1. Advanced beam dynamics experiments at SPARC Alberto Bacci on be half the SPARC group PITZ Collaboration meeting, 27-28 October 2011, Zeuthen (Berlin)

2. SPARC layout linac -TW S-band Gun 1.6 SW 130MV/m VB cavity for low energy bunch compression and solenoids to emittance compensation 6 onadulators seeding laser room photocathode laser room New beam lines under installation : Thoson – PWFA – LWFA FLAME laser input line (300TW) seeding line

3. FEL Single Spike THz Radiation LWFA_ext LASER COMB Velocity Bunching PWFA Thomson C_Band injector 4 pulses Narrow THz Rad 2 pulses FEL SPARC Velocity Bunching applications Progress towards high brightness beam: -Gun RF pulse shaping 130MV/m (Marco Bellaveglia) -Laser Profile Optimization (Giancarlo Gatti) -Higher thermal Stability

4. Before (11 MW - 112 MV/m – 4.7 MeV) – 5÷10 discharge per minute Now (14 MW – 130 MV/m – 6.2 MeV) – ~ 1 discharge per hour 10 -10 torr vacuum level inside the Gun are maintained Marco Bellaveglia, M. Ferrario, A. Gallo, RF pulse shaping optimization to drive low emittance RF photoinjector, to be published New RF pulse shaping for Gun feeding Goals: Increase the gun accelerating gradient Maintain the residual phase noise, respect to the main oscillator, below 100fs Have a breakdown rate as low as possible Solution: In the first 3us the RF level is kept as low as possible to make the PLL (Phase Locked Loop) working The RF is brought to the maximum level in the last 0.8 us

5. - P.O.Shea et al., Proc. of 2001 IEEE PAC, Chicago, USA (2001) p.704. - M. Ferrario. M. Boscolo et al., Int. J. of Mod. Phys. B, 2006 (Taipei 05 Workshop) Laser Comb: beam echo generation of a train bunches

6. Movie ext-link

7. The technique used for this purpose relies on a birefringent crystal, where the input pulse is decomposed in two orthogonally polarized pulses (ordinary, extraordinary) with a time separation proportional to the crystal length. Different crystal thickness are available (10.353 mm in this case). Putting more crystals, one can generate bunch trains (e.g. 4 bunches). The intensity along the pulse train can be modulated (e.g. PWFA) A train of laser pulses at the cathode by birefringent crystal Giancarlo Gatti

8. Experimental results

9. Systematic analysis by simulations (two bunches Train) GIOTTO (Genetic Interface for OpTimizing Tracking with Optics) φ gun =36.66 deg φ gun =11.45 deg Initial parameters: T separation at chathode = 4.27 ps Q = 80 pC + 80 pC σ x = σ y = 400 μm Tw cavity II–III on crest Final Condiction: T separation ≈ 1 ps current I = current II Minimum rms ε I - II Free parameters (Knobs) : Gun ijection phase VB ijection phase Bz field Gun Solenoid Bz field Tw cavity N. 1 The minimum total projected emittance (measurable) corresponds to a similar behaviour of both sub-bunches (emittance and current) x px x

10. GIOTTO – Stat. Analysis

11. Two bunches train caracterization Q t =166 pC (92+78) remarkable agreement ε x,y (100%) = 0.8,1.1 mm-mrad, E spread for each pulse < 0.1 % (170 MeV) ε x,y (90%) = 0.5,0.5 mm-mrad, σt1 ≈ σt2 ≈ 1ps 350 [A] σt=140 fs ε x,y (100%) = 4.5,3.3 mm-rad ε x,y (90%) = 3.6,2.6 mm-rad E spread 0.4% and 0.25% (110 MeV) Energy separation ≈ 1.5 MeV on crest maximum compression VB phase -90.4 T sep. = 4.27 ps σ t-pulses ≈150fs σ x = σ y = 400 μm

12. Over-compression VB phase -95.6 Two bunches train caracterization σt I =140 fs, σt II =270 fs T separation ≈0.8 ps ε x,y (100%) = 6.2,4.4 mm-rad ε x,y (90%) = 5.8,4.0 mm-rad E spread 0.16% and 0.4% Energy separation ≈ 1.2 MeV

