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Digital measurement system for the LHC klystron high voltage modulator

Digital measurement system for the LHC klystron high voltage modulator. D. Valuch, A. Mikkelsen , G. Ravida , O. Brunner. LHC RF power system. Total 16 klystrons Thales TH2167 400.8 MHz, 330 kW, at nominal 58 kV, 9.2 A One klystron per cavity Four klystrons supplied by one HV supply

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Digital measurement system for the LHC klystron high voltage modulator

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  1. Digital measurement system for the LHC klystron high voltage modulator D. Valuch, A. Mikkelsen, G. Ravida, O. Brunner

  2. LHC RF power system D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator Total 16 klystrons Thales TH2167 400.8 MHz, 330 kW, at nominal 58 kV, 9.2 A One klystron per cavity Four klystrons supplied by one HV supply Crowbar to protect klystrons in case of arcing

  3. LHC RF power system D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator

  4. Klystron modulator D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator Modulator contains filament transformer and modulation anode control Cathode and modulation anode voltage measured using dividers Currents measured using external DCCTs

  5. Present measurement system Example of a klystron cable fault D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator • Fast interlocks on voltage/currents • Observation/logging • 1 sample per second during rapid changes • 1 sample per 15 minutes during slow changes • Post-mortem analysis difficult due to granularity

  6. New measurement system in the HV modulator D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator Measurement system integrated in the modulator oil tank Voltages and currents measured directly at the klystron cathode Communication over optical fibre VME receiver card for readout, logging and post-mortem analysis Currents measured with DCCTs Voltages measured with present resistor dividers

  7. Improvements over present system • >1 ksps sample rate • 12 bit resolution • Measurements are done at the source, better accuracy • Over 100 second long post mortem buffer at full sample rate for all channels D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator

  8. Measurement board (floating at -57 kV, submerged in oil) Digital VME receiver board (for 4 klystrons) D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator

  9. High voltage testing D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator • The recipy: • Take some sensitive electronics such as CPLD, ADC, current transducers • Put it into a harsh high voltage environment with high fields in a normal operation • Or 6000 A, 10 kA/ms, 50 kV/ms in case of faults What could possibly go wrong…

  10. Observed problems D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator Current transducer develops a large analogue offset after crowbar. The offset dies-out after several hours Complete current transducer failure after tens of crowbars ADC upset after every few crowbars Crystal oscillator upset after every few tens of crowbars But… Not a single problem observed with the CPLD, DC/DCs, optical links!

  11. High voltage testing D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator High voltage compatibility testing at HV test stand Decoupling capacitor and solid state crowbar Motorized spark gap used to simulate klystron arcing

  12. Test set-up D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator

  13. Current transducer protection D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator Fault currents in case of klystron arc magnetized the current transducers Different transducers evaluated, all susceptible Transducer dies completely after multiple faults/crowbars Protection diodes eliminated problem

  14. Induced currents in board traces D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator Upset of the ADC was observed after spark gap firing Current transducer power trace found to be sensitive to induced noise from spark current Breaking the loop eliminated the problem

  15. Crystal oscillator upset D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator Board outputs serial data at ~20 times normal rate after arc Noise coupled through DC/DC converter. Grounding of board power input eliminated problem Use of a different crystal oscillator model (without PLL, operating at fundamental frequency)

  16. Measurement results with LHC klystron D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator Plot shows filament voltage and current waveform Time resolution allows individual 50 Hz mains cycles to be resolved Buffer length over 100 seconds at 1ms/point resulotion

  17. Measurement results with LHC klystron Cathode current flowing through the filament diminishes Crowbar fires Plot shows filament current waveform after crowbar

  18. Test stand power converter ripple D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator Test stand power converter known to have large amount of ripple FFT of cathode voltage shows 700 V RMS of 50 Hz ripple Measurement system gives useful information on power converter condition

  19. Project status D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator Tests with real klystron show very promising results System survived >160 crowbars without any upset or degradation of performance System installed in spare LHC modulator for long-term testing in 112 klystron test stand System is planned to be installed in one LHC modulator during Long Shutdown 1

  20. Summary D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator A new, state of the art digital measurement system for the klystron HV modulator was developped We have demonstrated that a sophisticated electronics could be successfully integrated into a harsh HV environment Increase of measurement precision, time resolution, measurement capabilities Early fault diagnostics with observation buffers (tube aging, connectors, HV supply health monitoring) Extensive post mortem data buffers for fault diagnostics in the LHC machine

  21. References D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator CERN-THESIS-2014-033 https://cds.cern.ch/record/1700591 Digital measurement system for the LHC klystron high voltage modulator. Author Mikkelsen, Anders (Department of Electronics Bergen University College)

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