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

Experimental Results on Bunched Experimental Results on Bunched Beam Schottky Signals and Possible Beam Schottky Signals and Possible

Implications for LHC Incoh. Tune Meas.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

20/Nov/2001 F. Caspers: Bunched Beam Transverse Schottky…[LHC-BI-

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Examples of incoherent tune measurements and Schottky Examples of incoherent tune measurements and Schottky signal observation on bunched beamssignal 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 couplers in the V-plane. Here we had a significantly higher sensitivity than at10.7 MHz.


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10.7 MHz SPS Schottky PU signals10.7 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


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460 MHZ SPS Schottky PU Signals (1)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


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460 MHZ SPS Schottky PU Signals (2)460 MHZ SPS Schottky PU Signals (2)

Proton bunch(A), H-planeCoh 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


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460 MHZ SPS Schottky PU Signals (3)460 MHZ SPS Schottky PU Signals (3)

D. Boussard, T.Linnecar, W.Scandale; Transverse tune observations using the 460 MHz Schottky device, Improvement report April 1986; SPS/DI-MST

Upper trace: Bunch(A)- Bunch(B), H-plane, incoh part at 460 MHzLower trace: Coh. Schottky measured at 10.7 MHz


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Tune Measurements in the AD Tune Measurements in the AD during decelerationduring deceleration

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]

M.Angoletta, V.Chohan, M.Ludwig, O.Marqversen, F. PedersenThe New Digital Reveiver-Based System for Antiproton Beam DiagnosticsPAC 2001 and CERN/PS 2001-044 (BD)


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Limiting factors and precautionsLimiting 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.


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Can we avoid the strong coherent Can we avoid the strong coherent revolution harmonicsrevolution 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)


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Implications for the LHCImplications for the LHC For the LHC we have to find a solution which

permits keeping 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)


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Preliminary ConclusionsPreliminary 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.


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AcknowledgementsAcknowledgements

I would like to thank 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.


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Comments by H.SchmicklerComments by H.Schmickler

1) Presently Schottky Monitors do not 1) Presently Schottky Monitors do not figure in the LHC instrumentation base figure in the LHC instrumentation base line line

2) No human resources within the SL-BI 2) No human resources within the SL-BI group with adequate profile and experience group with adequate profile and experience are available are available

3) If we consider Schottky observations 3) If we consider Schottky observations relevant for the LHC, this project has to relevant for the LHC, this project has to be startedbe started

- in collaboration with the PS division?- in collaboration with the PS division? - with outside collaboration partners?- with outside collaboration partners?

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

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