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SPS: Instabilities and impedance model in the longitudinal plane

SPS: Instabilities and impedance model in the longitudinal plane. Jose E. Varela on behalf of: T. Argyropoulos, T. Bohl, F. Caspers, A. Lasheen, B. Salvant, E. Shaposhnikova , H. Timko, C. Zannini. LIU Day, 11 th of April 2014, J.E. Varela. Multi-bunch instabilities (I).

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SPS: Instabilities and impedance model in the longitudinal plane

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  1. SPS: Instabilities and impedance model in the longitudinal plane Jose E. Varela on behalf of: T. Argyropoulos, T. Bohl, F. Caspers, A. Lasheen, B. Salvant, E. Shaposhnikova, H. Timko, C. Zannini LIU Day, 11th of April 2014, J.E. Varela

  2. Multi-bunch instabilities (I) • Instability threshold Nth~ 1/energy • Multi-bunch instability threshold during the rampfor bunches spaced at is much lower than for a single bunch • Similar Nth for 1, 2, 3 or 4 batches • No interactions between batches spaced by • Wake decay time (Q-factor) Average bunch length along cycle (4 batches, 50 ns spacing, 1 RF, Q26)

  3. Multi-bunch instabilities (II) 25 ns – Energy threshold 50 ns – Energy threshold • Energy threshold approximately scales with total current as • Emittance at flat top scales with bunch intensity ( + single bunch effect ? ) • 50 ns : with • 25 ns : with

  4. Single bunch instability • Bunch lengthening with intensity • To explain this bunch lengthening by potential well distortion one needs Im(Z)/n = 12 - 15Ω • Microwave instability due to some high frequency impedances? Bunch length at flat top

  5. High frequency impedance • Amplitude peaks correspond to SPS resonant impedances (on the top of 200 MHz harmonics) • Strong peak at 1.4 GHz for (observed also in 2001) • Reproduced in simulations with SPS impedance model impedances with high R/Q and Q in wide range Measured and simulated unstable spectrum of 25 ns long bunches with N (RF off) ?

  6. Vacuum flanges simulations and measurements • The vacuum flanges have been identified as sources of the 1.4GHz impedance peak A layout survey was carried out to count and classify vacuum flanges in the SPS. Many thanks to: Antoine Boucherie Jose A. Ferreira Sebastien Calvo Eric Montesinos

  7. The longitudinal impedance model (I) • Main known longitudinal impedance sources in the SPS* Travelling Wave Cavities BPM - B. Salvant * Approximated by resonant impedances + More details on the current longitudinal impedance model can be found in [ LIU-SPS BD WG, 27/03/2014 meeting ] Kickers - C. Zannini

  8. The longitudinal impedance model (II) • Current longitudinal impedance model in linear and logarithmic scale 200 MHz TWC 200MHz TWC HOM 800 MHz TWC Kickers 1.4 GHz flange resonance Serigraphy peak

  9. Estimation of the inductive impedance(measurements and simulations) • Quadrupole synchrotron frequency was measured from bunch length oscillations after injection of a single bunch into mismatched voltage. • Reduction of impedance observed with serigraphed MKEs (slope was reduced from for 2.2 ns ) Slopes as a function of bunch length Synchrotron frequency shift (εl = 0.1 eVs) Simulations (Hz) Slopes () 1Ω Measurements • Parabolic bunch • Gaussian bunch • Measurements • Estimation of effective Im(Z)/nby comparison with simulations (based on SPS model) • There is a strong dependence on bunch length => at 2.2 ns? • Still missing some impedance ( -1.5Ω for space charge ) • Measurements at higher energies. (ns)

  10. Conclusions • Instabilities are limiting the SPS performance • Impedance reduction would help to reach HL-LHC goals • More simulations and beam measurements to confirm the guilty impedance sources • From beam measurements, current impedance model is not complete • Travelling wave cavities – Measurements of the fundamental and higher order modes. • Other elements: • vacuum valves • unshielded pumping ports • …

  11. First thoughts on 1.4 GHz impedance reduction Damping the resonances does not help for single bunch instabilities since R/Q remains approximately constant. Any R/Q reduction strategy will imply opening 450 vacuum flanges… Shielding MBA / QF Bellows Current situation

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