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An RF photogun for external injection of electrons in a Laser Wakefield Accelerator

An RF photogun for external injection of electrons in a Laser Wakefield Accelerator. Seth Brussaard. People. Xavier Stragier Marnix van der Wiel ( AccTec ) Willem op ‘t Root Jom Luiten Walter van Dijk Seth Brussaard Walter Knulst (TUDelft) Fred Kiewiet

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An RF photogun for external injection of electrons in a Laser Wakefield Accelerator

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  1. An RFphotogun for external injection of electrons in a Laser Wakefield Accelerator Seth Brussaard

  2. People Xavier Stragier Marnix van der Wiel (AccTec) Willem op ‘t Root Jom Luiten Walter van Dijk Seth Brussaard Walter Knulst (TUDelft) Fred Kiewiet Eddy Rietman Bas van der Geer (Pulsar Physics) Ad Kemper Marieke de Loos (TU/e & Pulsar) Harry van Doorn Iman Koole Jolanda van de Ven

  3. Outline • Laser Wakefield Acceleration • External Injection • RF Photogun Design • RF Photogun Performance

  4. Laser Wakefield Acceleration Accelerating Fields: 100-1000 GV/m

  5. Injection max  1000  min  10 - 0  

  6. External Injection How many electrons can we get in? What will come out?

  7. Setup Incoming laser pulse: 300 mJ, 200 ps , 800 nm Compressed laser pulse: 150 mJ, 50 fs, 800 nm UV-pulse for photogun: 266-400 nm Plasma channel RF- photogun Solenoid (focusing electron bunch) Parabolic mirror 1.2 meter

  8. RF Photoguns Our approach: • Emittance growthdue to non-linearaccelerationfields: • full cylindricalsymmetry • notuningplungers • on-axis RF coupling single-diamondturning

  9. RFPhotogun Coaxial S-band input coupler: scaled down version L-band design DESY

  10. RF Photoguns Approach: • Emittance growthdue to non-linearaccelerationfields: • full cylindricalsymmetry • notuningplungers • on-axis RF coupling single-diamondturning 2nd generation: • Elliptical irises • Highest field strength on cathode; • Cavity parts are clamped, not braized • Easily replaced; • Copper cavity inside stainless vacuum can.

  11. RF Photoguns Clampedconstruction: cavityparts first (half) cell cathode plate second cell

  12. RF Photoguns Clampedconstruction: cavityparts single-diamond turning

  13. RF Photoguns Clampedconstruction: cavityinsidestainless steel vacuumcan

  14. RFPhotogun Cavitymountedinsidemainmagnet:

  15. RF Photoguns RF characterization: resonances f0=2.9980 GHz p-mode f0=2.9918 GHz 0-mode

  16. RF Photoguns RF characterization: onaxisfield profile

  17. RF Photoguns High power RF commissioning: • 80 MV/m at cathode (after one month of training) • Still occasional breakdown • 3 MeV electrons • QE ≈ 3·10-5→ bunch charge Qmax≈ 300 pC Conclusion: clamping is OK! ZFEL Workshop

  18. RF Photoguns • Water cooling for 1 kHz PRF • Presently operating @ 100 Hz (limited by Modulator/Klystron) ZFEL Workshop

  19. Emittance Quadrupole scan:

  20. Emittance Quadrupole scan: Q = 5 pC σx,cathode= 0.43 mm εn = 0.40(5) mm·mrad

  21. The RF photogun: 2.5 Cell RF power 266nm, 50fs Injector for Laser Wakefield Acceleration Three coupled pillboxes Resonant frequency of 2998 MHz RF power source: 10 MW peak power klystron Electron source: Photo-emission from cavity wall E-bunch

  22. Setup Incoming laser pulse: 300 mJ, 200 ps , 800 nm Compressed laser pulse: 150 mJ, 50 fs, 800 nm UV-pulse for photogun: 266-400 nm Plasma channel RF- photogun Solenoid (focusing electron bunch) Parabolic mirror 1.2 meter

  23. Beamline correction coils RF pulsed solenoid Faraday cup spectrometer 266nm 50fs phosphor screen

  24. Bunch Energy spectrometer E = 3.71 ± 0.03 MeV 1 Intensity (a.u.) 0.5 0 3.61 3.67 3.73 3.79 σ Energy (MeV) = 2 keV Emax

  25. Spot Size pulsed solenoid 700 εn~ 1-3 mm·mrad. 600 500 400 RMS Radius [μm] 300 200 100 0 0.0 0.1 0.2 0.3 0.4 focal length [m]

  26. Bunch Size at the Focus

  27. Spot Size & Stability pulsed solenoid 1 mm 0.75 mm

  28. Focus Stability 300 μm 100 μm

  29. Spot Size & Stability pulsed solenoid 20 15 10 Counts 5 0 -12 -6 0 6 12 -12 -6 0 6 12 1 mm ΔX centre focus [μm] ΔY centre focus [μm] 0.75 mm

  30. Simulations focus 20 mm inside plasma focus at entrance of plasma Einj = 3.71 MeV Plaser = 25 TW Eout = 900 MeV

  31. Simulations focus 20 mm inside plasma focus at entrance of plasma Einj = 3.71 MeV Plaser = 25 TW Eout = 900 MeV

  32. Simulations focus 20 mm inside plasma focus at entrance of plasma Einj = 3.71 MeV Plaser = 25 TW Eout = 900 MeV

  33. Simulations focus 20 mm inside plasma focus at entrance of plasma Einj = 3.71 MeV Plaser = 25 TW Eout = 900 MeV

  34. Simulations focus 20 mm inside plasma focus at entrance of plasma Einj = 3.71 MeV Plaser = 25 TW Eout = 900 MeV

  35. External Injection How many electrons can we get in? What will come out?  1 pC @ 3.7 MeV @ 25 TW: 8 fs bunch 900  40 MeV

  36. Conclusions & Outlook RF Photogun as external injector feasible ~ 1 pC accelerated bunches realistic Next: Condition to 6.5 MeV Inject behind the laser pulse

  37. Timing

  38. Timing Coherent Transition Radiation (CTR) CTR: radiallypolarized

  39. Bunch Length THz power & energy in focus Q = 70 pC τbunch < 2 ps

  40. Timing Coherent TransitionRadiation (CTR) RF phase 100 fsjitter

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