An experimental investigation on the mechanisms of the electrictriggerpulsegenerated bubble in a tri

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An investigation into the breakdown
mechanisms of a triggered water gap
switch     Mohsen Saniei     Institute for
Energy and Environment University of
Strathclyde Glasgow G1 1XW
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Introduction

• 1- Water has
• a- High dielectric constant (?81)
• b- High dielectric strength (1MV/cm)
• c- High energy storage density

2- Water is used as the dielectric in water gap
switches for pulsed power applications
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Aims and objectives
1-Investigating the effect of triggering on the
breakdown of a water gap 2- Investigating the
breakdown mechanism in triggered water gaps
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Electric set-up Schematic

• Main voltage is generated by the discharge of the
  80nF high voltage capacitor by activating the
  trigatron switch
• Trigger pulse for the water gap is generated by a
  4X Blumlein generator

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Triggering effect on the time lag to breakdown
Triggering decreases the time lag to breakdown
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Triggering effect on water gap breakdown
Triggering has decreased the time lag to
breakdown and the minimum breakdown voltage
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Triggering effect on water vapourisation

• Trigger pulse with trigger pulse energies of
  1-2J and a pulse duration of 500ns means
  available power of 2-4MW
• This energy could vaporise water and generate a
  bubble

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Methods of examining bubble generation
1- Measurement of time lags to the main gap
breakdown, when a delay time was applied between
the trigger pulse and the main voltage 2- Optical
procedure using a photo-detector and He-Ne
laser 3- Using a conventional camera working in
the open-shutter mode 4- Using a high speed
digital camera
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1- Time lag to the main breakdown measurement,
when a delay time was applied between the trigger
pulse and the main voltage
Time lag to breakdown vs. the delay time
,Plane-plane triggered water gap with a trigger
ring, gap voltage10kV, trigger pulse energy1J 
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2-Schematic of reflection and refraction of laser
beam due to the presence of a gas bubble
Refractive index differences gas phase 1.006
water 1.333 means bubble acts as a divergent
spherical lens. Laser beam diverges reducing
transmitted light intensity at the detector.
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Results from optical procedure using a
photo-detector and He-Ne laser
Intensity of a light beam transmitted through a
triggered plane-plane water gap as a function of
time after the application of a voltage pulse to
the trigger-pin
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3- Optical procedure using a conventional camera
working in the open-shutter mode
(a)
(b) Still Pictures taken by an open
shutter camera with a trigger pulse, but without
the main gap voltage Trigger pulse energy (a)
1.44J (b) 3.26J  
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4-An optical method using a high speed digital
camera
Successive frames at times of 200, 400, 600, and
800 ?sec showing the development of a bubble
produced in the electrode gap after the
application of a trigger pulse, energy 2.56J
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Simulated Equipotential lines in the parallel
plane water gap including a hemisphere bubble
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Electric field simulation in the water gap
containing a bubble using Quickfield software
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Pre-breakdown streamer images captured by the
high-speed digital camera
Pre-breakdown streamer at the plane-plane
triggered water gap with a trigger pin, the main
gap spacing8.7mm and the main voltage16kV,
trigger pulse energy1.44J
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Still picture from the conductive channel after
the main breakdown
Breakdown pictures taken by an open shutter
camera technique, with a main gap voltage of
16kV, gap spacing of 8.7mm and trigger pulse
energy1.44J
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Summary

• Bubble generation as a result of a trigger pulse
• Electric field intensification within the bubble
• Initiation of an electric avalanche within the
  bubble
• Propagation of the electric streamer toward the
  high voltage electrode
• Final breakdown in the water gap

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Acknowledgements
I would like to thank Dr R A Fouracre and
Professor S J MacGregor for their guidance and
supervision, and also from Professor G Woolsey
for his advice and assistance during this
research. I would like to thank the Ministry of
Research, Science and Technology of Iran for
their financial support.
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THANK YOU FOR YOUR ATTENTION AND PARTICIPATION
Msaniei_at_eee.strath.ac.uk