f

Specifications for the dark current kicker for the Specifications for the dark current kicker for the NML test facility at FermilabNML test facility at Fermilab

S. Nagaitsev, M. Church, P. Piot, C.Y. Tan, J. Steimel

FermilabMay 31, 2007

f

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NML injector layoutNML 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 mmKicker (flange to flange): 300 mmKicker (mid. point) to collimator: ~ 800 mm

1 23

4

5

6

7

8

f Kicker functionsKicker functions

1. Dark current removal2. 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.

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Steerer, no kick

Beam Collimator

Steerer with kick

Beam Collimator

20 mrad

1.6 cm

f Beam parameters at the kicker (mid. Beam parameters at the kicker (mid. point)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

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f Injector simulation by P. PiotInjector simulation by P. Piot

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Kicker location

f SteererSteerer

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%

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f Kicker (prelim.)Kicker (prelim.)

Strip-line geometry Dist btw plates: 35 mm Effective length: 250 mm

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f Analytic (2-d) approximation for a stripline

kicker

Deflection angleDeflection angle

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φ

b

+V

-V

pcb

keVL2

k is the geometry factor, depends on b and φ; ~1-1.2 for our geometry. For L = 25 cm, b = 17.5 mmand θ = 20 mrad, the voltage ampl.is V = 3 kV

Pulse-to-pulse stability required

(rms): 0.1%

f Timing and Flattop RequirementsTiming 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

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0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1

em

itta

nc

e g

row

th [

%]

pulse flattop variation [%]

f Calculation of Attenuation of Dark Current by Calculation of Attenuation of Dark Current by Kicker and CollimatorKicker and Collimator

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Relative amount of dark current survivingdistance from cathode [m]

No collimator 2 cm dia. coll. 1 cm dia. coll.2 cm coll. +

20mrad kicker1 cm coll. +

20mrad kicker@ cathode 0.000 1.119 1.119 1.119 1.119 1.119

exit gun cavity 0.190 1.039 1.039 1.039 1.039 1.039exit gun snout 0.525 1.000 1.000 1.000 1.000 1.000d.s. of kicker 0.825 0.996 0.996 0.996 0.945 0.945u.s. of coll. 1.400 0.850 0.850 0.850 0.558 0.558d.s. of coll 1.600 0.775 0.476 0.270 0.031 0.006

u.s. of CC-1 2.200 0.570 0.442 0.235 0.029 0.006d.s. of CC-1 3.500 0.443 0.421 0.228 0.021 0.005

power into collimator for 1 mA @ z=.525 [W]

5.6 8.8 7.4 8.3

0.00000

0.20000

0.40000

0.60000

0.80000

1.00000

1.20000

RF

Han

gaussian

RF phase (deg)

Time distribution of dark current at the photocathode (yellow curve)

f Transverse Wake from a Round Transverse Wake from a Round CollimatorCollimator

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1.0 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 < .1y jitter<20rad.

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.