39th meeting of the co-ordinating committee on fast wave heating and current drive arc detection...

Arc Detection Meeting - JET - 4th August 2005

39th Meeting of the Co-ordinating Committee on Fast Wave Heating and Current Drive

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Presented by

M. Graham

A. Walden, M.Nightingale, I. Monakhov

Arcing in RF systems



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Introduction

• Amplifiers– VSWR and optical systems assist with

arc detection

• Transmission Lines– VSWR provides adequate arc

protection. Targeted optical arc detection systems may help to locate arcs

• Antenna strap– Generally well protected by VSWR

Areas where arcing can occur



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IntroductionAreas where arcing can occur

• Vacuum (Everything between antenna strap and window).

– First voltage node produced in this section which is not protected by VSWR detection

• Screen bars, etc – generally don’t have real time protection

however effects are generally seen on camera. Different problem as arcs are induced arcs at lower power



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VSWR Trips• Vast majority of arcs are caught by VSWR systems.

• VSWR or reflected power systems give good protection however their effectiveness is dependant on the standing wave pattern and thus do not provide 100% protection

• A few arcs are missed due to a maladjusted system

• There are instances where VSWR changes are so small that they go undetected, for instance

– A3 VTL strap on JET A2 antenna

– High Power Prototype (HPP)

– Modelling of EP antenna.

• These missed arcs have had significant impact on experimental time, finance and resources as a result of the damage caused.



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Undetected Arcing

• The A3 VTL on JET is a case where arcing should not have occurred– The VSWR was considered

to be adequate protection as the field was expected to be too low (voltage node) for arcing to occur.



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EP System

• Low VSWR arcs seem to occur in vacuum conditions only.

• There are two distinct areas that are of concern in the EP antenna; VTL T-point and within the vacuum capacitors.

• If arcing occurs then in-vessel entry for repair of VTL components will be required



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Future Systems

• It is generally assumed that high voltage points will arc

• Load tolerant systems will hide the effects of arcing making detection harder

• Despite the fact that arcs are rare, the consequences in future installations will have a much greater impact on operational and economic issues



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Detection systems Independent of standing wave pattern

• Optical– Line of sight or reflected light can be used to

provide out of the way protection. Difficulties can be encountered due to plasma light and signal to noise level

• Acoustic– Acoustic systems are too slow as they are

limited by acoustic propagation speeds



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Detection systems Independent of standing wave pattern

• Sub-Harmonic The Asdex upgrade uses this system but it has

run into signal to noise problems with the latest plasmas

• RF comparative techniques Sensors required either side of the target area to

monitor inconsistencies. These techniques include both the EP antenna proposal from ERM and G. Bosia’s proposal



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Data Collection• Most of the operational data collection is too slow

to be able to adequately assess the arcing mechanism.

• Future data collection needs to be higher speed with good triggering

• Very desirable to use high power prototype to reproduce low VSWR arcs

• Storing raw data is good practice as various data processing/detection modelling can be applied



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Conclusion

• My conclusion is that there is currently no working economic arc detection system that reliably detects low VSWR arcs

• It is not presently clear what systems under consideration are likely to provide a practical solution

• It should be recognised that a combination of arc detection techniques may be required



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Conclusion

• Important to establish a wide shared database to help identify characteristics imposed by different ICRH systems and plasma machines.

• Concentration should initially be given to the EP antenna however a general solution is required for the ICRH community as a whole

39th meeting of the co-ordinating committee on fast wave heating and current drive arc detection...

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