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Velocity bunching experiment @ SPARC

Velocity bunching experiment @ SPARC. Daniele Filippetto on behalf of SPARC team. Outline. The velocity bunching concept SPARC hardware overview VB experiment @ SPARC Emittance degradation by solenoid misalignment Conclusions.

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Velocity bunching experiment @ SPARC

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  1. Velocity bunching experiment @ SPARC Daniele Filippetto on behalf of SPARC team

  2. Outline • The velocity bunching concept • SPARC hardware overview • VB experiment @ SPARC • Emittance degradation by solenoid misalignment • Conclusions D. Filippetto HBEB-MAUI_09

  3. The velocity bunching concept: • the beam is injected in a long accelerating structure at the 0 crossing field phase • Injection at low energies where The beam is slower than the phase velocity of the RF wave (typically the first LINAC after the gun) • it will slip back to phases where the field is accelerating, but at the same time it will be chirped and compressed. • Compression and acceleration take place at the same time within the same linac section At SPARC the beam is accelerated from 4-5 MeV up to 20-25 MeV (instead of 60-65) D. Filippetto HBEB-MAUI_09

  4. HIGH-C MEDIUM COMPRESSION LOW COMPRESSION OVER-C Peak current vs RF compression phase SPARC nominal case Initial parameters: 1 nC beam 10 ps long D. Filippetto HBEB-MAUI_09

  5. ... What happens to the transverse plane? SPARC L. Serafini, M. Ferrario, “Velocity Bunching in PhotoInjectors” , AIP CP 581, 2001, pag.87 may avoid the phase space degrading effects observed in magnetic compression experiments on photoinjector-derived beams If the transverse emittance can be preserved during the longitudinal focusing, the beam brightness can be increased D. Filippetto HBEB-MAUI_09

  6. 150 MeV S-band linac Velocity Bunching Diagnostic and Matching Undulators u = 2.8 cm Kmax = 2.2 r = 500 nm 10m 15m S-band Gun Long Solenoids Seeding THz Source SPARC overview: D. Filippetto HBEB-MAUI_09

  7. Iron joke (blue) for field lines guiding • 1 single and 4 triplet coils surrounding two LINAC section, • indipendently powered D. Filippetto HBEB-MAUI_09

  8. Diagnostic hardware: Time measurement resolution: » 50 fs / pixel @ 150 MeV » 33 fs / pixel @ 100 MeV SPARC typical parameters: s @ m 70 m y B @ V 1 . 5 MV DEFL @ E 150 MeV @ f 2 . 856 GHz RF @ L 4 m s 90 fs @ 150 MeV y s = @ B _ t RES V 60 fs @ 100 MeV B w L DEFL RF E / e screens Quadrupole triplet Dipole magnet RF deflector Spectrometer system: Θ=14 deg; Lm=26.7cm; Ld=2.13m; Pixel size=30um; Energy resolution: about 15keV @ 150MeV; Overall resolution (RMS): 10-4≤DE/E ≤ 10-2 D. Filippetto HBEB-MAUI_09

  9. VB run @ SPARC: Laser parameters: Xrms =358um Yrms =350um TFWHM=7.3ps Energy @ cathode = 170uJ Gun parameters: Gun Input Power=7.5MW Gun Peak Field=105MV/m e-Energy out of the gun=4.7MeV Working inj.phase=30 deg. e-beam charge @30deg=280pC D. Filippetto HBEB-MAUI_09

  10. C-Factor Vs RF compressor phase: First linac section used as compressor • C=3 chosen for characterization measurements: • Useful in a hybrid scheme with magnetic compressor (SPARX case) • Less sensitive to relative phase jitter C=15 240 fs rms C=3 1000 fs rms Maximum energy D. Filippetto HBEB-MAUI_09

  11. E-beam parameters @ LINAC exit, C=1: Max energy on crest 147.5MeV Total DErms0.16MeV DE/Erms 0.11% Charge 280pC Bunch Length RMS 3.01ps Slice Peak Current 30Amps Longitudinal emittance 159.6 keV*mm Beam slice current profile D. Filippetto HBEB-MAUI_09

  12. Effect of solenoid: TW solenoids OFF Vs ON (660Gauss) C=1 TW sol on TW-SOL on: εx=1.85um εy=1.65um Best emittance after solenoid scan with TW-SOL off: εx=1.4um εy=1.5um Isol=161 A D. Filippetto HBEB-MAUI_09

