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LHC RF Feedback

LHC RF Feedback. Donat Stellfeld John Molendijk Philippe Baudrenghien Pierre Maesen Urs Wehrle. SM18 tests, Aug-Sept 2005. Reported by P. Baudrenghien. Open Loop. 56 kV, 7.8 A. Q 20000 -> 180000. DC coupled Analog Fdbk (Digital Fdbk OFF). RF feedback Theory.

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LHC RF Feedback

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  1. LHC RF Feedback Donat Stellfeld John Molendijk Philippe Baudrenghien Pierre Maesen Urs Wehrle SM18 tests, Aug-Sept 2005 Reported by P. Baudrenghien SM18 tests Aug-Sept 05

  2. SM18 tests Aug-Sept 05

  3. Open Loop 56 kV, 7.8 A Q 20000 -> 180000 DC coupled Analog Fdbk (Digital Fdbk OFF) SM18 tests Aug-Sept 05

  4. RF feedback Theory • RF Feedback theory [1],[2] • Minimal cavity impedance (with feedback) scales linearly with T (600 ns) • Achieved for a gain value proportional to Q • Achievable fdbk BW inversely proportional to T 43.2 kW assumed single-cell 690 kHz 2-sided BW SM18 tests Aug-Sept 05

  5. Effect of coupling • Transmission Loss Pout/Pin [3]: • At resonance we get: • So Vout/Vin is proportional to sqrt[QL] SM18 tests Aug-Sept 05

  6. RF Feedback Module (eda-586.v2) Notch to damp the resonance of the second klystron cavity (404.8-405.45 MHz) + phase advance to increase closed loop BW. loop amplifier klystron 10 MHz 400.8 MHz SM18 tests Aug-Sept 05

  7. O.L. vs. Notch position • Left: notch well adjusted • Right: Notch at min and max positions (~800 kHz range) SM18 tests Aug-Sept 05

  8. Stability. Open Loop:What we expected… 10 dB gain margin Open Loop gain = 13 • Nyquist Plot using measurement of real klystron • Q=20000, real klystron, loop delay 450 ns (excluding klystron) • Low Level: Notch plus Phase Advance SM18 tests Aug-Sept 05

  9. O.L. Stability vs. CW power 200 kW CW 50 kW CW Q=20000 O.L. gain = 20 (26 dB) • Positive Fdbk -> unstable point = (+1,0) • 50 kW vs 200 kW • Gain drops by 2 dB • Phase does not change SM18 tests Aug-Sept 05

  10. O.L. stability vs. Q Q=60000, LL gain x 1.7 Q=20000 O.L. gain =60 O.L. gain =20 • All meas, 50 kW CW • To keep same gain margin, LL gain varies as SQRT(Q) • No significant phase change Q=180000, LL gain x 3 O.L. gain =180 SM18 tests Aug-Sept 05

  11. O.L. stability vs. detuning Cavity on tune Cavity detuned by 10 kHz • Q=60000, 50 kW CW • No change in gain neither phase • Detuning has no effect on stability SM18 tests Aug-Sept 05

  12. O.L. stability vs. Klystron HV • Q=60000, • Change HV 56 kV to 54 kV • Thomson TH2089 measurements: • But Cathode current does not change RF phase. But changes gain. SM18 tests Aug-Sept 05

  13. Closed Loop • Closes OK on first trial • Tracks Iref,Qref OK • But when large step in Iref, remains stuck with strong pure CW SM18 tests Aug-Sept 05

  14. Overdriving the Modulator • AD8345 wants IF levels max +-0.3 V offset by 0.7 V DC • Driven by AD8138 single-ended to differential • Adding clamping diodes HSMS-2820 (RF Schottky diodes) on input of AD8138 cured problem SM18 tests Aug-Sept 05

  15. Closed Loop. LL output noise All spectra with phase advance, Q20000, 130 kW, 1MVacc O.L. gain 20 O.L. gain 10 O.L. gain 5 • Pre-driver -20 dB out TP • Noise varies linearly with Low Level gain -> it is a Measurement noise: Analog Demodulator noise • High HF gain due to Phase Advance • Probably no effect on beam… but… SM18 tests Aug-Sept 05

  16. Phase Advance and LL output noise Both spectra loop closed, O.L. gain 20 linear, Q20000, 130 kW, 1MVacc With phase advance No phase advance • Pre-driver -20 dB out TP • Phase Advance network boosts noise by 5 linear at +-1 MHz offset • Probably no effect on beam… but… SM18 tests Aug-Sept 05

  17. Closed Loop:What we expected… 2-sided -3 dB BW = 600 kHz linear phase response in <1 MHz band • Mathematica using measurement of real klystron • Q=20000, real klystron, loop delay 450 ns (excluding klystron) • Low Level: Notch, no Phase Advance. Gain set for 10 dB margin SM18 tests Aug-Sept 05

