0. An RF photogun for external injection of electrons in a Laser Wakefield Accelerator
Seth Brussaard

1. 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

2. Outline Laser Wakefield Acceleration External Injection
RF Photogun Design
RF Photogun Performance

3. Laser Wakefield Acceleration
Accelerating Fields:
GV/m

4. Injection
max  1000 
min  10 -  

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

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

7. Our approach: RF Photoguns
Emittance growth due to non-linear acceleration fields:
full cylindrical symmetry
no tuning plungers
on-axis RF coupling
single-diamond turning

8. RF Photogun Coaxial S-band input coupler:
scaled down version L-band design DESY

9. Approach: 2nd generation: RF Photoguns
Emittance growth due to non-linear acceleration fields:
full cylindrical symmetry
no tuning plungers
on-axis RF coupling
single-diamond turning
2nd generation:
Elliptical irises
Highest field strength on cathode;
Cavity parts are clamped, not braized
Easily replaced;
Copper cavity inside stainless vacuum can.

10. RF Photoguns Clamped construction: cavity parts first (half) cell
cathode plate
second cell

11. RF Photoguns
Clamped construction:
cavity parts
single-diamond turning

12. RF Photoguns Clamped construction:
cavity inside stainless steel vacuum can

13. RF Photogun
Cavity mounted inside main magnet:

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

15. RF Photoguns
RF characterization: on axis field profile

16. Conclusion: clamping is OK!
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

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

18. Emittance
Quadrupole scan:

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

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

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

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

23. Bunch Energy σ E = 3.71 ± 0.03 MeV spectrometer = 2 keV Emax 1 0.51
0.5
Intensity (a.u.)
3.61
3.67
3.73
3.79
Energy (MeV)
E = 3.71 ± 0.03 MeV σ
Emax
= 2 keV

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

25. Bunch Size at the Focus

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

27. Focus Stability
300 μm
100 μm

28. Spot Size & Stability pulsed solenoid 1 mm 0.75 mm -12 -6 6 12 5 10 156 12 5 10 15 20
Counts
ΔY centre focus [μm]
ΔX centre focus [μm]
0.75 mm

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

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

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

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

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

34.  1 pC @ 3.7 MeV @ 25 TW: 8 fs bunch 900  40 MeV External Injection
How many electrons can we get in?
What will come out?
 MeV
@ 25 TW:
8 fs bunch
900  40 MeV

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

36. Timing

37. CTR: radially polarized
Timing
Coherent Transition Radiation (CTR)
CTR: radially polarized

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

39. Timing
Coherent Transition Radiation (CTR)
RF phase
100 fs jitter