Recent advances and novel ideas for high brightness electron beam production

1. Recent advances and novel ideas for high brightness electron beam production based on photo-injectors Massimo Ferrario INFN - LNF

2. e+e- Linear Colliders X-Ray SASE FEL

3. Brief Review of Beam Dinamycs in Photo-Injectors Ultra-Short Bunch and High Peak Current Beam Generation Flat Beam Production

4. Brief Review of Beam Dinamycs in Photo-Injectors The beam undergoes two regimes along the accelerator, from photocathodetolinac exit, determined by the: Laminarity parameter : Laminar Beam ==> (Space charge dominated beam) Thermal Beam ==> (Emittance dominated)

5. Typical X-FEL Beam I=4 kA Potential space charge emittance growth rl I=1 kA Gas Beam I=100 A

6. Laminar - Beam (Space Charge dominated) Transverse Laminarity ==> Trajectories do not cross each other Longitudinal Laminarity ==> Diffrerent slices do not mix Projected emittance >> Slice emittance

7. Transverse Dynamics of a quasi-laminar beam The rms envelope equation for a beam subject to strong acceleration where Normalized focusing gradient (solenoid +RF foc.) has an exact analytical solution The associated plasma frequency is

8. This solution includes (at ) the so-called Brillouin flow (rigid rotation at constant spot-size in a solenoid field) Perturbed trajectories oscillate with the same frequency Unless energy spread is considered

9. Invariant Envelope => Beam confinement even without external focusing This solution represents a beam equilibrium mode that turns out to be the transport mode for achieving minimum emittance at the end of the emittance correction process (L.S and J.B.R., PRE55 (1997) 7565)

10. Emittance Oscillations in Laminar Beams Envelope Oscillationsdrive emittance oscillations( ) Damped Oscillations (emittance correction) if the beam is transported under two possible equilibrium conditions connected to each other Brillouin FlowInvariant Envelope

11. S-band photoinjector up to 150 MeV, HOMDYN simulation (RF Gun + 2 Traveling Wave Structures) Q=1nC, L=10ps, R=1 mm, Epeak=140 MV/m, TW Eacc = 25 MV/m Matching onto the Local Emittance Max. Final emittance = 0.4 mm

12. Ultra-Short Bunch and High Peak Current Beam Generation How to overcome present limitations in peak current of Photo-Injectors without sacrificing the transverse Emittance? Ballistic Compression (i.e. in drifts) Radio Frequency Compression(i.e. while accelerating)

13. Alternative options magnetic bunch compression high brightness sub-ps beams • Compression is rectilinear (no Coherent Synch. Radiation effects), • Performed at low energy (10-80 MeV), fully integratedinto the emittance correction process (for maximum brightness)

14. Ballistic Compression (in drifts) Provide a correlated energy spread enough to produce, in a drift of length , a path difference equal to half the bunch length (total compression)

15. Transverse Dynamics of a laminar beam subject to Ballistic Bunching • Assuming a current growing linearly with the beam propagation (far from the focus of the longitudinal phase space rotation) and a ramping solenoid field • The envelope equation still has an exact solution similar to Brillouin flow (rigid rotation with increasing plasma freq.)

16. Emittance Oscilations in Ballistic Bunching • We find that the envelope mismatch has an increasing oscillation frequency • implying a larger number of emittance oscillations to reach the end of laminar regime (where the emittance correction process is halted). Non-linearities driving wave-breaking and anharmonic effects in emittance oscillations eventually prevent a full correction of the emittance

17. Ramped solenoid drift Example of hybrid bunching (velocity + ballistic) in the ORION photoinjector Velocity Ballistic Final current 1.3 kA RF Gun PWT X-band TW Higher plasma frequency Lower Final emittance 1.5 mm S.Anderson-J.Rosenzweig Parmela/UCLA

18. ==> P. Musumeci talk

21. Compression during acceleration Current scaling with energy Radio Frequency compressor (L. Serafini & M. F.) an example on LCLS injector:

22. Trapped trajectories in a slow wave If the phase velocity of the wave is ~c

23. A quarter synchrotron oscillation gives phase compression By Injecting at and extracting at we perform an energy spread enhancement associated to a phase spread reduction

24. Performances @ full compression I T

25. Transverse Dynamics of a laminar beam subject to RF Compression • Assuming a current growing at the same rate as the beam energy the envelope equation becomes • and the new (exact) solution is • with same plasma frequency asthe IE RF Compression Invariant Envelope No beam confin. without external focusing

26. Emittance Oscillations This gives rise to oscillations of the rms projected normalized emittance, that are dampedas for the IE, but of constant amplitude for the RFC-IE

27. LCLS Photoinjector with RF Compression 3 solenoids for additional focusing 2 solenoids for additional focusing

28. Three Conditions to preserve emittance while bunching • current growing at the same rate as the beam energy • additional external focusing to match onto a parallel envelope (I.E. RFC solution) • RF compressor accelerating section longer than a plasma wavelength (2-3 m) • Needs a dedicated well optimized lay-out (presently not available): motivation for SPARC project at LNF

29. SPARC Project @INFN-LNF Initial and final phase spectrum. C. Ronsivalle - PARMELA

30. C. Ronsivalle - PARMELA

31. Tolerances and Phase Jitters Introducing a random error (due to fabrication tolerances) in the iris-to-iris distance (0.1 % and 1 %)

32. undulators linac (off) linac 5 MeV 75 MeV 75 MeV DUVFEL dump dump RF Compression at DUVFEL (B. Graves & Ph. Piot) 1 pC

33. A variation to the RF-gun concept: the pulsed photodiode 2 MV HV 1 ns pulse on a 2 mm diode gap: 100 pC @ 100 fs bunch Bn=1.2.1015 M. Van der Wiel et al., T.U. Eindhoven,

34. Flat Beam Production In 1998 Y. Derbenev invented a linear optics device for transforming a beam with high ratio of horizontal to vertical emittance (flat beam) to one with a vortex motion (rigid rotor). After injecting such a beam into a matched solenoid this vortex motion can be canceled to create a magnetized beam with equal emittances in the transverse degrees of freedom (round beam) (UM-HE-98-04). In 1999 R. Brinkmann, K. Flottmann and Y. Derbenev proposed to reverse the process: obtain a flat beam from a round beam produced from a cathode in a magnetic field: A Flat Beam Source for Linear Colliders (TESLA-99-09) In 2000 H. Edwards and the A0 collaboration have seen the first flat beam from a photo-injector.

35. Vortex Motion

36. Clockwise rotation Identity matrix in x 90o phase advance in y D1 D2 Q3 Q1 Q2 By choosing b=1/k, particles end up with equal displacement and angles in x and y This is a flat beam 45o inclined. D. Edwards

37. Flat Electron Beam Production from A0 Photo-injector Round beam image on fluorescent screen Flat beam image on fluorescent screen Flat beam measurements Fermilab/NICADD Photoinjector Laboratory (FNPL): Demonstrated large emittance ratio (50:1) with small emittance 0.9 mm-mrad @ 1 nC ==> S. Lidia Talk Beam image through slits for emittance measurement

38. CONCLUSIONS • Lot of R&D ongoing on technical issues: High duty, quasi-CW operations, SC RF gun, higher frequencies ultra-high gradients (W-band) • Within a quite short time more experimental data will be available on RF compression (UCLA, LLNL, BNL, LNF) and Flat Beam Production (Fermilab, LBNL)