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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...

  2. ... On behalf of SPARC team D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  3. Outline • The velocity bunching concept • SPARC hardware overview • VB experiment @ SPARC • Emittance degradation by solenoid misalignment • Conclusions D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  4. The velocity bunching concept: If the beam injected in a long accelerating structure at the 0 crossing field phase and it is slightly slower than the phase velocity of the RF wave , 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, actually the first section following the gun, that typically accelerates the beam, under these conditions, from a few MeV (> 4) up to 20-25 MeV. D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  5. Limitations to the compression: • Δγ0 • Δφ0 (rf non linearities) Phase space rotation: S. G. Anderson et al., “Velocity bunching of high-brightness electron beams”, Phys. Rev. ST-AB, 8, 014401 (2005). D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  6. 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 -VB@SPARC-FEL09-Liverpool-UK

  7. ... What happens in 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 !!! RF Compression is made at lower energy than magnetic compression. Can be used at SPARC to shorten the beam and use it also for other applications foreseen at SPARC (ICS, THZ production, etc... ) D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  8. 150 MeV S-band linac Velocity Bunching Diagnostic and Matching Undulators u = 2.8 cm Kmax = 2.2 r = 500 nm 10 m 15 m S-band Gun Long Solenoids Seeding THz Source SPARC overview: D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  9. Iron joke (blue) for field lines guiding • 1 single and 4 triplet coils surrounding two LINAC section, • indipendently powered D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  10. 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: 8keV/pix @ 150MeV; Overall resolution (RMS): 10-4≤DE/E ≤ 10-2 D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  11. VB run @ SPARC (high charge case): 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 -VB@SPARC-FEL09-Liverpool-UK

  12. WP for next measurements: C=3 C-Factor Vs RF compressor phase: First linac section used as compressor 240fs rms Maximum energy D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  13. 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 • The slice is chosen to be the FEL slippage length (Nupxλr≈210μm; Sl=250um) • The sliding slice approach is used in data analysis D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  14. Effect of solenoid: TW solenoids OFF Vs ON (66Amps, 590Gauss) 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 Isol=158 A D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  15. 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 -VB@SPARC-FEL09-Liverpool-UK

  16. Beam after compression @C3 • Beam loses more than 45MeV; • Increase of correlated energy spread ; • Increase of slice current by factor approx 4 D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  17. Btr=1.1x1014 A/m2 Beam after compression @C3 Emittance without TW solenoids (Gun solenoid current=157Amps): Ex=6.2 mm mrad Ey=4.0 mm mrad 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) B/C=0.9 D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  18. 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 -VB@SPARC-FEL09-Liverpool-UK

  19. 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 B≈ 2x1014 Amps/m2 TW solenoids OFF Gun sol Current(151Amps): Ex=4.1 mm mrad Ey=3.4 mm mrad ! Slice length equal to the minimum time resolution of the system (32fs) D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  20. 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 -VB@SPARC-FEL09-Liverpool-UK

  21. 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 -VB@SPARC-FEL09-Liverpool-UK

  22. Effect on emittance measurement: QS for projected emittance QS for slice emittance same quad currents higher emittance value Simulated X e Y vs phi at linac output X-phi Y-phi Beam dimensions Quad strength D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  23. Slice centroid spread exclusion: Projected emittance from slice αn, βn, γn, εntwiss parameters of slice n Dattoli et al. “Slice Emittance, Projected emittance and properties of the FEL SASE radiation”, MOPC32, this conference D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  24. 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 -VB@SPARC-FEL09-Liverpool-UK

  25. 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 Slice centroids Vs Z Transverse phase space distorsion due to beam misalignment 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 -VB@SPARC-FEL09-Liverpool-UK

  26. Conclusions: • Demonstrated transverse emittance preservation in the VB regime for medium compression factors; • At extreme compressions the emittance is not perfectly compensated but still good enough to further accelerate the beam and use it for FELs. • Higher total energy spreads make the beam emittance sensible to magnetic components misalignment (quads, sol., etc...) • The centroid emittance contribution can be isolated and measured Next steps • BBA on TW solenoids • emittance studies as function of TW solenoid fields • Longitudinal phase space detailed studies • FEL radiation experiments with velocity bunched beams D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  27. SPARC results @ different energies resolution 0.9182 [ps/mm] 1.6119 [ps/mm] @142 MeV Increasing energy 1.3745 [ps/mm] RFD: RFD off SPARC typical parameters: D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  28. RF compression experiments in the world: D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  29. Operational consideration on Machine tuning: 1 mm sx=555.6 (3.0) mm Not VB data Isol=157 A sx=515.5 (0.93) mm Isol=158 A sx=595(1.3) mm Isol=160 A D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

  30. Homdyn singlet doublet singlet Emittance degradation induced by chromatic effects in quadrupoles Single quadrupole Quadrupole doublet D. Filippetto -VB@SPARC-FEL09-Liverpool-UK

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