13. FEL Comb at SPARC (two bunches train) From the spectrum dt ≈ 0.615 ps; comparable with data = 0.8 ps

14. 4-pulses-time-structure time orizontal dim energy vertical dim 4-levels-energy-spectrum energy longitudinal phase space time Four pulses COMB structure (200 pC) Laser pulse @ gun cathode whole train length ≈ 9 ps σ t (per spike) ≈ 200 fs

15. 4 comb pulses and long. phase space rotation Over-compression region: The sub-bunches are well separated; their distance can be controlled by VB phase injection φ VB =115.7 deg

16.  (S1)= -91.5 deg  (S1)= -89.5 deg 4 comb pulses last simulations by Tstep C. Ronsivalle Dog-leg effects are under study

17. THz radiation can be easily produced by means of CTR It is difficult to put high charge in sub-ps bunches A laser comb structure in the longitudinal laser profile can solve this problem The SPARC THz source – narrow band

18. o Operating spectral range: 100 GHz-5 THz o It allows to reconstruct the beam profile o First test with pyroelectric detector; foreseen Golay cell or bolometers Martin-Puplet interferometer Silicon Aluminated screen (40 nm coating) The SPARC THz source Interferogram CTR spectrum by Fourier trasforming

19. Interferogram Measured Expected Spectrum Narrow THz radiation measured Interferogram for bunches train show 2N-1 peaks (inter-distance = sub-bunches distance) Radiation spectrum is strongly suppressed outside the comb rep. ferquency Analysis by Chiadroni

20. Conclusion The SPARC linac has improved the machine stability and the gun gradient We have demonstrated, from experimental point of view, that one can control pulse spacing, length, current and energy separation by properly setting the accelerator. A very good agreement with simulations

21. Thanks for your attention

22. using an RF switch controlled by a trigger signal coming from the machine trigger distribution, we can decide when the RF level jump takes place. One can decide the power ratio between the two levels combining the DC coupling in the input branch and the value of the fixed attenuation in the low power branch. The phase shifter is used to minimize the phase jump during the switching transition, due to the two different paths of the signal. RF Gun feeding schema Marco.bellaveglia@lnf.infn.it

23. Time jitter relative to the main RF clock (PLLs ON) – Linac RF devices phase noise (standard phase detection): 40÷100 fs RMS – Photo-cathode LAM measured time jitter (resonant Laser Arrival Monitor): <250fs RMS – e-bunch time jitter BAM (resonant Bunch Arrival Monitor): <250fs RMS RF deflector centroid jitter (image analysis): <150fs RMS Laser amplitude stability (from new timing) – Laser amplification timing locked to machine trigger – Amplitude jitter always <5% Jitter performance achieved

24. VB Longitudinal compression >> no CSR >> avoid emittance degradation high improving in the bunch’s brightness Beam in injected ahead of peak accelerating phase, and compresses as it slips back in phase Beam executes 1/4 “synchrotron” oscillation in longitudinal phase space. It permits to reach very high compression (~50 times) The VB causes a non-linear longitudinal phase space deformation that limits the compression capability of the method Cause of deformation is the non-linear RF accelerating fied - S-band (2856 MHz) SLAC type structures solution A short X-band (11424 MHz) accelerating structure

25. The first section to compress, the following sections to accelerate and to reduce relative energy spread. Considering very short bunches the compressing factor can be pushed at higher values; -Short bunches show a lower longitudinal phase space deformation, which is evident directly from analitical considerations Initial debunching: Non-negligible for A=(R/L) >> 1

26. Compression  <90 deg Over-compression 180 deg<  <90 deg Deep over-compression  >180 deg  =beam rotation in the longitudinal phase space COMPUTED COMPRESSION CURVES OF THE SINGLE BUNCHES OF THE FOUR-PULSES TRAIN 0.0 Deg -90.0 Deg -105.8 Deg

27. TSTEP simulations Operational principle

28. FEL Comb at SPARC - 2 Two bunches train Along the shift 200 shots have been acquired IV typology of spectrums have been found Simulations agree with the day jitters 65+65 pC, Є x =2.8, γ=232 Only RF def. ; VB phase - 92 Deg