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Electron Cloud Density Measurement using Microwave Modes in the SPS and LHC beam-pipe

Electron Cloud Density Measurement using Microwave Modes in the SPS and LHC beam-pipe. XXX January 23, 2003. Fritz Caspers, Tom Kroyer, CERN AB-RF. Motivation: Reflectometer for obstacle detection.

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Electron Cloud Density Measurement using Microwave Modes in the SPS and LHC beam-pipe

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  1. Electron Cloud Density Measurement using Microwave Modes in the SPS and LHC beam-pipe XXX January 23, 2003 Fritz Caspers, Tom Kroyer, CERN AB-RF

  2. Motivation: Reflectometer for obstacle detection • Feasibility of a waveguide reflectometer for obstacle detection on the LHC beam-pipe is currently examined • Operation in the first TM mode (cut-off 5.3GHz), since TE mode heavily attenuated by axial slots and interconnects • Attenuation of TM01 mode found to be roughly 0.05dB/m • Using a modern network analyser with 120dB dynamic range we should be able to cover 1 octant (1.7km) in transmission or half an octant in reflection mode F. Caspers, T.Kroyer: Integral Measurement of electron cloud

  3. Alternative use of the reflectometer pick-ups and kicker electrodes • In situ operation in the LHC would be need th installation of 32 high frequency pick-ups (used from 3 to 8GHz) as common BPM buttons do not work well above 0.5GHz due to their high capacity to ground (roughly 10pF @100MHz) • look for alternative use of these electrodes • A (transverse) 2 button pick-up/kicker could be used for an integral measurement of the electron cloud density • In the SPS this measurement may done already this summer F. Caspers, T.Kroyer: Integral Measurement of electron cloud

  4. Feasibility of the measurement Suppose a measurement in transmission mode over one sextant in the SPS (roughly 1km): • The one-way attenuation for the fundamental TE mode is estimated to be roughly 50dB @ 2 to 3GHz; this is well within the dynamic range of the vector spectrum analyser • W.r.t. the amplitude of S21 the electron cloud is not presumed to give large effects, but a measureable phase shift should occur • The repetition frequency of the proton batches leads to periodic phase modulation (electron cloud on/off) with 43kHz F. Caspers, T.Kroyer: Integral Measurement of electron cloud

  5. Expected phase shift The phase shift for an angular frequency  is given by with the plasma frequency e=1012/m3 designating the electron volume density, re the classical electron radius and c the speed of light For the SPS @ f=2 to 3GHz over 1km this would give a phase shift of roughly –25 to -17º. F. Caspers, T.Kroyer: Integral Measurement of electron cloud

  6. Analogy to Ionosphere In the ionosphere the situation is quite similar with maximum e= 1012/m3 as for our electron clouds. GPS (Global positioning system) encounters the same problems At f=1.575GHz through 200km of ionosphere  1m phase delay observed.  Scaling to 1km length and 2GHz gives a phase shift of 8º. F. Caspers, T.Kroyer: Integral Measurement of electron cloud

  7. Frequency modulation • The SPS revolution frequency of 43.23kHz causes a phase and subsequent frequency modulation of the transmitted CW signal • For =10º we got modulation index =0.2  “narrow band” FM, i.e. carrier plus 2 side bands, 20dB smaller than the carrier • Measurement sensitivity can go down to few millidegrees assuming carrier to sideband ratio of 80dB (dynamic range of a standard spectrum analyser) • However these weak sidebands may be masked by beam-related signals F. Caspers, T.Kroyer: Integral Measurement of electron cloud

  8. Microwave pick-up and kicker structures for this experiment in the SPS • Measurement in fundamental TE mode • 2 button (Ø 20 to 30mm) transverse pick-up type structure in opposite phase mode, should mainly couple with TE type signals and have little response with the TEM field of the beam • Operation in between Schottky bands (if possible; band overlap??) to avoid interference with the beam F. Caspers, T.Kroyer: Integral Measurement of electron cloud

  9. Cyclotron resonance effect • Cyclotron resonance of electrons occurs at 28GHz/T, thus at more the 3GHz with 0.117T at injection • S21 might go through a strong resonance absorption in this frequency • Remark by Frank Zimmermann: the impact on the electron may be small since the field of the fundamental TE mode is parallel to the static magnetic field. Higher modes could be used instead. F. Caspers, T.Kroyer: Integral Measurement of electron cloud

  10. Conclusion • In the SPS an integral measurement of the electron cloud effect over a sextant seems possible • At frequencies of roughly 3GHz narrow-band absorptions due to cyclotron resonance might be observed F. Caspers, T.Kroyer: Integral Measurement of electron cloud

  11. Discussion • Comments to the proposed methods • What other effects could be potentially observed in this frequency range? (2 to 5GHz) • Whatever you have in mind… F. Caspers, T.Kroyer: Integral Measurement of electron cloud

  12. Acknowledgements • The authors would like to thank Flemming Pedersen, Roland Garoby and Trevor Linnecar for support as well as Elena Chapochnikova, Frank Zimmerman, Noel Hillary and Thomas Bohl for useful informations and very helpful discussions F. Caspers, T.Kroyer: Integral Measurement of electron cloud

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