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M. Barnes CERN TE/ ABT Contributions by J. Holma (CERN)

An Inductive Adder as a Low-Jitter, Ultra-Flat, DR Extraction Pulser. M. Barnes CERN TE/ ABT Contributions by J. Holma (CERN). Overview. Specifications for kickers for CLIC Damping Ring (DR) Extraction Kickers Striplines Low beam coupling impedance Excellent field homogeneity

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M. Barnes CERN TE/ ABT Contributions by J. Holma (CERN)

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  1. An Inductive Adder as a Low-Jitter, Ultra-Flat, DR Extraction Pulser M. Barnes CERN TE/ABT Contributions by J. Holma (CERN) LεR2011, October 3-5, 2011

  2. Overview • Specifications for kickers for CLIC Damping Ring (DR) Extraction Kickers • Striplines • Low beam coupling impedance • Excellent field homogeneity • Pulse generator • Schematic of an Inductive Adder • Modulation schemes • Status of R&D • Measurement challenges • Summary LεR2011, October 3-5, 2011

  3. CLIC General Layout Phase measurement TA kicker Loop phase compensation kicker Dump Dump DR Extraction Kicker kicker • A total of approximately 300 kickers will be required for CLIC ! ; • Damping Rings: one injection and extraction system per ring and per beam (8 kicker systems); • Damping rings reduce beam emittance; hence kickers must be high stability (low ripple and droop) with excellent integrated field homogeneity and very low beam coupling impedance. LεR2011, October 3-5, 2011

  4. DR Kickers: Selected CLIC, ILC & DAΦNE Parameters LεR2011, October 3-5, 2011

  5. Taken from: D. Alseni, LNF-INFN, “Fast RF Kicker Design”, April 23-25, 2008. Ceramic Support Elliptical cross-section (increases deflection efficiency). Feedthru Beam DAΦNE Striplines (~0.9m) Beam-Line Kicker Element The specifications for the pre-damping rings and the damping rings include: • low longitudinal and transverse beam coupling impedances; • high stability and reproducibility of the field; • excellent field homogeneity; • ultra-high vacuum. • Stripline structures will be used for the beam-line kicker element; • IFIC, in conjunction with CIEMAT & CERN, are carrying out a complete optimization of the design of the DR striplines (C. Belver-Aguilar: R&D on Striplines for the CLIC DR Kickers, LER2011); • Spanish Industry (TRINOS) will produce manufacturing drawings and a set of prototype DR striplines; • The striplines will be supplied with suitable high voltage vacuum feedthroughs. Note: each taper ≈ 30% of overall length. LεR2011, October 3-5, 2011

  6. CLIC Damping Ring Pulse Definition • Rise time: time needed to reach the required flattop voltage (but includes settling time). DR extraction 1000 ns rise time allowed, ~100 ns desired; • Settling time: time needed to damp oscillations to within specification; • Beam: 160 ns time window during which any ripple and droop (i.e. flattop stability) must be within specification; • Flattop stability: within ±2x10-4, for combined ripple and droop for DR extraction. This corresponds to a maximum, combined, ripple and droop of ±2.5 V for a 12.5 kV output pulse for the DR extraction kicker; • Reproducibility: maximum difference allowed between any two pulses, of ±1x10-4; • Fall time: time for voltage to return to zero. DR ext.  1000 ns allowed, ~100 ns desired. • Minimizing rise and fall times reduces stress on kicker system. • To minimize settling time, impedance of system has to be well matched. LεR2011, October 3-5, 2011

  7. DR Kicker with an Inductive Adder Beam upstream end of striplines Schematic of CLIC DR Kicker System with an Inductive Adder • An extensive literature review of existing pulse generators has been carried out: an inductive adder is a very promising means of achieving the demanding specifications for the DR extraction kickers. • Two of the challenges of DR kickers • Impedance matching of ALL parts/components over a wide frequency range (… striplines are particularly challenging); • Stability of ALL parts/components (with time, temperature, ….). Capacitor MOSFETor IGBTs Gate Driver ½ Layer of an Inductive Adder Inductive Adder of 14 Layers LεR2011, October 3-5, 2011

  8. Inductive Adder (IA) Schematic of an Inductive Adder • Inductive Adder • Semiconductor switches; • Gate-drive circuit referenced to ground - no electronics referenced to high voltage despite the high voltage output pulse of the Inductive Adder (IA); • Modularity: • the same design can potentially be used for DR and PDR kickers despite the different specifications. However the PDR version will require more layers in series; • the possibility to generate positive or negative output pulses with the same adder: the polarity of the pulse can be changed by grounding the other end of the secondary winding of the IA; • Source impedance is low, hence minimizing the number of layers required; • Output voltage can be modulated during the pulse; • Redundancy and machine safety: if one switch or layer fails, the adder still gives a significant portion of the required output pulse. Analogue Modulation Layer Gate Drive Circuit Digital Modulation Layer Constant Voltage Layers (N-2) layers LεR2011, October 3-5, 2011

