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HEADTAIL simulation during the accelerating ramp in the PS

HEADTAIL simulation during the accelerating ramp in the PS. S. Aumon - EPFL & CERN Acknowledgement to B. Salvant , G. Rumolo. Instability believed to be an TMCI Instability during the acceleration around transition

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HEADTAIL simulation during the accelerating ramp in the PS

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  1. HEADTAIL simulation during the accelerating ramp in the PS S. Aumon - EPFL & CERN Acknowledgement to B. Salvant, G. Rumolo

  2. Instability believed to be an TMCI Instability during the accelerationaround transition Instrumentation: wide band pickup, band width 2.5MHz-1GHz(See the “A 1.5 GHz wide-band beam position and intensity monitor for the electron-positron accumulator (EPA)”) 10MHz RF cavity height 145kV instead of 200kV (ToF beam) TMC Instability measurements without gamma jump Vertical Longitudinal

  3. TMC Instability measurements without gamma jump: chromaticity settings Obtained? The chromaticity is not measurable around 15 ms both side of transition Programmed

  4. HEADTAIL code Broad band impedance model Flat chamber No longitudinal and transverse space charge Acceleration No higher orders in momentum compaction. Chromaticity as in measurements 1 kick per turn Instability simulations

  5. TMCI simulation without chromaticity • Benchmark the rise time of the instability with/without chromaticity • Systematic offset of 30 turns in the measurements compared to the simulations

  6. TMCI simulation without chromaticity • Set a broad band impedance model in HEADTAIL • Rs=1.4MOhm/m, fr=1 GHz, Q=1 • No coupling, no resistive wall (length set to 0) • Vertical chromaticity set to 0 Measurements HEADTAIL

  7. TMCI simulation without chromaticity Before transition 400 turns before Before transition 1000 turns before @transition 200 turns after transition

  8. TMCI simulation without chromaticity Look like two interfering waves Spectrum 2 frequencies 600MHz and 800MHz

  9. TMCI simulation without chromaticity – Static bucket • Simulation in a stationary bucket (no acceleration) • Same bunch length at a given energy before transition with the same bunch length and momentum spread as initial condition as in the accelerating bucket • No node

  10. TMCI simulations with chromaticity • Routine implementation in HEADTAIL which change the vertical chromaticity (Qprime). • Not a very elegant way • The chromaticity is printed in the inph file

  11. TMCI simulations with chromaticity Before transition Through transition After transition (200 turns)

  12. TMCI simulation with chromaticity Eta= -0.0005 (below transition) ~100 turns before transition xiV=0.02 More we go far from transition (below) , more the rise time of headtail instability is increasing Rise time: Mode 0 : 0.9ms Mode 1: 1.7ms Mode 2: 2.6 ms Mode 3: 3.3 ms Rise time (s) Mode m

  13. TMCI simulation: tune shift • FFT with sussixnot suitable with acceleration since the bunch length is changing, how many turns? • Try with a sliding windows and 700 turns (not enough for a FFT). • Any idea, suggestion? Qy

  14. TMCI simulation: tune shift Qy ?? Sliding windows

  15. Gamma jump in headtailCrossing Transition in the CERN PS • Asymmetrical gamma jump • In the past, avoid 'negative mass instability' and longitudinal space charge effect on bunch length. • Doublets and triplets (combined doublets) quadrupoles in non zero dispersion locations. • Designed for a Qx,y=6.25 with no tune shift. • Δγt=-1.24 in 500μs • RF stable phase jump at transition to keep the longitudinal focusing after transition energy

  16. Gamma jump in headtailno chromaticity • Implementation of gamma jump in headtail with interpolation equation by part (eta/ gammatranstion as a function of the energy) • NOT elegant at all… • If In this case, it would be nice to have a passerelle between MAD and headtail

  17. Outlooks • Strange nodes in hdtl file which are not present in the static bucket and no chromaticity (not headtail mode) • Any suggestion/idea/help is very welcome (thanks already to Benoit, Elena for their suggestions) • Is this phenomena appears in the simulations with gamma jump? • …sometimes the documentation is not up to date..

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