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Ideas for BE.KFA14L1 Impedance I mprovement

Ideas for BE.KFA14L1 Impedance I mprovement. M.J. Barnes and L. Sermeus Acknowledgements: L.S. Ducimetière, L. Miguel Coralejo Feliciano. Motivation: Source of Horizontal Instability in the PSB. Outline of Presentation. Motivation for Impedance Improvement

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Ideas for BE.KFA14L1 Impedance I mprovement

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  1. Ideas for BE.KFA14L1 Impedance Improvement M.J. Barnes and L. Sermeus Acknowledgements: L.S. Ducimetière, L. Miguel Coralejo Feliciano IWG: 11/12/2018 (MJB)

  2. Motivation: Source of Horizontal Instability in the PSB Outline of Presentation • Motivation for Impedance Improvement • Existing KFA14L1 kicker system • Potential changes to kicker system and expected influence • Disclaimers: • Ideas presented here for changes to the KFA14L1 are initial ideas only and would need proper studies by both ABT and HSC; • There are no ABT resources presently available for major study; • Most important, before any studies commence in ABT, is there justification of the work on the basis of the performance? For example, if the transverse feedback supresses the instability, this removes any urgent motivation for modifying the KFA14L1 system; • The main purpose of giving this presentation is to ask the IWG if they see any possible impedance issues with these possible changes….. IWG: 11/12/2018 (MJB)

  3. Motivation: Source of Horizontal Instability in the PSB • Long-standing head-tail instability with its source being unknown for years • Cured in daily operation with transverse feedback system • With Linac4, could limit high intensity and constrain working point at 160 MeV injection Slide adapted from one by Eirini Koukovini Platia, following MD of 12/11/2018, for KFA14 Kicker in nominal configuration (R3) Modified kicker termination for MD (R3) No sign of instability! Modified Nominal configuration • Up to 100% losses for horizontal tunes between 4.22 and 4.30 on a 160 MeV plateau • Nowadays controllable with feedback ON • Kicker termination modified for MD –not usable in operation • No instability observed even with feedback OFF IWG: 11/12/2018 (MJB) The extraction kicker IS the impedance source of the instability!

  4. Existing KFA14 Kicker System (1 ring) ~90ns one way delay During MD of 12/11/2018, this filter was replaced by a 6.25Ω resistor Open to short delay: ~125ns+  λ(c)≈150m, and hence F1≈2MHz-(~1.8MHz measured 12/11/2018: see slide 13) IWG: 11/12/2018 (MJB)

  5. JPARC Approach, at 3GeV RCS, for kicker impedance reduction Thyratron Without diode-resistor Tx≈770ns With diode-resistor • Diode-resistor network : • degradation of current waveform rise time and flat-top; • A potential source of failure…. Tm≈95ns Short-circuit transmission line magnet Special diode-resistor network on thyratron end of cables IWG: 11/12/2018 (MJB)

  6. Measurements made at TRIUMF on a prototype magnet Tx≈192ns (2) But measurements demonstrated that the DISI was effective at suppressing some “low”-frequency (<45MHz) resonances for a short-circuit kicker magnet: Or replace by ground connection (short-circuit) Longitudinal Impedance (Ω) 7F1 5F1 (1) Original purpose of Displacement Current suppression Saturating Inductor (DISI) was to reduce 1% - 99% field rise time (measurement): With DISI: ~26ns  λ(c) =31m, hence F1(DISI)= 9.7MHz. Tm≈26ns 3F1 M.J. Barnes, G.D. Wait, “Effect of Saturating Ferrite on the Field in a Prototype Kicker Magnet”, proc. EPAC’94,London, June 27-July 1, 1994, pp2544-2546. F1 Without DISI: ~218ns  λ(c) =260m, hence F1= 1.1MHz. ~70ns H. J. Tran, M. J. Barnes, G. D. Wait, and Y. Yan, “Longitudinal impedance of a prototype kicker magnet system”, proc. PAC-1993, Washington, DC, 1993 (IEEE, New York, 1993), pp 3402-3404. Fig. Longitudinal impedance of short-circuited kicker magnet without and with a DISI (12cm2 ??) on its input. Note: for KFA14, a suitable DISI would move the first resonance to ~7MHz IWG: 11/12/2018 (MJB)

  7. But… PHYS. REV. ACCEL. BEAMS 21, 061003 (2018) - Plots Tx+TM≈ 770ns*+95ns*≈ 865ns λ(c)≈ 1040m, hence F1≈ 0.29MHz. TM ≈ 95ns * λ(c)≈ 114m, hence F1≈ 2.6MHz. 9F1 Significant (~x15) increase in magnitude F1 7F1 5F1 5F1 7F1 9F1 3F1 Significant (~x15) increase in magnitude F1 11F1 3F1 F1 FIG. 3. Theoretical results for the longitudinal (left) and horizontal (right) beam impedances for a relativistic beam, when the cable terminals are open. The red dashed and blue solid lines show the real and imaginary parts of the beam impedances, respectively. FIG. 4. Measurements of the horizontal beam impedances, where all cables are simply detached from the kicker. The red dashed and blue solid lines show the real and imaginary parts of the impedance. *J. Kamiya,T. Takayanagi, and M. Watanabe, “Performance of extraction kicker magnet in a rapid cycling synchrotron”, Physical Review Special Topics - Accelerators and Beams, 12, 072401 (2009), DOI: 10.1103/PhysRevSTAB.12.072401. IWG: 11/12/2018 (MJB)

