CLARA Gun Cavity Optimisation NVEC 05/06/2014

1. CLARA Gun Cavity OptimisationNVEC05/06/2014 P. Goudket G. Burt, L. Cowie, J. McKenzie, B. Militsyn

2. CLARACompact Linear Advanced Research Accelerator • Beam Energy ~250MeV • SASE Saturation length <15m • Seed with Ti:Sa 800nm, lase up to 8th harmonic • Seeding with HHG at 100nm also possible • Single spike SASE, electron bunch length ~50fs FWHM and charge <20pC • Seeding, peak current ~400A, flat top ~300fs and charge <200pC

3. Photocathode Guns Laser Pulse • Allows the production of extremely short bunches dependent on laser pulse length. • High RF gradients and solenoidal fields allow for emittance preservation. • Removable photocathode inserts allow for higher quantum efficiency through the use of metal photocathode surfaces. RF TM010 p-mode Photocathode Electron Bunch Solenoid Bucking coil

4. Basic parameters

5. Choice of number of cells In order to remain below the 10 kW average power limit, and the 10 MW peak power limit set by the klystron, the only option that allows peak fields of 120 MV/m to be reached is a 1.5 cell gun. Target power 7MW peak due to losses in transmission system.

6. a = minor radius b = major radius r = iris radius blend = blending radius C1 length = length of 1st cell including iris blend a b r C1 length

7. Changing ellipticity Ratio is , at this point the maximum field on the cavity wall is no longer on the cell to cell iris. This is the chosen ellipticity. Major radius = 14 mm Ellipticity is . Minor radius is held at 8mm (min cooling channel dimensions) and major radius is changed. Maximum surface H changes by less than 1% over the whole range. R/Q changes by ~2% over the whole range. Q increases with higher ellipticity. Ellipticity (MHz) Mode separation is ~22 MHz Ellipticity

8. Mode separation • Mode separation is the only thing that gets worse at lower iris radius • Choose a lowest acceptable mode separation and go with that iris radius • No lower than 20 MHz • Iris radius = 13.4 mm 15 MHz/m was required at LCLS to reduce beating between modes

9. Superfish optimisation: final results

10. E H

11. Probe aperture optimisation • In order to minimise the peak H-field, a large probe aperture was chosen • This has the effect of changing the cell frequency, necessitating a retune of that cell • The peak magnetic field in the cell is now 2.54x105 A/m. • The calculated temperature increase for that location is 25K • The baseline H-field in that location is 1.94x105 A/m. • The calculated temperature increase for that location is 14K

12. Cathode plug: 10 mm diameter Cathode plug (10 mm diameter) 3rd generation plug

13. Cathode plug optimisation 0.6 ratio chosen Additional optimisation was performed on the cathode plug profile. An elliptical profile was adopted to maximise the (E field at centre)/(max E field on cathode) ratio.

14. Field flatness with cathode and probe coupler.

15. Peak field distribution - Electric

16. Peak field distribution - Magnetic

17. External Q coupZ0 = distance between C2 iris and coupler tip CST gives the Q0 as being 14970. The target Qe should be 15000. CoupZ0 = 11.5 mm Some adjustability should built-in in order to allow for adjusting the Qe to the effectively measured Q0.

18. Minimising coupler parasitic mode transmission

19. Tuneable alternative: H feed Short • Shorts are now used which can be adjusted and used for pumping. • Tee sections provide a reflective element to provide matching. RF in RF in

20. Mechanical Design • Up to 10kW average RF power will need to be handled. • Thermal simulations have been extensively performed on all gun components in order to ensure power handling capability. • Solutions to integrate the gun with the solenoids, photocathode transfer system and other components are mostly complete.

21. Conclusion • The RF design is approaching its final phase and mechanical design is already under way. • Planned manufacture later this year. • Planned installation from early 2015.