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Longitudinal impedance identification

Longitudinal impedance identification. T. Argyropoulos, T. Bohl, J. E. Muller, H. Timko, E. Shaposhnikova Thanks to H. Damerau for preparing the beam LIU-SPS BD WG 30/08/2012. Motivation. The increase of the average bunch length of the batches along the flat bottom

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Longitudinal impedance identification

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  1. Longitudinal impedance identification T. Argyropoulos, T. Bohl, J. E. Muller, H. Timko, E. Shaposhnikova Thanks to H. Damerau for preparing the beam LIU-SPS BD WG 30/08/2012

  2. Motivation • The increase of the average bunch length of the batches along the flat • bottom • Observations of high frequency pattern on the nominal SPS bunches at flat • bottom during the single bunch measurements for threshold instabilities

  3. Measurements • Same method used in the past (Measuring the Resonance Structure of Accelerator Impedance with Single Bunches, T. Bohl, T. Linnecar, and E. Shaposhnikova, PRL, 1998) • Experimental conditions: • Long single proton bunches (εL(90%) ~ (0.23 – 0.26) eVs – τinj~ (25-30) ns) • Small momentum spread (to be more unstable and debunch slowly) • Intensity scan from 0.5x1011 to 2.0x1011 p • SPS RF off • Method: • Acquisitions of beam profiles for a period of ~90 ms after injection each 10 turns. • followed by a Fourier analysis  The presence of resonant impedances with high R/Q and low Q leads to line density modulation at the resonant frequencies Example with Np=1.58x1011 and τinj(4σ) = 28.5 ns 1.4 GHz Video of bunch profile

  4. Example at average (90 ms) Np= 8x1010 Contour plot Mountain range Projection Mode Amplitude vs Time

  5. Comparison with old measurements Contour plot Projection T. Bohl, 2007 Np = 8x1010 ? 2012

  6. Simulations • SPS long. impedance model: • TWC 200 • TWC 200 HOM (629 MHz) • TWC 800 • all kickers + resistive impedance (C. Zannini) • Initial particle distribution: • Saved at PS (tomoscope – for low intensity  smaller bunch length) • Simulated (with ESME) until extraction (Helga) • Simulations here have been done with my code (in Matlab): • (Helga is currently working with HEADTAIL)

  7. Comparison with Simulations (preliminary) Simulations with SPS long. impedance model + fr = 1.4 GHz – R/Q = 58 kOhms – Q = 7 for Np = 1x1011 Good agreement but more fine tuning is needed

  8. Summary Mode amplitude ratio of 1.4/0.2 versus Intensity Simulations were done with the SPS impedance model + fr = 1.4 GHz – R/Q = 35 kOhm – Q = 20 • Strong peak at 1.4 GHz observed in the beam • spectrum • Threshold around Np~8x1010 p: observed also • in old measurements (2001 and 2007)  this • impedance was always there! • Impedance evaluation through simulations  • Using low Q and high R/Q  more fine tuning is • necessary Any ideas about the possible impedance source would be very welcome!

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