  9. Principle of IA Modulation Layers No capacitor here Linear switch • PROBLEM: During output current pulse charge is removed from capacitor banks, hence capacitor bank voltage reduces, causing droop of output pulse. • ANALOGUE MODULATION: layer is used to compensate voltage droop of capacitors, and can also significantly reduce the required capacitance per layer. • No energy storage capacitor on modulation layer, but there is a resistor Ra in parallel with the transformer core (magnetizing inductance Lm); • Resistor Ra is effectively in series with the load; • PASSIVE MODULATION: during the pulse, current through Lm increases, hence current through Ra decreases (τ=Lm/Ra). Therefore, voltage over Ra decreases, compensating voltage droop caused by storage capacitor voltage droop of other layers. • ACTIVE MODULATION: alinear switch provides a shunt path for the current through resistor Ra. Therefore, the voltage over Ra can be controlled by controlling the current through the switch. • DIGITAL MODULATION: switching “On” and “Off” provides coarse modulation and hence will not be used for droop compensation. However turn-on of layers at different times may be used to reduce ripple. ic LεR2011, October 3-5, 2011

  10. Predictions for Various Modulation Schemes • Without modulation 320 µF, per layer, is required to achieve 0.02% droop over 160 ns. • An analogue modulating layer is very effective at controlling droop, even with a relatively small value of the capacitor banks; • Critical design issues include: • low inductance for capacitor bank circuit, • small parasitic capacitances, • impedance matching (e.g. of pulse transformer) to minimize reflections • temperature stability of magnetic characteristics of transformer cores and careful choice of material; • For tape-would transformer cores, adequate interlaminar insulation; • etc.. • For a prototype inductive • adder it is proposed to use • ~80 µF, per layer. 20µF - am 160ns 40µF- pm 320µF 20µF - pm 40µF 20µF PSpice simulations of the effect of the value of the capacitance per layer upon the flattop droop, with: no modulation, active analogue modulation (am); passive analogue modulation (pm). Ripple may still be present at a level above ±0.02%: hence, a double kicker system may be required as well….. this will be better known once a prototype DR ext. system is tested. LεR2011, October 3-5, 2011

  11. Status of the IA Design • IA adder expert (Ed Cook – now “retired” from LLNL) invited to collaborate in design and lab testing; • Specifications have been defined for the main IA components, based on existing applications, discussions with Ed Cook, and simulations; • Simulations on-going: • Compensation of droop and ripple using analogue modulation; • Samples of main components ordered: • Storage capacitors, MOSFETs (switch-type and linear devices) and transformer cores; • Components will be tested starting autumn 2011 and the most suitable candidates will be chosen for the prototype; • The first prototype layers are scheduled to be ready for lab testing in the first quarter of 2012; • The goal is that one or more prototype adder(s) will be ready by the end of year 2012; • The prototype kicker system will be tested in a suitable facility, e.g. at ATF. Sample components: transformer cores, capacitors, MOSFETs and their gate driver circuits. LεR2011, October 3-5, 2011

  12. Measurement Challenges • The ±0.02 % requirement for the pulse flattop stability, for the DR extraction kicker, is an extremely demanding specification from both the design and measurement perspective. • Commercially available Current Transformers (CT) are promising for the measurement device, e.g.: • Short coaxial cables, terminated in their characteristic impedance, to minimize attenuation, dispersion and reflections. • But commercially available oscilloscopes are not capable of measuring shape of pulse flattop to required (relative) accuracy, e.g. because of ~1% amplifier droop over 200 ns. • High speed, 14-bit, Analogue to Digital converter to be investigated… • Pulse measurement experts contacted (e.g. Technical Research Institute of Sweden and ETH Zürich) and discussions commenced. • To confirm effect of analogue modulation by lab measurements (e.g. for droop compensation), two systems, with output currents in opposite sense though a CT (~zero total field without modulation), will probably be required. • Se • Ipeak > 250 A; • Droop over 160 ns < 0.002%; • Rise time < 10 ns, hence measurement ripple introduced by CT is expected to be insignificant after 100 ns target rise-time. +I or −I I LεR2011, October 3-5, 2011 I

