1 Short Electron Pulses from RF Photoinjectors Massimo Ferrario INFN - LNF

2 Schematic View of the Envelope Equations (HOMDYN model)

3 Emittance Compensation: Controlled Damping of Plasma Oscillation Brillouin Flow Hokuto Iijima L. Serafini, J. B. Rosenzweig, Phys. Rev. E 55 (1997) 100 A ==> 150 MeV

4 Final emittance = 0.4  m Matching onto the Local Emittance Max., Example of an optimized matching M. Ferrario et al., “HOMDYN Study For The LCLS RF Photo-Injector ”, Proc. of the 2 nd ICFA Adv. Acc. Workshop on “The Physics of High Brightness Beams”, UCLA, Nov., 1999, also in SLAC-PUB-8400

5 Coherent Synchrotron Radiation in bending magnets  Powerful radiation generates energy spread in bends  Causes bend-plane emittance growth (DESY experience)  Energy spread breaks achromatic system   x = R 16 (s)  E/E bend-plane emittance growth e–e–e–e– R zzzz coherent radiation for   z overtaking length: L 0  (24  z R 2 ) 1/3  s s xx xx  L0L0L0L0

6 Pulsed photodiodes Ballistic bunching Velocity bunching Bunch slicing Talk Outline

7 Q = pC  z  z = 20  m  x ~  m ==>  nx < 5  m  < 1 %  ~   ~150 MeV e - beam requirements

8 maximum amount of charge that can be extracted from a photocathode illuminated by a laser the induced rms energy spread on the electron bunch: the actual beam current at the gun exit will be almost independent on the initial peak current L. Serafini, “The Short Bunch Blow-out Regimein RF Photoinjectors” Pulsed photodiode + femtoseconds laser High gradient required !

9 2 MV HV 1 ns pulse on a 2 mm diode gap: 1 GV/m, 100 pC ==> 200 fs bunch,

10 Provide a correlated energy spread enough to produce, in a drift of length L drift a path difference equal to half the bunch length L o Bullistic Bunching

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12 Bullistic Bunching experiment at UCLA (Rosenzweig)

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15 Velocity bunching concept

16 Quarter wavelength synchrotron oscillation

17 Limitation: longitudinal emitance growth induced by RF non-linearities

18 Average current vs RF compressor phase LOW COMPRESSION MEDIUM COMPRESSION HIGH COMPRESSION OVER- COMPRESSION

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20 B. Spataro et al, PAC05 ==>

21 = 860 A = 860 A  nx = 1.5  m C. Ronsivalle et al., “Optimization of RF compressor in the SPARX injector”, PAC05

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26 To be published on JJAP

27 Streak Images of Electron Bunch Injected Phase -70 O Minimum! 200 psec range 50 psec range Injected Phase -1 O

28 Stability of Velocity Bunching (-1 degree) Streak images at injection phase of –1 degree. Fluctuation is 0.4 ps (rms) for 30 shots. 1.1 psec1.4 psec0.9 psec 0.5 psec1.1 psec0.8 psec

29 Current sensitivity for 1 degree error in the RF compressor phase with IV harmonic cavity D. Alesini, PAC05

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31 Rectilinear Bunching Experiments Summary BNLUCLABNL-DUVFELUTNL-18LLLNL MethodeBallistic Velocity Bunching Acc. Structure S-bandPWT4 S-band 1 S-band 4 S-band Measurement zero- phasing method CTR zero-phasing method Femotsecond Streak Camera CTR Charge0.04 nC0.2 nC 1 nC0.2 nC Bunch width 0.37 ps (rms) 0.39 ps (rms) 0.5 ps (rms) 0.5 ps (rms) < 0.3 ps Comp. Ratio 615> 3> Solenoid field No Yes

32 ==> D. Giulietti talk tomorrow

33 Exercise for this workshop  z = 200  m ==> < 25  m  x =175  m ==> < 20  m  = 0.2%,  nx < 0.3  m Q = 20 pC

34 HOMDYN movie

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37 C. Vaccarezza et al., EPAC_04 Bunch slicing Q = 1nC ==> 25pC L b =10 ps ==> 100 fs  x = 0.5 mm ==> 5  m  < 0.2%

38 Short pulses delivered by RF photoinjectors could meet the plasma acceleretor requirements Within a quite short time more experimental data will be available on RF compression in optimized layout Conclusions

39 Physics and Applications of High Brightness Electron Beams Organizers: L. Palumbo (Univ. Roma), J. Rosenzweig (UCLA), L. Serafini (INFN-Milano).