  8. Drawing of KFA14 present connection box Cable connector IWG: 11/12/2018 (MJB)

  9. Drawing of KFA14 connection box with DISI Ferrite ring overall length to be minimised to fit the limited space (CMD5005 available: OD=60mm, ID=20mm, L=4x20mm shown  A=16cm2). Possible extension of cable connector IWG: 11/12/2018 (MJB)

  10. Scope of studies needed….. • HSC et al: can existing (upgraded) feedback cope with this resonance ? • For DISI: • Cross-sectional area of ferrite required? (need information from HSC for beam induced current/voltage in electrical circuit); • HSC expect significant benefit from shifting the dipolar impedance resonance to a higher frequency, i.e. 1.6MHz ~7MHz… But is ~x15+ increase in transverse impedance OK?; • Impedance experts need to confirm that it is OK (beneficial?) to shift the longitudinal impedance resonanceto higher frequency; • Influence of DISI on field ripple (a short, fat, DISI may be better for low ripple); • If DISI is to be used, should the new (existing design) tank still be installed during LS2? (postponing could save need to open SF6 connections twice) – what is the latest date for decision? • Space is very limited by KFA14L1 tank – integration studies are required; • …… • Other ideas for eliminating resonance at ~1.6MHz (suggestions???)…. • Timescale for studies/“fix”… • Follow-up framework: LIU-PSB beam dynamics meeting IWG: 11/12/2018 (MJB)

  11. Spare slides IWG: 11/12/2018 (MJB)

  12. PHYS. REV. ACCEL. BEAMS 21, 061003 (2018) - Plots IWG: 11/12/2018 (MJB)

  13. VNA measurement on KFA14.L1, R3, on 12/11/2018 Courtesy of C. Zannini IWG: 11/12/2018 (MJB)

  14. Other possible sources? • A narrow-band resonator impedance? • A line at this frequency had been singled out on a spectrum analyzer during previous instability measurements at the PSB (M. Chanel, C. Carli) • This value is also likely to be associated to the lowest resonance due to the unmatched terminations on the PSB ejection kickers G. Rumolo, MSWG 27/08/10 The beam coupling impedance due to unmatched terminations of the extraction kicker has been a long-lived suspect for the horizontal instability… IWG: 11/12/2018 (MJB) 27 of 30 E. Koukovini-Platia MSWG meeting #14, 28/09/2018

  15. Influence of DISI on flattop (predicted) G.D. Wait, M.J. Barnes, K.D. Metzmacher, L. Sermeus, “The Application of Saturating Inductors for Improving the Performance of the CERN PS Kicker Systems”, proc. of 17th Particle Accelerator Conf. (PAC’97), Vancouver, Canada, May 12-16, 1997, pp1328-1330. IWG: 11/12/2018 (MJB)

  16. Other possible sources? Miscellaneous Notes (1) • From minutes of 154th HSC section meeting (17/09/2018), https://indico.cern.ch/event/754245/: • The worst case is for Qh ~ 4.26, which is very close to the tune which we want to use for space charge considerations… Furthermore, during the first milliseconds the damper will not work well, so it is really a potential worry for LIU…A possibility could be to use a slightly higher horizontal tune during the first milliseconds. • From Eirini’s presentation at the MSWG of 28/09/2018 (Summary slide), https://indico.cern.ch/event/758054/: • Need to fully rely on the TFB from the very start of the cycle  what about the saturation of the Beam Orbit Signal Suppressor (BOSS) unit? IWG: 11/12/2018 (MJB)

  17. Other possible sources? Miscellaneous Notes (PSB) • Acceleration from 50MeV to 160MeV: Beta increases from 0.3 to 0.9. Bunch width reduces from ~1us to 180ns (4 sigma). • Critical tune is dependent upon revolution frequency; • Bunch width reduction with energy results in higher frequency content of pulse. IWG: 11/12/2018 (MJB)

  18. Summary • Experimental data have been collected and analyzed for different intensities, chromaticity and tune values at 160 MeV • A range of horizontal tunes between 4.21 and 4.30 causing detrimental beam loss has been identified when the TFB is off • Chromaticity increase, which is a common cure for head-tail instability, will not be useful for qh = 4.26, given the sextupole’s range • Need to fully rely on the TFB from the very start of the cycle  what about the saturation of the Beam Orbit Signal Suppressor (BOSS) unit? • Expected linear dependency of the rise time with intensity was found • The head-tail modes with chromaticity have been measured • Close to natural chromaticity a head-tail mode m = 3 is observed • As chromaticity increases, the head-tail mode increases to m = 13 • Higher-order modes showed slower rise times • The normal rf cavities C02, C04 and C16 have now been excluded as possible source of the instability • Other potential impedance candidate is the peaked impedance at 1.7 MHz from unmatched terminations on the extraction kicker cables IWG: 11/12/2018 (MJB) 29 of 30 E. Koukovini-Platia MSWG meeting #14, 28/09/2018

  19. Other possible sources? Measurements at TRIUMF Longitudinal Impedance (Ω) 7F1 5F1 With DISI: ~26ns  λ(c) =31m, hence F1(DISI)= 9.7MHz. 3F1 F1 Without DISI: ~218ns  λ(c) =260m, hence F1= 1.1MHz. Fig. Longitudinal impedance of terminated kicker magnet with speed-up networks (no DISI). Fig. Longitudinal impedance of short-circuited kicker magnet without and with a DISI (12cm2 ??) on its input. IWG: 11/12/2018 (MJB) H. J. Tran, M. J. Barnes, G. D. Wait, and Y. Yan, “Longitudinal impedance of a prototype kicker magnet system”, proc. PAC-1993, Washington, DC, 1993 (IEEE, New York, 1993), pp 3402-3404.

  20. IWG: 11/12/2018 (MJB)

  21. IWG: 11/12/2018 (MJB)

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