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

1
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

2
NML injector layout
6
4
8
1
2
5
3
7

• 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

3
Kicker functions

• Dark current removal
• 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.

Collimator
Beam
20 mrad
1.6 cm
Steerer, no kick
Beam
Collimator
Steerer with kick
4
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

5
Injector simulation by P. Piot
Kicker location
6
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

7
Kicker (prelim.)

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

8
Deflection angle

• Analytic (2-d) approximation for a stripline
  kicker

V
f
b
k is the geometry factor, depends on b and f
1-1.2 for our geometry. For L 25 cm, b
17.5 mm and ? 20 mrad, the voltage ampl. is V
3 kV Pulse-to-pulse stability required (rms)
0.1
-V
9
Timing and Flattop Requirements

• Flattop width 40 ps or more
• Flattop variation lt 0.6 over 20 psec for
  emittance growth lt 10
• Rate 3 MHz
• Number of pulses in a macropulse up to 3000
• Macropulse rate 5 Hz

10
Calculation of Attenuation of Dark Current by
Kicker and Collimator
Time distribution of dark current at the
photocathode (yellow curve)
RF phase (deg)
S. Nagaitsev
10
11
Transverse Wake from a Round Collimator
Calculated with program ABCI
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.
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
Requirement Limit transverse jitter of
downstream beam to lt .1sy ? Dqjitterlt20mrad. Usi
ng average wake field ½ peak wake field Q3.2
nC P4.7 MeV/c 2 cm dia. collimator
amplifies kicker jitter by 4 collimator
position must be stable to lt 370 mm 1 cm dia.
collimator amplifies kicker jitter by 20
collimator position must be stable to lt 80
mm Requirement Limit emittance blowup of
downstream beam to lt 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)
S. Nagaitsev
11