter  Vs. roughness period  (h=10nm)

11. 11 Dependence on Electric Field Thermal emittance growth due to the cathode roughness as function of electron affinity and roughness period  (h=10nm)

12. 12 Dependence on Electric Field Single Bump Model Electric field of the bumped surface Conformal transformation plane capacitor Emission points for simulations w w,a.u. u,a.u. z,a.u. x,a.u. Conformal transformation

13. 13 Dependence on Electric Field Simulated divergence (h=10nm)  nm 10nm 100nm  nm Ecath, MV/m E0=Ecath*sin(  emission ), MV/m 4224 6042

14. 14 Dependence on Electric Field Maximum divergence vs. Emission Field E0 for various roughness width  (roughness depth h=10nm) Ecath, MV/m E0=Ecath*sin(  emission ), MV/m 4224 6042 p x,max, 

15. 15 Conclusion Thermal emittance growth due to the cathode roughness “Geometric” roughness factor ~10-50% Induced by the electric field increase ~30% Emittance growth ~10÷65%