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

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

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D. Filippetto HBEB-MAUI_09 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)

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

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D. Filippetto HBEB-MAUI_09 If the transverse emittance can be preserved during the longitudinal focusing, the beam brightness can be increased 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 SPARC... What happens to the transverse plane?

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

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

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D. Filippetto HBEB-MAUI_09 Diagnostic hardware: Dipole magnetRF deflector Quadrupole triplet screens 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 Time measurement resolution: SPARC typical parameters: mL GHzf MeVE MVV m MeVpixelfs MeVpixelfs RF DEFL y B 4 856.2 150 5.1 70 100@/33 150@/50 MeVfs MeVfs L eE V RF DEFL y RESt B B 100@60 150@90 / _

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D. Filippetto HBEB-MAUI_09 VB run @ SPARC: Laser parameters: X rms =358um Y rms =350um T FWHM =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

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

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D. Filippetto HBEB-MAUI_09 E-beam parameters @ LINAC exit, C=1: Max energy on crest 147.5MeV Total DE rms 0.16MeV DE/E rms 0.11% Charge 280pC Bunch Length RMS 3.01ps Slice Peak Current30Amps Longitudinal emittance 159.6 keV*mm Beam slice current profile

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

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D. Filippetto HBEB-MAUI_09 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;

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D. Filippetto HBEB-MAUI_09 Beam after compression @C3 No compressionCompression @C3 Bunch charge280 pC Injection phase (S1)0 deg (on crest)-87deg Beam Energy147.5 MeV101 MeV Total energy spread0.11%1.1% Bunch length3.01ps RMS0.97ps RMS TW Solenoid field0450 Gauss

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D. Filippetto HBEB-MAUI_09 Beam after compression @C3 B sl =1.1x10 14 A/m 2 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 1.1x10 14) ΔB/C=0.9

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D. Filippetto HBEB-MAUI_09 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 Extreme compression WP

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D. Filippetto HBEB-MAUI_09 TW solenoids OFF Gun sol Current(151Amps): Ex=4.1 mm mrad Ey=3.4 mm mrad Beam @ C-17 (TW sol 45Amps): Energy=97.6 MeV DE/Erms =1% I peak =217.5 Amps Ex=1.52 mm mrad Ey=1.62 mm mrad Proj. emittance B ≈ 2x10 14 Amps/m 2 Preliminary

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D. Filippetto HBEB-MAUI_09 Critical point: Proj. emittance degradation due to solenoids misalignment The solenoid force is energy dependent: K L =qB 0 /2m 0 cβγ 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 Induced longitudinal-transverse correlation, proj. emittance increase Lower Energies higher Energies

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

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

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D. Filippetto HBEB-MAUI_09 Slice centroid spread exclusion: Projected emittance from slice α n, β n, γ n, ε n twiss parameters of slice n

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D. Filippetto HBEB-MAUI_09 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. ε x env =0ε x cent =0 ε x cross ≠0 Slice centroid contribution to the emittance:

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D. Filippetto HBEB-MAUI_09 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 Measured H. Projected emittance @157A (red dot)= 2.3um ε x env =1.5 um ε x cent =0.52 um ε x cross =1.72 um ε x tot =√(ε x env ) 2 + (ε x env ) 2 +(ε x env ) 2 =2.34um Slice centroids Vs Z Slice mean divergence Vs Z Transverse phase space distorsion due to beam misalignment

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D. Filippetto HBEB-MAUI_09 Conclusions: 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 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

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