Transcript EiC and Session Leader Talks 2004

39th Meeting of the Co-ordinating Committee on Fast Wave Heating and
Current Drive
Arcing in RF systems
Presented by
M. Graham
A. Walden, M.Nightingale, I. Monakhov
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Arc Detection Meeting - JET - 4th August 2005
39th Meeting of the Co-ordinating Committee on Fast Wave Heating and
Current Drive
Introduction
Areas where arcing can occur
• 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
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Arc Detection Meeting - JET - 4th August 2005
39th Meeting of the Co-ordinating Committee on Fast Wave Heating and
Current Drive
Introduction
Areas 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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Arc Detection Meeting - JET - 4th August 2005
39th Meeting of the Co-ordinating Committee on Fast Wave Heating and
Current Drive
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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Arc Detection Meeting - JET - 4th August 2005
39th Meeting of the Co-ordinating Committee on Fast Wave Heating and
Current Drive
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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Arc Detection Meeting - JET - 4th August 2005
39th Meeting of the Co-ordinating Committee on Fast Wave Heating and
Current Drive
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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Arc Detection Meeting - JET - 4th August 2005
39th Meeting of the Co-ordinating Committee on Fast Wave Heating and
Current Drive
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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Arc Detection Meeting - JET - 4th August 2005
39th Meeting of the Co-ordinating Committee on Fast Wave Heating and
Current Drive
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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Arc Detection Meeting - JET - 4th August 2005
39th Meeting of the Co-ordinating Committee on Fast Wave Heating and
Current Drive
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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Arc Detection Meeting - JET - 4th August 2005
39th Meeting of the Co-ordinating Committee on Fast Wave Heating and
Current Drive
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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Arc Detection Meeting - JET - 4th August 2005
39th Meeting of the Co-ordinating Committee on Fast Wave Heating and
Current Drive
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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Arc Detection Meeting - JET - 4th August 2005
39th Meeting of the Co-ordinating Committee on Fast Wave Heating and
Current Drive
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
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Arc Detection Meeting - JET - 4th August 2005