Digital measurement system for the LHC klystron high voltage modulator
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Transcript of 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
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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• Crowbar to protect klystrons in case of arcing
CWRF2014, Trieste, May 2014
D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator
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LHC RF power system
Crowbar tankSeparator tank
To other modulators and klystrons
+
–
HV supply0 – 100 kV
40A
Modulator tank
H
H+K
MA
Capacitor tank
Ground level HV bunker UX45 Klystron gallery UX45
CWRF2014, Trieste, May 2014
D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator
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Klystron modulator
• Modulator contains filament transformer and modulation anode control
• Cathode and modulation anode voltage measured using dividers
• Currents measured using external DCCTs
CWRF2014, Trieste, May 2014
D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator
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Present measurement system• 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
CWRF2014, Trieste, May 2014
D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator
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New measurement system in the HV 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
Modulation anode control
Filament transformer
Optical fibre
Optical fibre
Cathode + Filament
Filament
MAC10 klystron modulator
Filament power
-HV from separator
Measurement data
Modulation anode
setpoint
10 Ω
Live
Neutral
- HV
Filter + protection
CPLD
Fibre TX
ADC
Measurement card
CWRF2014, Trieste, May 2014
D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator
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Improvements over present system
Channel Range Resolution
Cathode voltage 0 – 100 kV 25 V
Modulation anode voltage
0 – 100 kV 25 V
Cathode current 0 – 15 A 10 mA
Heater voltage ± 50 V 25 mV
Heater voltage RMS
0 – 50 V RMS
25 mV
Heater current ± 50 A 30 mA
Heater current RMS
0 – 50 A RMS
30 mA
• >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
CWRF2014, Trieste, May 2014
D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator
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D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator
Digital VME receiver board (for 4 klystrons)Measurement board (floating at -
57 kV, submerged in oil)
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High voltage testing• The recipy:
1) Take some sensitive electronics such as CPLD, ADC, current transducers
2) Put it into a harsh high voltage environment with high fields in a normal operation
3) Or 6000 A, 10 kA/ms, 50 kV/ms in case of faults
What could possibly go wrong…
CWRF2014, Trieste, May 2014
D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator
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Observed problems• 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!
CWRF2014, Trieste, May 2014
D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator
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High voltage testing• High voltage compatibility testing
at HV test stand• Decoupling capacitor and solid
state crowbar• Motorized spark gap used to
simulate klystron arcing
CWRF2014, Trieste, May 2014
D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator
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Test set-up
CWRF2014, Trieste, May 2014
D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator
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Current transducer protection• 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
CWRF2014, Trieste, May 2014
D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator
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Induced currents in board traces• 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
CWRF2014, Trieste, May 2014
D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator
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Crystal oscillator upset• 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)
CWRF2014, Trieste, May 2014
D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator
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Measurement results with LHC klystron
CWRF2014, Trieste, May 2014
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
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Measurement results with LHC klystron• Plot shows filament current waveform after crowbar
Crowbar fires
Cathode current flowing through the filament diminishes
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Test stand power converter ripple• 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
CWRF2014, Trieste, May 2014
D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator
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Project status• 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
CWRF2014, Trieste, May 2014
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Summary• 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
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D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator
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References
CERN-THESIS-2014-033 https://cds.cern.ch/record/1700591Digital measurement system for the LHC klystron high voltage modulator.Author Mikkelsen, Anders (Department of Electronics Bergen University College)
CWRF2014, Trieste, May 2014
D. Valuch et al.: Digital measurement system for the LHC klystron high voltage modulator