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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