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Examples of incoherent tune measurements and Schottky signal observation on bunched beams

Experimental Results on Bunched Beam Schottky Signals and Possible Implications for LHC Incoh. Tune Meas. Examples of incoherent tune measurements and Schottky signal observation on bunched beams Limiting factors and precautions Implications for the LHC Preliminary conclusions.

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Examples of incoherent tune measurements and Schottky signal observation on bunched beams

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  1. Experimental Results on Bunched Beam Schottky Signals and Possible Implications for LHC Incoh. Tune Meas. • Examples of incoherent tune measurements and Schottky signal observation on bunched beams • Limiting factors and precautions • Implications for the LHC • Preliminary conclusions F. Caspers PS-RF

  2. Examples of incoherent tune measurements and Schottky signal observation on bunched beams • In the SPS two incoherent Schottky diagnostics systems were tested, one at 10.7 MHz (availability of quartz filters) and another around 460 MHz. • The 10.7 MHz system used 3 meter long resonating strip-line plates (adaptive centering to the beam) with an unloaded Q around 350 and the passive quartz filter was ahead of the first low noise amplifier to avoid inter-modulation from the strong revolution harmonics. This system has been also operated in the BTF mode for signal improvement (gives a certain beam blow-up!) at low beam intensities and for low emittance • For the 460 MHz a HOM in one of the main accelerating cavities was taken in the H-plane and 2 pairs of strip-line couplersin the V-plane. Here we had a significantly higher sensitivity than at10.7 MHz. F. Caspers: Bunched Beam Transverse Schottky…[LHC-BI-WS Nov 2001].

  3. 10.7 MHz SPS Schottky PU signals The scanned images shown below {SPS data} are Fig 3 and Fig 4 from “ A Transverse Schottky Noise Detector For Bunched Beams ” by T. Linnecar and W. Scandale [IEEE-NS-28 June 1981] Passive response BTF response F. Caspers: Bunched Beam Transverse Schottky…[LHC-BI-WS Nov 2001].

  4. 460 MHZ SPS Schottky PU Signals (1) • The data shown below are from the SPS MST seminar Dec 17th 1985 by P.Baudrenghien (SPS/Di-MST/JG/EEK) • Title: A New 460 MHz Schottky System for the observation of coherent and incoherent transverse signals of the p- bunch. • The strong 50 Hz harmonics (magnet ripple, left image) were removed by a comb filter F. Caspers: Bunched Beam Transverse Schottky…[LHC-BI-WS Nov 2001].

  5. 460 MHZ SPS Schottky PU Signals (2) Proton bunch(A), H-plane Coh and incoh part Proton bunch(A) - bunch(B) H-plane, incoh. part D. Boussard, T.Linnecar, W.Scandale; Transverse tune observations using the 460 MHz Schottky device, Improvement report April 1986; SPS/DI-MST F. Caspers: Bunched Beam Transverse Schottky…[LHC-BI-WS Nov 2001].

  6. 460 MHZ SPS Schottky PU Signals (3) Upper trace: Bunch(A)- Bunch(B), H-plane, incoh part at 460 MHz Lower trace: Coh. Schottky measured at 10.7 MHz D. Boussard, T.Linnecar, W.Scandale; Transverse tune observations using the 460 MHz Schottky device, Improvement report April 1986; SPS/DI-MST F. Caspers: Bunched Beam Transverse Schottky…[LHC-BI-WS Nov 2001].

  7. Tune Measurements in the AD during deceleration M.Angoletta, V.Chohan, M.Ludwig, O.Marqversen, F. Pedersen The New Digital Reveiver-Based System for Antiproton Beam Diagnostics PAC 2001 and CERN/PS 2001-044 (BD) The problem of fast tune measurements on a small beam with small emittance during deceleration has been solved the following way: Use of a very sensitive, high Q, low noise transverse PU system operating near 6 MHz A special “M” shape noise distribution excitation (essentially acting on the imaginary part of the BTF) in order to enhance the signal/noise ratio [with minimum beam-blowup] F. Caspers: Bunched Beam Transverse Schottky…[LHC-BI-WS Nov 2001].

  8. Limiting factors and precautions The extraction of the incoherent tune requires sophisticated signal treatment and may be limited by signal to noise problems (small beams and small emittance) as well as dynamic range limitations (intermodulation) The beam should be in the electrical center of the PU (or the center of the PU should follow the beam) in order to minimize the impact of the strong coherent revolution harmonics A passive high Q notch filter (preferably low loss) can be very helpful to avoid inter-modulation at the front end of the active electronics (pre-amplifier) chain. F. Caspers: Bunched Beam Transverse Schottky…[LHC-BI-WS Nov 2001].

  9. Can we avoid the strong coherent revolution harmonics Theoretically above the “single-bunch cutoff frequency” coherent signals should decay very rapidly This assumption was the basis for bunched beam stochastic cooling attempts both in the SPS and at Fermilab. However in practice it was found that this rapid decay is not all rapid and that effect was the KILLER for bunched beam stochastic cooling in big machines!!! A high frequency cavity PU operating around 2 GHz for the Tevatron has been designed in 1987 (Goldberg et al, LBL 22273) F. Caspers: Bunched Beam Transverse Schottky…[LHC-BI-WS Nov 2001].

  10. Implications for the LHC For the LHC we have to find a solution which permitskeeping the beam exactly in the electrical center of the PU or vice versa. The maximum possible frequency for a cavity is given by band-overlap and waveguide cutoff. We cannot rely to get rid of the undesired strong coherent signals staying well above the SBCF (single bunch cutoff frequency) Towards lower frequencies we have to watch out for impedance aspects (Z/n) Unless considering very low intensity pilot bunches we should not have too much of a sensitivity problem (thus no BTF needed) F. Caspers: Bunched Beam Transverse Schottky…[LHC-BI-WS Nov 2001].

  11. Preliminary Conclusions Based on the practical experience gained over the last 20 years, an incoh. Schottky system for tune measurements during the ramp in the LHC appears to be possible. However keeping in mind all the difficulties that had to be overcome on such systems in the past, it may be a realistic strategy not to rely at this moment on the full design performance of such a diagnostic tool. Instead one may consider to install a suitable PU and after a certain number of measurements with the LHC beam take a decision to set up the complete system. F. Caspers: Bunched Beam Transverse Schottky…[LHC-BI-WS Nov 2001].

  12. Acknowledgements I would like tothank T.Linnecar as well as W. Scandale for very helpful discussions and providing several references that would have been hard to find otherwise. E.Angoletta, Mike Ludwig and O.Marqversen gave me important information on specific aspects of the AD Schottky system Also thanks to H.Schmickler for the invitation to this workshop and R. Garoby for support. F. Caspers: Bunched Beam Transverse Schottky…[LHC-BI-WS Nov 2001].

  13. Comments by H.Schmickler 1) Presently Schottky Monitors do not figure in the LHC instrumentation base line 2) No human resources within the SL-BI group with adequate profile and experience are available 3) If we consider Schottky observations relevant for the LHC, this project has to be started - in collaboration with the PS division? - with outside collaboration partners? F. Caspers: Bunched Beam Transverse Schottky…[LHC-BI-WS Nov 2001].

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