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Optimization c athode design with gun5

Optimization c athode design with gun5. D. Lipka , V. Vogel, DESY Hamburg, Germany. Tilted backplane. To decrease field on cathode edge and keep high performance for beam dynamics: backplane to cathode are tilted, here 10°, On blue line field strength can be monitored. Tilted backplane.

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Optimization c athode design with gun5

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  1. Optimization cathode design with gun5 D. Lipka, V. Vogel, DESY Hamburg, Germany

  2. Tilted backplane To decrease field on cathode edge and keep high performance for beam dynamics: backplane to cathode are tilted, here 10°, On blue line field strength can be monitored

  3. Tilted backplane The cathode holder and spring for this simulation is used like present FLASH case

  4. 1D Results > S-Parameters Length of first cell is adapted to reach 1299.9 MHz resonance frequency of p-mode

  5. E-field on axis With this adaption the balance is almost perfect, here the maximum field strength is scaled to 60 MV/m, but The field strength on cathode is reduced due to the tilted backplane

  6. E-field on cathode E-field strength on cathode edge is maximum with 63.11 MV/m. This is a factor of 1.37 higher compared to cathode center field strength, Compared with present case: factor is 1.39, only slight reduction

  7. E-field on surface • Diagram shows E-field strength on surface. Compared to present case: • Lower field on cathode • Relatively high field on iris • Surface is longer due to tilt and adaption to get balance and resonance frequency

  8. ASTRA simulation • To check the beam performance with the tilted backplane ASTRA simulations are performed • Laser beam settings: • Q=1nC, longitudinal flat-top with 20 ps time duration and 2 ps rise and fall time • Transverse size sx,y=0.75 mm radial symmetry, no thermal emittance

  9. ASTRA simulation: fields • E-field maximum scaled to 60 MV/m, will be compared with gun5 design • Solenoid field taken from standard FLASH type • For optimization of emittancerf-phase of E-field and solenoid strength will be varied Solenoid field E-field gun5 design E-field tilted backplane Fields as a function of distance to cathode

  10. ASTRA simulation results For optimization of emittance following optimization results are found: • Tilted backplane: rf-phase between laser and highest energy gain has to be +20°, at 0° electrons are lost due to backwards acceleration, solenoid maximum strength has to be 0.23 T • Design gun5: rf-phase between laser and highest energy gain has to be +10°, solenoid maximum strength has to be 0.25 T

  11. ASTRA results: tilted backplane A small emittance observed, no electron loss, energy 6.3 MeV still with 20° rf phase due to longer first cell

  12. ASTRA result: default gun5 design A small emittance observed too, energy 5.9 MeV with 10° rf phase

  13. ASTRA results: emittance • Tilted backplane minimum emittance of 0.8 p mm mrad at z=1.54 m • Design gun5 minimum emittance of 1.05 p mm mrad at z=1.25 m → still smaller emittance with tilted backplane and reduced field at cathode Design gun5 tilted backplane Emittance as a function of distance to cathode

  14. Change design: smaller rounding holder • To reduce field on cathode edge the holder rounding are reduced from bending radius 3 mm to 1 mm

  15. E-field on z-axis • Similar field distribution along z-axis like before because of only small change • Resonance frequency still at 1299.9 MHz

  16. E-field on cathode • Field maximum strength on edge is 57.9 MV/m, a factor of 1.31 higher compared to center • The factor is reduced from 1.39 (present case) • Summary: field strength on edge is reduced from 84.6 to 63.1 (10°) and to 57.9 MV/m for smaller rounding

  17. E-field strength on resonator surface • Field strength in units of E/m • higher field on iris visible in both optimized design visible • Smaller field strength due to smaller bend radius • Even smaller bend increases field again

  18. H-field strength on cathode H-field strength on cathode due to modification is reduced: • Default: 31.5 kA/m • 10°: 28.7 kA/m • Smaller rounding: 21.1 kA/m Field strength maximum at connection between spring and cathode Default case

  19. ASTRA simulation results • Similar settings for optimal emittance: Bmax=0.23 T, phase +20° • Emittance 0.89 p mm mrad at 1.57 m distance to cathode

  20. Compare shunt impedance • The shunt impedance for each case is: • Default case: 7.2 MΩ • 10° case: 9.0 MΩ • 10° and smaller rounding: 9.1 MΩ

  21. Conclusion • Emittance reduced from 1.05 to 0.8 (10°) and 0.89 p mm mrad (last case) • Maximum E-field strength on cathode surface reduced from 84.6 to 63.1 (10°) and to 57.9 MV/m (last case) • Maximum H-field strength on cathode surface reduced from 31.5 to 28.7 (10°) and 21.1 kA/m (last case)

  22. Slice emittances • Slice emittances are shown as a function of longitudinal slices at the z-position with best projected emittance • Best slice emittances reached for default case in the center (about 0.2 p mm mrad), only front and end contributes to the larger projected emittance • Other both slice emittances at the center higher, at the front and end are smaller compared to default case Tilted backplane at z=1.54 m Default case at z=1.25 m Tilted backplane and smaller rounding at z=1.57 m

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