  13. TW solenoids Off VS ON, slice emittances: • The solenoid misalignment leads to an increase of the projected emittance, which is not found looking at the slice emittances; • the mismatch parameter is similar in the two cases; • The difference is due to slice centroid misalignement (will be treated more in detail further on in the presentation); • A beam based alignment is mandatory to reach lower projected emittances; D. Filippetto HBEB-MAUI_09

  14. Beam after compression @C3 D. Filippetto HBEB-MAUI_09

  15. Bsl=1.1x1014 A/m2 Beam after compression @C3 For a compression factor C=3: Gain of a factor 3.7 on the maximum slice current (30 Vs 110) Loss of a factor 1.15 on the minimum slice emittance (1.2 Vs 1.4) Gain of a factor 2.7 on the slice Max Brigthness (0.41 Vs 1.1x1014) ΔB/C=0.9 Emittance without TW solenoids (Gun solenoid current=157Amps): Ex=6.2 mm mrad Ey=4.0 mm mrad D. Filippetto HBEB-MAUI_09

  16. Extreme compression WP • Low charge/max Compression Case: Bunch Charge= 60pC Bunch length rms= 1.95 ps Longitudinal emittance= 54.2 keV*mm Laser spot size rms= 250um D. Filippetto HBEB-MAUI_09

  17. Beam @ C-17 (TW sol 45Amps): Energy=97.6 MeVDE/Erms =1% Ipeak=217.5 Amps Ex=1.52 mm mrad Ey=1.62 mm mrad Proj. emittance Preliminary B≈ 2x1014 Amps/m2 TW solenoids OFF Gun sol Current(151Amps): Ex=4.1 mm mrad Ey=3.4 mm mrad D. Filippetto HBEB-MAUI_09

  18. Critical point: • Proj. emittance degradation due to solenoids misalignment • The solenoid force is energy dependent: • KL=qB0/2m0cβγ • strong energy-time correlation in VB conditions • different focusing forces for different time slices • if the beam is propagating off axis respect to the magnetic field, the slice centroids will experience time dependent kicks Lower Energies higher Energies Induced longitudinal-transverse correlation, proj. emittance increase D. Filippetto HBEB-MAUI_09

  19. Example: 1mm solenoid misalignment (H) Effect on transverse beam shape along the Linac: measurements Out linac1 On crest VB conditions Out linac2 PARMELA runs simulating the two TW solenoids 1mm off axis respect to the rf cavity, on crest and in the VB conditions VB conditions D. Filippetto HBEB-MAUI_09

  20. Effect on emittance measurement: QS for projected emittance RFD off QS for slice emittance RFD on same quad currents Y time X X higher emittance value Simulated X e Y vs phi at linac output X-phi Y-phi Beam dimensions Quad strength D. Filippetto HBEB-MAUI_09

  21. Slice centroid spread exclusion: Projected emittance from slice αn, βn, γn, εntwiss parameters of slice n D. Filippetto HBEB-MAUI_09

  22. Slice centroid contribution to the emittance: M.Ferrario, V.Fusco, M.Migliorati, L. Palumbo,Int. Journal of Modern Physics A ,Vol 22, No. 3 (2007) 4214-4234 uses the slice centroid different from 0 only if slice centroids do not lie on the same axis correlation between slice centroid spread and single slice dimension in ph.sp. εxenv=0 εxcent=0 εxcross≠0 D. Filippetto HBEB-MAUI_09

  23. Measured H. Projected emittance @157A (red dot)= 2.3um εxenv=1.5 um εxcent=0.52 um εxcross=1.72 um εxtot=√(εxenv)2+ (εxenv)2 +(εxenv)2=2.34um Transverse phase space distorsion due to beam misalignment Slice centroids Vs Z Slice mean divergence Vs Z • From the slice emittance with the quad scan, the values of alpha beta and emittance for each slice are calculated at one precise position • From the QS measurements also the system for slice centroids (both in X and X’) can be written and solved (first order system) • All the 3 emittance terms can be calculated D. Filippetto HBEB-MAUI_09

  24. Conclusions: • Demonstrated transverse emittance preservation in the VB regime for medium compression factors; • Preliminary studies on high CF show an emittance decrease, but still work to do to fully compensate. • Higher total energy spreads make the beam emittance sensitive to magnetic components misalignment (quads, sol., etc...) • The slice centroid spread contribution to the projected emittance can be isolated and measured Next steps • BBA on TW solenoids • emittance study as function of TW solenoid fields (field shaping) • Longitudinal phase space detailed studies (slice DE) • THZ production, ICS experiments, FEL single spike, laser comb D. Filippetto HBEB-MAUI_09

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