1. 1 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

2. 2 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

3. 3 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

4. 4 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

5. 5 Triggering effect on the time lag to breakdown Self-breakdown Triggered- breakdown Triggering decreases the time lag to breakdown

6. 6 Triggering effect on water gap breakdown Triggering has decreased the time lag to breakdown and the minimum breakdown voltage

7. 7 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

8. 8 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

9. 9 Time lag to breakdown vs. the delay time,Plane-plane triggered water gap with a trigger ring, gap voltage=10kV, trigger pulse energy=1J 1- Time lag to the main breakdown measurement, when a delay time was applied between the trigger pulse and the main voltage

10. 10 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.

11. 11 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 3.24J 0.8J

12. 12 3- Optical procedure using a conventional camera working in the open-shutter mode High Voltage Electrode Earth Electrode Bubble Trigger Pin (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

13. 13 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

14. 14 Simulated Equipotential lines in the parallel plane water gap including a hemisphere bubble

15. 15 Electric field simulation in the water gap containing a bubble using Quickfield software Gas/liquid boundary

16. 16 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 spacing=8.7mm and the main voltage=16kV, trigger pulse energy=1.44J

17. 17 Still picture from the conductive channel after the main breakdown Generated Bubble Conductive Channel Breakdown pictures taken by an open shutter camera technique, with a main gap voltage of 16kV, gap spacing of 8.7mm and trigger pulse energy=1.44J

18. 18 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 Summary

19. 19 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.

20. 20 THANK YOU FOR YOUR ATTENTION AND PARTICIPATION Msaniei@eee.strath.ac.uk