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ATF Fast Kicker R&D at LBNL
Stefano De Santis
Center for Beam Physics
ILCDR06, Cornell University
September 26th, 2006
S. De Santis
ILCDR06
Sept. 26-28, 2006
Summary
• Motivation of the project and its main
requirements
• Calculation of kicker’s main parameters
• Computer simulations of stripline
electrodes
• Conclusions
S. De Santis
ILCDR06
Sept. 26-28, 2006
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
ILCDR06
Sept. 26-28, 2006
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: 1.4 + 0.8 m
• Kicker field falls to <0.07% of max value
before next bunch
S. De Santis
ILCDR06
Sept. 26-28, 2006
Pulser and bunch separation
bunch 0
tk
bunch 1
bunch 2
tb -tk
tk< [tb- max(tr, td)]/2
FID pulsers:
- V up to 10’s of kV
- tr down to 50 ps (100-200 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
V
ILCDR06
Sept. 26-28, 2006
tr
tft=2Lk/c td
3 MHz
1.16 GHz
5.6 ns/3 MHz pulse train spectrum
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 Eb / e  h

k / 3
2 4  Lk  g
Vk/2 ≈ 21 kV
2
Vk2
 7 kW
The peak power is Pk 
Rs
and the average power at 3 MHz is Pavg ≈ 100 W
S. De Santis
ILCDR06
Sept. 26-28, 2006
Rs ≈ 250 kΩ
kΩ
and the shunt impedance
 2g 
 sin 2 (k  Lk )
R
s  2Zc 
k  h 
MHz
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
ILCDR06
Sept. 26-28, 2006
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
ILCDR06
Sept. 26-28, 2006
S-parameters and stored energy
trailing bunch enters module
Excitation: tr=150 ps, tft=5.45 ns, td=300 ps
S. De Santis
ILCDR06
Sept. 26-28, 2006
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)
S. De Santis
ILCDR06
Sept. 26-28, 2006
… at midlength
… leaves kicker
Transverse field uniformity
variation < 1% over 5 mm
S. De Santis
ILCDR06
Sept. 26-28, 2006
Field decay
Average and rms values are of the order
of 5 10-3. 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
ILCDR06
Sept. 26-28, 2006
Longitudinal impedance and loss factor
We can simulate the classic coaxial wire measurement (Walling’s formula for
dut
ref
/ S21 ) )
distributed impedances Z//  2Zc ln( S21
Loss factor of the order of 0.1 V/pC
S. De Santis
ILCDR06
Sept. 26-28, 2006
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.
Z xfr 
downstream
S. De Santis
ILCDR06
Sept. 26-28, 2006
Vp
Ib
 Zc
Sp1
S21
... and upstream port
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
ILCDR06
Sept. 26-28, 2006
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
ILCDR06
Sept. 26-28, 2006