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 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

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

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

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

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

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: 266-400 nm 1.2 meter

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

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

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.

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

RF Photoguns Clamped construction: cavity parts single-diamond turning

RF Photoguns Clamped construction: cavity inside stainless steel vacuum can

RF Photogun Cavity mounted inside main magnet:

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

RF Photoguns RF characterization: on axis field profile

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 11-02-2011

RF Photoguns Water cooling for 1 kHz PRF Presently operating @ 100 Hz (limited by Modulator/Klystron) ZFEL Workshop 11-02-2011

Emittance Quadrupole scan:

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

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

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: 266-400 nm 1.2 meter

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

Bunch Energy σ E = 3.71 ± 0.03 MeV spectrometer = 2 keV Emax 1 0.5 1 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

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.

Bunch Size at the Focus

Spot Size & Stability pulsed solenoid 1 mm 0.75 mm

Focus Stability 300 μm 100 μm

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

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

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

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

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

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

 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?  1 pC @ 3.7 MeV @ 25 TW: 8 fs bunch 900  40 MeV

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

Timing

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

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

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