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

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 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 RMS normalized emittance in XFEL injector

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

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

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

7 Cathode Roughness Models

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

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

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

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

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

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 p x,max, 

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%