Diagnostic of Coating Results: Microwave Measurements

1. Diagnostic of Coating Results: Microwave Measurements F. Caspers, S. Federmann, E. Mahner, B. Salvant, D. Seebacher

2. Contents • Introduction • Ecloud and coating • Measurement possibilities • Modulation of signals • Realisation of the experiment • Problems and Solutions • Latest results and preliminary conclusions • Summary and outlook

3. Introduction • High intensity beams may build up ecloud • Undesired effect • Reduces luminosity, beam stability • Mitigation: Coating of beampipe • reduction/supression of ecloud expected • testing required

4. Measurement Configurations • Pick up electrodes to monitor ecloud (talk by C. Yin Vallgren) • Local measurement only • Pressure measurements (talk by M. Taborelli) • Measurement of phase modulation of a microwave (MW) signal due to ecloud

5. MW Transmission Measurements - Theory • Measurement of phase modulation (PM) of MW signal could give information of integrated ecloud density: • Δφis proportional to the electron cloud density: • Expect a phase shift of Δφ = 2.3 10-3rad(order of magnitude) wp…. Plasma frequency w……Injected frequency (f = 2.68 GHz) wc…. Cutoff frequency (f = 1.23 GHz) L……length of transmission path (6.5 m) ne…typical electron cloud density ≈ 1012

6. Modulation • Consider a continuous, pure sinusoidal wave (CW): Amplitude Modulation PhaseModulation

7. AM Modulation where: AC…. constant determining overall signal amplitude a ……modulation index [0≤|a|≤1] m(t)... normalized modulation signal ωC….. frequency of carrier If: Taken from: R. Witte, Spectrum and Network Measurements, 1991

8. AM Modulation Frequency domain: Time domain: ωC - ωm ωC ωC + ωm ω Result in time domain

9. PM Modulation In case of sinusoidal (and narrowband) modulation: β.... modulation index 180° Phase shift!

10. PM Modulation Frequency domain: (narrowband FM) Time domain: ωC - ωm ωC ωC + ωm ω Result in time domain

11. Modulation • Combination of modulation: Unequal height of sidebands! This is the practical situation in SPS MW experiment (BA5) and we have to separate the undesired AM signal from the desired PM signal by suitable instruments of the Vector Spectrum Analyzer (VSA)

12. Experimental Realisation • Excite preferably the TE10mode, couple to Magnetic field – due to space constraints in the pumping port regions • Not much influence of beam signals since TEM like mode of beam considerably decays on the side of the beampipe E field of the TE10 mode H field of the TE10 mode

13. Experimental Realisation • Coupling antennas (loops) on left and right side of beampipe in the pumping ports

14. First experimental setup VSA … Vector Spectrum Analyzer BP …… Band pass DC ….. Direct current bypass

15. Problems with this Setup • Main problem: Intermodulation distortion (IMD) caused by: (43.3 kHz = SPS revolution frequency) • ~43.3 kHz on power supply • ~43.3 kHz induced on cables down to tunnel • beam harmonics (around 2.68 GHz) • PM to AM conversion caused by different sideband attenuation in the hardware transfer function (HTF) • Change of CW signal amplitude and phase over the magnetic cycle (just of academic interest since we stay on the flat bottom)

16. New Setup • To get rid of this effect: • Concentrated on only one section: uncoated/coated • Installed high pass filters on surface as well as new amplifiers and capacitors in the tunnel

17. Observation with New Setup • IMD still present • Change of carrier found to be effect of deformations in beam pipe – tests on stand alone magnet confirmed this Evolution of carrier amplitude over cycle without beam dBm Time [s]

18. Latest Changes • Installed DC blocks (corner frequency ~100 MHz) in tunnel • Reduced hardware on surface (only DC blocks at the end of the cable used)

19. Preliminary results Beam 1 batch: PM signal in uncoated magnet 10 dB above noise Uncoated magnet Coated magnet

20. Preliminary results Uncoated magnet, 3 batches

21. Preliminary results Coated magnet, 3 batches

22. Preliminary Results Beam 3 batches: Signal in uncoated magnet increases about 3 dB with each injection On average signal 13 dB over noise Uncoated magnet Coated magnet

23. Preliminary Results Beam 3 batches, 10% less intensity: Signal in uncoated magnet increases about 3 dB with each injection On average signal 10 dB over noise Uncoated magnet Coated magnet

24. What quantity of the ecloud is measured? • The carrier is modulated by the ecloud (shape relative unknown) in time domain. The modulation repeats as the ecloud every revolution • What we seen on our instrument is the peak value of the fundamental wave of the modulation (ecloud) • Measured value has to be multiplied with an, shape dependent and not precisely known factor to get the peak value in time domain! Offset Harmonics Fundamental

25. Preliminary Results • We see a demodulated signal of about 10 dB above noise(1 batch) or about 14 dB above noise (3 batches) for the demodulated PM signal on the coated section • For a beam with 90% nominal intensity we obtained a reduction o modulation signal with respect to nominal beam by 3 dB for 3 batches in both cases • Increase of 3 dB of modulation sideband with each injection • For 2 batches we should see an increase in modulation signal strength by 3 dB with respect to 1 batch and about 5 dB for 3 batches

26. Summary and Outlook • Latest measurements have delivered promising results (no modulation seen in the coated section and a clear signal in uncoated beam pipe for the same beam and instrument settings) • Need to exclude any potential sources of error and signal contamination • Tests foreseen in SPS with 75 ns beam • Ecloud testbench in PS 84 where we have a clearing electrode and MW transmission to be used for comparison

27. Thanks for your attention!