# Presentation: 407879

## 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

## NML injector layout

-  S. Nagaitsev

- 	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

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## 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.

-  S. Nagaitsev

- Steerer, no kick

- Beam

- Collimator

- Steerer with kick

- Beam

- Collimator

- 20 mrad

- 1.6 cm

## 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

## Injector simulation by P. Piot

-  S. Nagaitsev

- Kicker location

## 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

## Kicker (prelim.)

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

-  S. Nagaitsev

## Deflection angle

-  S. Nagaitsev

*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%

## 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

## Calculation of Attenuation of Dark Current by Kicker and Collimator

-  S. Nagaitsev

**RF phase (deg)**

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

## Transverse Wake from a Round Collimator

-  S. Nagaitsev

**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 < .1********y************  ********jitter****<20********rad.**
**Using average wake field ~ 1⁄2 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.**