f Specifications for the dark current kicker for the NML test facility at Fermilab S. Nagaitsev, M. Church, P. Piot, C.Y. Tan, J. Steimel Fermilab May 31, 2007

f S. Nagaitsev 2 NML injector layout 1 – Gun cavity, 2 – Coaxial rf coupler, 3 – dark current kicker and air-core steerers, 4 – laser mirror cross, 5 – current monitor & BPM, 6 - collimator, 7 – Tesla cavity (CC1), 8 – Tesla cavity (CC2) Dimensions: Cathode to kicker(mid. point ): ~ 750 mm Kicker (flange to flange): 300 mm Kicker (mid. point) to collimator: ~ 800 mm

f Kicker functions 1.Dark current removal 2.Fast beam inhibit  A fast kicker is combined with a dc air-core steering coil, which normally deflects the beam vertically down onto a collimator. The kicker pulse allows to pass the bunch through the hole in the collimator. S. Nagaitsev 3 Steerer, no kick Beam Collimator Steerer with kick BeamCollimator 20 mrad 1.6 cm

f Beam parameters at the kicker (mid. point)  For a 3.2 nC bunch  Beam energy: pc = 5 MeV  Emittance (norm, rms): 6 µm  Beam size: ~3 mm  Uncorrelated beam angular spread: 0.2 mrad S. Nagaitsev 4

f Injector simulation by P. Piot S. Nagaitsev 5 Kicker location

f Steerer  The steerer needs to deflect a 5-MeV/c beam by 40 mrad (max), 20 mrad (nominal)  Coil length: 20 cm  B = 35 G  Pulse-to-pulse stability (rms): better than 0.1% S. Nagaitsev 6

f Kicker (prelim.)  Strip-line geometry  Dist btw plates: 35 mm  Effective length: 250 mm S. Nagaitsev 7

f  Analytic (2-d) approximation for a stripline kicker Deflection angle S. Nagaitsev 8 φ b +V -V k is the geometry factor, depends on b and φ; ~1-1.2 for our geometry. For L = 25 cm, b = 17.5 mm and θ = 20 mrad, the voltage ampl. is V = 3 kV Pulse-to-pulse stability required (rms): 0.1%

f Timing and Flattop Requirements  Flattop width: 40 ps or more  Flattop variation: < ± 0.6% over ± 20 psec for emittance growth < 10%  Rate: 3 MHz  Number of pulses in a macropulse: up to 3000  Macropulse rate: 5 Hz S. Nagaitsev 9

f Calculation of Attenuation of Dark Current by Kicker and Collimator S. Nagaitsev 10 RF phase (deg) Time distribution of dark current at the photocathode (yellow curve)

f Transverse Wake from a Round Collimator S. Nagaitsev cm dia: peak wake = 750 V/pC/m 1.2 cm dia: peak wake = 520 V/pC/m 1.4 cm dia: peak wake = 370 V/pC/m 1.6 cm dia: peak wake = 270 V/pC/m 1.8 cm dia: peak wake = 200 V/pc/m 2.0 cm dia: peak wake = 160 V/pC/m Calculated with program ABCI Requirement: Limit transverse jitter of downstream beam to <.1  y    jitter <20  rad. Using average wake field ~ ½ peak wake field; Q=3.2 nC; P=4.7 MeV/c 2 cm dia. collimator: amplifies kicker jitter by 4%; collimator position must be stable to < 370  m 1 cm dia. collimator: amplifies kicker jitter by 20%; collimator position must be stable to < 80  m Requirement: Limit emittance blowup of downstream beam to < 10%. emittance increase from offset collimator adds in quadrature with initial emittance  collimator should be centered on beam to within 1 mm (for 1 cm dia. collimator) Beam travelling off-center through a round collimator (iris), will experience a transverse kick due to the wake field. Position jitter at the collimator due to kicker jitter will be amplified by the collimator.