1. D. Lipka, MDI, DESY Hamburg, July 2012 Simulation of fields around spring and cathode for photogun D. Lipka, MDI, DESY Hamburg

2. D. Lipka, MDI, DESY Hamburg, July 2012 Simulate fields in gun (here gun design version 5 is used) Monitor fields at spring and cathode region to find unexpected high fields Goal

3. D. Lipka, MDI, DESY Hamburg, July 2012 Setup Gun version 5 Frequency domain solver around 1.3 GHz Fields are scaled to 60 MV/m at cathode surface Body copper

4. D. Lipka, MDI, DESY Hamburg, July 2012 Present cathode region setup Materials: Cathode: Molybdenum Spring: CuBe (conductivity 25∙10 6 S/m) or Silver Backplane (blue): stainless steel Blue line: on this line field strength will be monitored Holder at backplane is simplified to increase the field resolution (more mesh cells possible)

5. D. Lipka, MDI, DESY Hamburg, July 2012 Setup 85 lamella around cathode Contact verified on inner spring to cathode and lamella to the other Contact of spring to copper body too

6. D. Lipka, MDI, DESY Hamburg, July 2012 Mesh view Use tetrahedral mesh Large cells in resonator, very small cells at cathode region for high resolution

7. D. Lipka, MDI, DESY Hamburg, July 2012 Boundary and symmetry Symmetry on yz-plane with magnetic H t =0 Magnetic boundary at transverse coordinates Electric boundary at z=max Open boundary at z=min

8. D. Lipka, MDI, DESY Hamburg, July 2012 Reflection result Each frequency domain simulation produces reflection like shown here,  -mode is at 1299.82 MHz

9. D. Lipka, MDI, DESY Hamburg, July 2012 Scaled E-field The fields are scaled such that E=60 MV/m at the cathode surface is generated, here the absolute E- field along the z-axis is shown, almost perfect field balance visible

10. D. Lipka, MDI, DESY Hamburg, July 2012 Scaled E-field Here monitored the E- field along the x-axis at the surface of the cathode and further Higher field of almost 90 MV/m found at the corner of the cathode, smaller at the corner of the copper wall

11. D. Lipka, MDI, DESY Hamburg, July 2012 Scaled H-field distribution The maximum of H-field is at the rounding of the resonators, H max =133 kA/m At the cathode it is much smaller, details will be given for different cases

12. D. Lipka, MDI, DESY Hamburg, July 2012 Different cases Beside the default setup different cases are investigated: One missing lamella 11 missing lamella 11 shifted lamella cut sphere (no contact to (in addition to cathode) shifted lamella)

13. D. Lipka, MDI, DESY Hamburg, July 2012 E-field on curve Absolute E-field along the cathode line shown; unit: V/m 1.Default: ends at about 13.5 mm 2.Missing lamella: slightly higher field 3.11 missing lamella: more field toward other end of cathode 4.Shifted lamella and cut sphere (similar): more field behind cathode Similar for silver and CuBe

14. D. Lipka, MDI, DESY Hamburg, July 2012 E-field distribution Shown for default and cut sphere case Results: no significant higher field at the additional peak for the last case

15. D. Lipka, MDI, DESY Hamburg, July 2012 H-field on curve Absolute H-field along the cathode line shown; unit A/m 1.Default: high field spike at connection to spring 2.1 missing lamella: field smeared out 3.11 missing lamella: more smearing 4.Shifted lamella and cut sphere (similar): longer behind cathode Similar to silver and CuBe Field distributions in the following slides

16. D. Lipka, MDI, DESY Hamburg, July 2012 H-field distribution default case H-field strength at cathode is smaller compared to cavity, but at the junction between spring to cathode or holder higher local fields visible

17. D. Lipka, MDI, DESY Hamburg, July 2012 H-field distribution default case Here the local field spikes are visible at the cathode and spring

18. D. Lipka, MDI, DESY Hamburg, July 2012 H-field distribution: missing lamella At the monitoring line the field strength is reduced but at the other contacts the field strength is slightly higher

19. D. Lipka, MDI, DESY Hamburg, July 2012 H-field distribution: 11 missing lamella A larger area get a reduced field, but at the starting spring the field is higher compared to the default case

20. D. Lipka, MDI, DESY Hamburg, July 2012 H-field distribution: 11 shifted lamella Like it is shown along the curve on the slide before the field strength is more smeared, but still higher field at the first contact

21. D. Lipka, MDI, DESY Hamburg, July 2012 H-field maximum at cathode Taking the maximum field strength on the cathode the surface current can be calculated with: Surface current I=H∙2  r/√2, with r the radius For the different cases the maximum current are: Default: 1207 A Missing lamella: 1268 A 11 missing lamella: 1634 A 11 shifted lamella: 1926 A

22. D. Lipka, MDI, DESY Hamburg, July 2012 Summary Simulation of gun 5 with present cathode design and different cases E-field: at corner higher field, behind is lower H-field: strong difference between different cases, higher field and therefore higher surface current when spring does not have a good contact to cathode

23. D. Lipka, MDI, DESY Hamburg, July 2012 I got the new holder and spring design, see 3D model on right, Simulation: same settings New spring and holder design

24. D. Lipka, MDI, DESY Hamburg, July 2012 View around spring, 45 lamellas New spring and holder design

25. D. Lipka, MDI, DESY Hamburg, July 2012 On the right side the mesh view, low mesh sizes for cathode and spring and holder around defined Mesh view

26. D. Lipka, MDI, DESY Hamburg, July 2012 E-field on right plot in V/m Similar field along z-axis, similar field on cathode surface in resonator compared to present design, a little bit longer field to the backside of cathode due to spring position is more behind, amplitude on cathode side few MV/m and lower E-field along line

27. D. Lipka, MDI, DESY Hamburg, July 2012 H-field on right plot in A/m Seems to be no spike at cathode, but see next slide … H-field along line

28. D. Lipka, MDI, DESY Hamburg, July 2012 Before the contact with spring high field: 28 kA/m (line was just between two lamellas) Corresponds to 1061 A surface current, lower than present design (1207 A) But … H-field on cathode

29. D. Lipka, MDI, DESY Hamburg, July 2012 Introducing a bad contact of spring, I removed 6 lamellas, see design on right Missing lamella

30. D. Lipka, MDI, DESY Hamburg, July 2012 A strong H-field at the first contact on the cathode is produced with 67.6 kA, which corresponds to 2552 A surface current (higher compared to the failure setting with present design 1926 A) H-field on cathode with missing lamellas