  18. C.L. vs. LL gain O.L. gain 20 O.L. gain 28 Group delay compensated • No Phase Advance Network • Q=20000, 1MVacc, 135 kW • O.L. gain = 20 linear for 10 dB gain margin SM18 tests Aug-Sept 05

  19. C.L. vs. CW power 135 kW CW (1 MVacc) 50 kW CW (0.62 MVacc) • No Phase Advance Network • Q=20000 • O.L. gain = 20 linear for 10 dB gain margin SM18 tests Aug-Sept 05

  20. C.L. vs. Q Q20000 1 MVacc (135 kW) Q180000 2 MVacc (60 kW) • No Phase Advance Network • O.L. gain = proportional to Q (keep 10 dB gain margin) • Low Level gain = proportional to Sqrt[Q] • Significant change in phase distortion SM18 tests Aug-Sept 05

  21. C.L. with/without Phase Advance No Phase Advance With Phase Advance • Q=20000, O.L. gain set to keep 10 dB gain margin, 135 kW CW -> 1 MVacc • Phase Advance • increases gain outside 3 dB BW • reduces non-linear phase distortion SM18 tests Aug-Sept 05

  22. Impedance Reduction • First calibrate with Feedback Off • Then measure response SM18 tests Aug-Sept 05

  23. Impedance Reduction:What we expected… Reduction by 13 linear at the tune Reduction in a 300 kHz band (2-sided) Increase • Mathematica using measurement of real klystron • Q=20000, real klystron, loop delay 450 ns (excluding klystron) • Low Level: Notch, no Phase Advance. Gain set to 13 (linear) for 10 dB margin SM18 tests Aug-Sept 05

  24. Impedance Reduction vs. LL gain Gain set to 7 dB margin Gain set to 10 dB margin • Q=60000, zero CW. No Phase Advance • 10 dB margin is the “best” • Modulus of Z is reduced in a +- 150 kHz band SM18 tests Aug-Sept 05

  25. Impedance Reduction vs. CW power Zero CW 2 MVacc (190 kW) • Q=60000. No Phase Advance • Marginal effect of CW power. SM18 tests Aug-Sept 05

  26. Klystron ripples reduction Cavity field phase ripples at 50 Hz and 600 Hz Fdbk OFF. ~3 degrees pkpk Fdbk ON. ~ 0.2 degree pkpk Enlargment Fdbk ON. ~0.2 degrees pkpk • Using vector voltmeter 8508A Analog Out on high Z (1 kHz BW) • Q=20000, 1 MVacc, 130 kW, with phase advance, O.L. gain = 20 linear • Data in..\..\Modules\RFfeedback\Tests\SM18\Week35\PhaseNoise.xls SM18 tests Aug-Sept 05

  27. Klystron ripples reduction Cavity field amplitude ripples at 50 Hz and 600 Hz Fdbk OFF. ~25 kVacc pkpk Fdbk ON. ~15 kVacc pkpk Left with 50 Hz measurement noise ? • Using HP423A X-tal detector plus 100 kHx LPF on high Z Analog Out on high Z • Q=20000, 1 MVacc, 130 kW, with phase advance, O.L. gain = 20 linear • Data in..\..\Modules\RFfeedback\Tests\SM18\Week35\AmplNoise.xls SM18 tests Aug-Sept 05

  28. Klystron ripples reduction Cavity field phase ripples at 50 Hz and 600 Hz • Using ZLW-1W mixer plus SLP-100 onto 50 ohm • Q=60000, 2 MVacc, 170 kW, no phase advance • Data in..\..\Modules\RFfeedback\Tests\SM18\Week37\PhaseNoise.xls SM18 tests Aug-Sept 05

  29. Cavity field noise AC coupled Vcav I and Q vs time Vcav on I/Q plot Noise ~ 13 kVacc pkpk around 2 MVacc (0.4 degrees pkpk, 0.65 % pkpk) • Using Independent Analog I/Q demodulator plus 20 MHz LPF (1 mV-> 6.6 kVacc) • Q=60000, 2 MVacc, no phase advance, O.L. gain = 40 • Data in..\..\Modules\RFfeedback\Tests\SM18\Week39\IndepNoiseMeas.xls SM18 tests Aug-Sept 05

  30. Beam Loading Test 50 kV step in quadrature with 1 MVacc 1 ms falltime 600 ns delay 1 ms 50 kV step in 1 ms (+ 600 ns delay) • 400 MHz rectangular burst on Beam Loading Test input. (see page 22) • Measure Voltage error signal, that is RF feedback I/Q inputs • After transient, beam loading perturbation reduced by O.L. gain • Q=20000, 1 MVacc in I, 130 kW, with phase advance, O.L. gain = 20 • Data in..\..\Modules\RFfeedback\Tests\SM18\Week35\BeamLoading.xls SM18 tests Aug-Sept 05

  31. Beam Loading Test 5 ms falltime 50 kV step in phase with 1 MVacc 50 kV step in 1 ms (+ 600 ns delay) 1 ms risetime • Very asymmetric • After transient, beam loading perturbation reduced by O.L. gain • Q=20000, 1 MVacc in I, 130 kW, with phase advance, O.L. gain = 20 • Data in..\..\Modules\RFfeedback\Tests\SM18\Week35\BeamLoading.xls SM18 tests Aug-Sept 05