  13. Summary • The DR extraction kickers are particularly challenging as they require: • low longitudinal and transverse beam coupling impedances; • good integrated field homogeneity; • excellent stability (±0.02 % requirement for the pulse flattop stability). • An Inductive Adder is promising for both the PDR & DR kickers: • for achieving a reliable design: multiple layers are good for machine protection; • for achieving adequately low ripple and droop – good predictions with a circuit model of active analogue modulation. • Progress and plans: • samples of main components ordered, to start testing in autumn 2011; • a prototype layer scheduled to be ready for testing in the first quarter of 2012; • discussions ongoing with experts and potential collaborators re IA design; • experts and potential collaborators have been contacted re measurement issues; • the prototype kicker system (pulse generator and striplines) will be tested in a suitable facility, e.g. ATF. LεR2011, October 3-5, 2011

  14. Thank you for your attention.QUESTIONS ? LεR2011, October 3-5, 2011

  15. DoubleKickerSystem: Concept (Extraction) Extraction with two kicker magnets: • Two “identical” pulses are required; • One power supply sends the pulses to 2 “identical” kickers. • 1st kicker system for beam extraction; • 2nd kicker system for compensation of jitter of deflection angle (ripple & droop) from 1st kicker; • Figure shows 1st and 2nd kickers separated by a betatron phase of 2nπ: for a betatron phase of (2n−1)π the 2nd kick is in the other direction. (Anti-Kicker) Time of flight Extraction with one kicker magnet: • Requires a uniform and stable magnetic field pulse. (Kicker) KEK/ATF achieved a factor of 3.3 reduction in kick jitter angle, with respect to a single kicker, with single-bunch measurements. LεR2011, October 3-5, 2011

  16. Example of Double Kicker System for DR Extraction Beam Time-of-Flight compensation. • 1stkicker system (in damping ring) for beam extraction; • 2nd kicker system (in extraction line) for jitter compensation. • In order that beam bunches and kicker field are synchronized in time at the 2nd kicker system, the two kicker systems are powered in parallel. However, additional lengths of transmission line are required to compensate for the beam-of-flight between the 1st and the 2nd kickers. • Potential problems • Different attenuation & dispersion of stripline waveforms (due to length of transmission lines); • Differences between magnetic characteristics of kicker & anti-kicker; • Imperfections in beam-line elements/alignment between kicker & anti-kicker. LεR2011, October 3-5, 2011

  17. Virtual Ground +/-ve +ve -ve Stripline Design: Longitudinal Impedance Longitudinal beam coupling impedance for untapered (Chao) and tapered stripline kicker (S. Smith, SLAC): Without dielectric or magnetic materials: • Striplines driven to same magnitude, but opposite polarity, voltage, to extract beam •  ODD mode characteristic impedance. • Total capacitance (C) is given by: • capacitance between a stripline and virtual ground (C11) • capacitance between a stripline and beam-pipe ground (2C12) • Same polarity and magnitude of current / voltage induced on both striplines by beam. •  EVEN mode characteristic impedance • Capacitance (C) is given by: • capacitance between a stripline and beam-pipe ground (C11) Beam pipe Ground Beam pipe Ground C11 C11 2C12 2C12 C11 C11 Beam +/-ve LεR2011, October 3-5, 2011

  18. Stripline Design: Field Homogeneity 20 mm • CLIC DR specifications: Field inhomogeneity: • ±0.1 % (1e-3) for DR injection (over 3.5 mm radius); • ±0.01 % (1e-4) for DR extraction (over 1 mm radius). 7 mm STRIPLINE STRIPLINE 2 mm • Contour plots of field inhomogeneity in the kicker aperture for the optimized design • Sensitivity of field homogeneity to parameter variations from the optimized design (Courtesy of C. Belver-Aguilar) LεR2011, October 3-5, 2011

  19. Other Issues • If only one of the two striplines is powered, beam will receive ~1/2 deflection; high intensity beam could cause considerable damage to other equipment. This could result if a “single” switch were used for each stripline: an inductive adder (multiple primary switches) will help to avoid this problem. • Fast rise and fall times of field are desirable; e.g. if beam is mis-timed, with respect to the kick pulse, a fast rise/fall time will result in beam being swept faster across downstream materials/devices, minimizing potential damage. • Se LεR2011, October 3-5, 2011

  20. Deflection due to Electric Field: Strip-line at positive voltage Fe From CTF3 CR Beam (e-) To CLEX Strip-line at negative voltage B B B B B B Deflection due to Magnetic Field: Strip-lines fed from CLEX end I +V Fm From CTF3 CR Beam (e-) To CLEX -V I Tail Clipper: Deflection LεR2011, October 3-5, 2011

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