1. ATF Fast Kicker R&D at LBNL ILCDR06, Cornell University
Stefano De Santis
Center for Beam Physics
ILCDR06, Cornell University
September 26th, 2006
S. De Santis ILCDR Sept , 2006

2. Summary Motivation of the project and its main requirements
Calculation of kicker’s main parameters
Computer simulations of stripline electrodes
Conclusions
S. De Santis ILCDR Sept , 2006

3. The ATF as test-bench for ILC DR technologies
The challenge:
Developing a pulser capable of generating elevated voltages over few nanoseconds, with short rise and fall times and relatively high repetition rate.
Developing a kicker structure capable to efficiently transform the voltage pulse into a deflecting field, maintaining the pulse’s time structure and without introducing undesired beam impedance.
S. De Santis ILCDR Sept , 2006

4. Fast kicker’s main specifications
Deflection angle k: 5 mrad
Bunch spacing tb: 5.6/2.8 ns
Pulse repetition rate frep: 3 MHz
Total kicker length LT: m
Kicker field falls to <0.07% of max value before next bunch
S. De Santis ILCDR Sept , 2006

5. Pulser and bunch separationV tr td
tft=2Lk/c
bunch 0
bunch 1
bunch 2 tk
tb -tk
tk< [tb- max(tr, td)]/2
1.16 GHz
3 MHz
5.6 ns/3 MHz pulse train spectrum
FID pulsers:
- V up to 10’s of kV
- tr down to 50 ps ( typ.)
- tft up to 10’s of ns
- frep up to 100’s of kHz
… and kicker’s electrodes can only make things worse
S. De Santis ILCDR Sept , 2006

6. Calculation of stripline kicker’s main parameters (tb=5.6 ns)
Max. stripline length: ~84 cm
Lk= 65 cm (allows 2 modules in the 1.4 m sector)
Trying to minimize the impedance, we choose the plates separation h=24 mm ( same as beam pipe diameter), with a traditional angle of 120° (coverage factor g=0.93)
The required pulser voltage is
Vk/2 ≈ 21 kV
MHz kΩ
Rs ≈ 250 kΩ
and the shunt impedance
The peak power is
and the average power at 3 MHz is Pavg ≈ 100 W
S. De Santis ILCDR Sept , 2006

7. 2D Computer Simulation
Determination of the beam pipe radius required to obtain a 50  characteristic impedance for the striplines. We get an outer pipe radius of 22 mm, with a 120º coverage angle and a 0.5 mm thickness for the striplines.
even mode
odd mode
S. De Santis ILCDR Sept , 2006

8. 3D electromagnetic modelling
Detail of the mesh (with coax for impedance measurements)
Kicker module with feedthroughs and tapers as seen by Microwave Studio™
S. De Santis ILCDR Sept , 2006

9. S-parameters and stored energy
trailing bunch enters module
Excitation: tr=150 ps, tft=5.45 ns, td=300 ps
S. De Santis ILCDR Sept , 2006

10. Deflecting field along bunch orbit
Since the kicker is substantially shorter than half the bunch separation, it is possible to prefire the pulser, at the price of a slightly higher power dissipation, to ensure deflecting field uniformity along the bunch path.
bunch enters kicker
(avg. 695)
… at midlength
… leaves kicker
S. De Santis ILCDR Sept , 2006

11. Transverse field uniformity
variation < 1% over 5 mm
S. De Santis ILCDR Sept , 2006

12. Field decay
Average and rms values are of the order of Anyway we have to consider that this is a free oscillating field and things will get better
- Attenuation from ohmic losses
- Better feedthroughs
- Effects of different modules don’t add
trailing bunch at kicker’s midlength
S. De Santis ILCDR Sept , 2006

13. Longitudinal impedance and loss factor
We can simulate the classic coaxial wire measurement (Walling’s formula for distributed impedances )
Loss factor of the order of 0.1 V/pC
S. De Santis ILCDR Sept , 2006

14. Transfer impedance
We can simulate this measurement as well in order to evaluate the field levels on the feedthroughs, loads, etc induced by the circulating beam.
downstream
... and upstream port
S. De Santis ILCDR Sept , 2006

15. Shorter kicker electrodes 650 -> 350 mm
residual field
If power dissipation is not a concern and/or more powerful pulsers are available this could be a viable method for reducing perturbations on the trailing bunch
S. De Santis ILCDR Sept , 2006

16. Conclusions
For 5.6 ns bunch spacing the specifications seem to be attainable.
tb = 2.8 ns is much harder, due to (non) commercial availability of high voltage pulsers with appropriate rise and fall times.
The frep = 3 MHz is likely the hardest requirement on the pulsers in the 5.6 ns scenario (FID say they are being developed).
Shorter modules, if power figures allow, are beneficial, yet more expensive.
The effects of modules in different locations on the ring need to be investigated, as well as computer simulations at higher frequencies.
S. De Santis ILCDR Sept , 2006