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Instabilities observed in the LHC e -cloud MD 26 th August 2011

Instabilities observed in the LHC e -cloud MD 26 th August 2011. H. Bartosik, G. Arduini , R. de Maria, B. Goddard, W. Höfle , E. Métral , B . Salvant, D. Valuch , F. Zimmermann. Overview.

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Instabilities observed in the LHC e -cloud MD 26 th August 2011

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  1. Instabilities observed in the LHC e-cloud MD 26th August 2011 H. Bartosik, G. Arduini, R. de Maria, B. Goddard, W. Höfle, E. Métral, B. Salvant, D. Valuch, F. Zimmermann

  2. Overview • Analysis of ADT pickup data for the 2 injection with 48 bunches during the LHC 25ns injection MD (21:58h and 22:44h) • First injection of 48 bunches at 21:58h with ADT dampers on  beam was lost after ~1000 turns • Second attempt of injecting 48 bunches without the ADT dampers  beam was lost after ~500 turns (due to fast losses) • Injections themselves looked clean (see summary by B. Goddard) • No particular observation on BBQ data (in particular for the second dump) • Analysis of the 73 turns before the dump from the ADT (damper) pickups of all bunches as recorded in the post mortem (bunch by bunch)

  3. 1st injection (with transverse damper) • Growing motion along the train (mostly V) • Coupled bunch motion not evident as very high frequency motion is observed Horizontal Vertical

  4. “Mountain range” • Small oscillation amplitude for the first 20 bunches • Increasing amplitude for last 28 bunches, especially in the vertical plane (note the different scales!) Horizontal Vertical

  5. Single bunch FFTs • FFT of the 73 turns for each bunch • Vertical tune appearing in horizontal plane for last bunches Horizontal Vertical

  6. FFT along the bunch train for each turn • Low frequency part of the spectrum is suppressed by the transverse dampers Horizontal Vertical

  7. 2nd attempt (22:44h) – dampers off • Fast growing losses in vertical plane  beam dump

  8. ADT pickup signals (B2-Q9) • ADT pickup signal for the 48 bunches (22:44h) • Increasing oscillation amplitude after the first ~20 bunches … • Coupled bunch motion? (losses already occurred before this data) Horizontal Vertical

  9. “Mountain range” • Small oscillation amplitude for the first 20 bunches • Significantly larger amplitude for last 28 bunches Horizontal Vertical

  10. Single bunch FFTs • FFT of the 73 turns for each bunch • Small sidebands in horizontal plane (maybe linked to large losses) Horizontal Vertical

  11. FFT along the bunch train for each turn • Low frequency part of spectrum is dominant • Indication for coupled bunch motion Horizontal Vertical

  12. Summary • Unstable motion of the second half of 48 bunches can be observed on the ADT pickups • Low frequency part can be handled by the dampers  high frequency oscillation remains mainly in vertical plane • Without dampers, coupled bunch motion is observed in both planes • Note that delay setting up of the dampers for 25ns bunch spacing was not done yet!

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