Short Electron Pulses from RF Photoinjectors Massimo Ferrario

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

Schematic View of the Envelope Equations (HOMDYN model) 2

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

Example of an optimized matching Matching onto the Local Emittance Max. , Final emittance = 0. 4 m M. Ferrario et al. , “HOMDYN Study For The LCLS RF Photo-Injector”, Proc. of the 2 nd ICFA Adv. Acc. Workshop on 4 “The Physics of High Brightness Beams”, UCLA, Nov. , 1999, also in SLAC-PUB-8400

Coherent Synchrotron Radiation in bending magnets coherent radiation for > zz e– L 0 R bend-plane emittance growth s E E < 0 overtaking length: L 0 (24 z. R 2)1/3 E E = 0 x x = R 16(s) E/E · Powerful radiation generates energy spread in bends · Energy spread breaks achromatic system · Causes bend-plane emittance growth (DESY experience) 5

Talk Outline Pulsed photodiodes Ballistic bunching Velocity bunching Bunch slicing 6

e- beam requirements Q = 20 -100 p. C z < �~ 250 m ==> z = 20 m x ~ 20 -30 m ==> nx < 5 m < 1 % ~ �~150 Me. V 7

Pulsed photodiode + femtoseconds laser 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 High gradient required ! L. Serafini, “The Short Bunch Blow-out Regimein RF Photoinjectors” 8

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

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

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Bullistic Bunching experiment at UCLA (Rosenzweig) 12

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Velocity bunching concept 15

Quarter wavelength synchrotron oscillation 16

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

Average current vs RF compressor phase OVERCOMPRESSION HIGH COMPRESSION MEDIUM COMPRESSION LOW COMPRESSION 18

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B. Spataro et al, PAC 05 ==> 20

<I> = 860 A nx = 1. 5 m C. Ronsivalle et al. , “Optimization of RF compressor in the SPARX injector”, 21 PAC 05

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To be published on JJAP 26

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

Stability of Velocity Bunching (-1 degree) 1. 1 psec 0. 5 psec 1. 4 psec 0. 9 psec 1. 1 psec 0. 8 psec 28 Streak images at injection phase of – 1 degree. Fluctuation is 0. 4 ps (rms) for 30 shots.

Current sensitivity for 1 degree error in the RF compressor phase with IV harmonic cavity D. Alesini, PAC 05 29

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Rectilinear Bunching Experiments Summary BNL UCLA BNL-DUVFEL UTNL-18 L LLNL Methode Ballistic Velocity Bunching Acc. Structure S-band PWT 4 S-band 1 S-band 4 S-band Measurement zerophasing method CTR zero-phasing method Femotsecond Streak Camera CTR Charge 0. 04 n. C 0. 2 n. C 1 n. C 0. 2 n. C Bunch width 0. 37 ps (rms) 0. 39 ps (rms) 0. 5 ps (rms) < 0. 3 ps Comp. Ratio 6 15 > 3 > 13 10 Solenoid field No No No Yes 31

==> D. Giulietti talk tomorrow 32

Exercise for this workshop Q = 20 p. C z = 200 m ==> < 25 m x =175 m ==> < 20 m = 0. 2% , nx < 0. 3 m 33

HOMDYN movie 34

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Bunch slicing Q = 1 n. C ==> 25 p. C Lb=10 ps ==> 100 fs x = 0. 5 mm ==> 5 m < 0. 2% 37 C. Vaccarezza et al. , EPAC_04

Conclusions 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 38

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