  32. Step Response 70 kV step in quadrature with 1 MVacc 1 ms risetime 70 kV step in 1 ms -> limit saturation (with 1 MVacc CW) • Q=20000, 1 MVacc in I, 130 kW, no phase advance, O.L. gain = 20 • Data in..\..\Modules\RFfeedback\Tests\SM18\Week39\RefStepVcav.xls SM18 tests Aug-Sept 05

  33. Step Response 70 kV step in phase with 1 MVacc 1 ms risetime 5 ms falltime saturation when step adds to 1MVacc CW • Q=20000, 1 MVacc in I, 130 kW, no phase advance, O.L. gain = 20 • Data in..\..\Modules\RFfeedback\Tests\SM18\Week39\RefStepVcav.xls SM18 tests Aug-Sept 05

  34. Step Response 70 kV step in phase with 1 MVacc saturation at 280 kW when step adds to 1MVacc CW • Q=20000, 1 MVacc in I, 130 kW, no phase advance, O.L. gain = 20 • Data in..\..\Modules\RFfeedback\Tests\SM18\Week39\RefStepVcav.xls SM18 tests Aug-Sept 05

  35. Huge Step Response +- 0.5 MV MV step in quadrature with 1 MVacc 1 MV step in 10 ms -> severe saturation (with 1 MVacc CW) • Q=20000, 1 MVacc in I, 130 kW, no phase advance, O.L. gain = 20 • During heavily saturated transients, measurements from input coupler do not make sense!!! • Data in..\..\Modules\RFfeedback\Tests\SM18\Week39\RefStepVcav.xls SM18 tests Aug-Sept 05

  36. Huge Step Response 1 MV step in phase with 1 MVacc (from 0.5 MV to 1.5 MV) From 0.5 MV to 1.5 MV in 25 ms From 1.5 MV to 0.5 MV in <10 ms • Q=20000, I from 0.5 MV to 1.5 MV, no phase advance, O.L. gain = 20 • During heavily saturated transients, measurements from input coupler do not make sense • Data in..\..\Modules\RFfeedback\Tests\SM18\Week39\RefStepVcav.xls SM18 tests Aug-Sept 05

  37. Why we have been lucky… Klystron used in SM18 tests Typical klystron curve Courtesy of Olivier Brunner • During SM18 tests, the klystron could never go above saturation peak… SM18 tests Aug-Sept 05

  38. Overdriving the klystron (ramp) Klystron input clamped Klystron over-driven Courtesy of Janne Holma SM18 tests Aug-Sept 05

  39. Overdriving the klystron (steps) Klystron input clamped Klystron over-driven Courtesy of Janne Holma SM18 tests Aug-Sept 05

  40. If the LL gain scales with sqrt[Q] we have Closed loop BW 550 kHz, independent of Q and CW power -> Apparent Q < 1000 Apparent cavity impedance ~45 kohm, independent of Q Reduction of apparent cavity impedance modulus in a 300 kHz band, independent of Q Phase advance is not worth it…rid of it in eda586.v3 1% ripple in HV causes 8.4 degrees phase shift @400.8 MHz. That is acceptable for stability. (At least in the absence of 1-T fdbk). Close Loop phase characteristic OK for 1-T feedback in a 1 MHz band (2-sided) Reduction of klystron phase ripples by O.L. gain (as expected) Conclusions (good) SM18 tests Aug-Sept 05

  41. With 1 MVacc, Q=20000, step 70 kV in quadrature in 1 ms (OK for long damper at injection?) No problem when klystron saturates on transients Rare trips: Main Coupler Vacuum (always cured by additional conditioning) and Klystron Body Overheat when running in saturation for long time. RF feedback and Tuner Loop fully compatible Conclusions (good) SM18 tests Aug-Sept 05

  42. Conclusions (bad) • Non-linear phase distortion changes with Q • Without phase advance the phase distortion is bigger • Close Loop phase characteristic NOT OK for 1-T feedback in a >1 MHz band (2-sided). Foresee Phase Equalizer in 1-T feedback? • Reduction of klystron amplitude ripple by less than loop gain (measurement noise at 50 Hz?) • Measurements from the cavity input coupler do not make sense during heavily saturated transients • What if the klystron is allowed to go over saturation peak? SM18 tests Aug-Sept 05

  43. References [1] Control of cavities with high beam loading, D. Boussard, IEEE Tr. On N.S. Vol NS 32 No.5, p. 1852, 1985 [2] Low Level RF Systems for Synchrotrons, Part 2, P. Baudrenghien, CERN SL-Note-2001-028 HRF [3] Microwave Measurements, Edward L. Ginzton, McGraw-Hill Book Company Inc., New-York, 1957, pp 404-405 SM18 tests Aug-Sept 05

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