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Measurement and Compensation of Microphonics in CW-Operated TESLA Type Cavities

Measurement and Compensation of Microphonics in CW-Operated TESLA Type Cavities. Oliver Kugeler. Motivation. Why is microphonics an issue? Energy Recovery Linacs:. Beam loading small / close to zero Cavity run at high loaded Q Small cavity bandwidth

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Measurement and Compensation of Microphonics in CW-Operated TESLA Type Cavities

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  1. Measurement and Compensation of Microphonics in CW-Operated TESLA Type Cavities Oliver Kugeler

  2. Motivation Why is microphonics an issue? Energy Recovery Linacs: • Beam loading small / close to zero • Cavity run at high loaded Q • Small cavity bandwidth •  Any small shift in the cavity frequency • requires signficant increase in power to maintain constant field • produces phase errors that affect the beam. •  Must understand what microphonics should be expected •  Must investigate mechanics of the cavity-tuner system to be able to compensate for the microphonics

  3. Contents • Experimental setup at HoBiCaT • Microphonics measurements • Comparison of tuner systems • Microphonics compensation • Conclusion

  4. HoBiCaT • Purpose: • CW adaptation of TESLA technology • Cryostat for two cavities • Cryogenics 80 W @ 1.8 K • 10 kW Klystron • 30 kW IOT CW transmitter • Low level-RF • 4 different cavities tested so far

  5. RF-operation of cavity circulator TTF3-coupler master oscillator pin-diode 1.3 GHz attenuator amplifier, klystron, IOT 3-stub-tuner levelled amplifier pickup antenna phase shifter mixer cavity Dj low-pass filter instrument amplifier microphonics phase-error signal

  6. RF-operation of cavity circulator TTF3-coupler master oscillator pin-diode 1.3 GHz attenuator amplifier, klystron, IOT 3-stub-tuner vary QL from 3 x 107 … 3 x 109 or bandwidth from 0.2 … 22 Hz levelled amplifier pickup antenna phase shifter mixer cavity Dj low-pass filter instrument amplifier microphonics phase-error signal

  7. RF-operation of cavity circulator TTF3-coupler master oscillator pin-diode 1.3 GHz attenuator amplifier, klystron, IOT 3-stub-tuner levelled amplifier pickup antenna phase shifter mixer cavity Dj optional feedback loop RF ( PLL ) low-pass filter instrument amplifier microphonics phase-error signal

  8. RF-operation of cavity circulator TTF3-coupler master oscillator pin-diode 1.3 GHz attenuator amplifier, klystron, IOT 3-stub-tuner levelled amplifier pickup antenna phase shifter mixer cavity Dj piezo tuner optional feedback loop piezo low-pass filter piezo amplifier instrument amplifier RT-labview microphonics phase-error signal

  9. Sources of Microphonics Integrated Fast Fourier Transform from microphonics spectra

  10. Sources of Microphonics – liquid Helium pressure Cross-correlation between pressure in liquid helium bath and phase error p(lHe) = 16 mbar maximum correlation 0.8 Hz thermoacoustic fluctuations in LHe pressure 0.4 s instrument measurement-time • Aiming at microphonics rms value < 1 Hz

  11. Frequency – Pressure Dependence Importance of pressure stability: 1 mbar pressure change shifts cavity resonance by several bandwidths Pressure stability in HoBiCaT now better than 30mbar pk-pk

  12. Sources of Microphonics Observed strong correlation between cavity resonance frequency (upper picture) and cryogenics parameters, here a badly coupled part of the liquid nitrogen shielding

  13. Strong correlation between cavity resonance frequency (upper picture) and cryogenics parameters, i.e. the slowly relaxing temperature at a badly thermally coupled stepper motor tuner

  14. Tuning Range Hysteresis Group delay small High Stiffness Boring transfer function Resolution Requirements for the tuner

  15. Tuners tested in HoBiCaT Saclay I tuner Modified piezo holder frame: Higher wall thickness

  16. Tuners tested in HoBiCaT Saclay I tuner Modified piezo holder frame: Higher wall thickness

  17. Tuners tested in HoBiCaT Saclay II tuner

  18. Tuners tested in HoBiCaT Saclay II tuner

  19. Tuner comparison Pre-stress released upon tuning to 1.3 GHz

  20. Motor tuner – tuning range Saclay I Saclay II working point working point

  21. Motor tuner hysteresis Saclay I Saclay II * Effect smaller towards lower frequencies

  22. Piezo tuning – tuning range Saclay I (high voltage piezo) Saclay II (low voltage piezo) * Used high pre-stress on piezos and stiffer piezo frame than DESY

  23. Transfer functions Saclay I Saclay II

  24. Transfer functions Saclay I Saclay II Double resonance

  25. Overview of tuner properties what works, what must be improved

  26. Microphonics compensation

  27. Microphonics with feedback and feed-forward compensation Df (rms, feedback) = 4.33 Hz, maximum=13.4 Hz Df (rms, feedback+FF) = 0.77 Hz, maximum at 2.9 Hz

  28. Conclusion • Measured microphonics in HoBiCaT, 1-3 Hz rms, 10-15 Hz pk-pk • 50 % is due to LHe bath pressure  feedback possible •  Stability of liquid Helium pressure critical for microphonics • Analyzed two tuners for suitability of compensation • Pro Saclay I: motor control, pro Saclay II: piezo control • Motor control for Saclay II needs improvement • Microphonics reduction demonstrated by factor 3

  29. Acknowledgment Bessy: Axel Neumann Jens Knobloch Wolfgang Anders Michael Schuster Sascha Klauke Saclay: Michel Luong Guillaume Devanz Eric Jacques Pierre Bosland DESY: Bernd Petersen Rolf Lange Kay Jensch Clemens Albrecht Lutz Lilje

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