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Hello Donald, For brief pulses (a few microseconds or less), pure water is an excellent dielectric. However, the water needs to be continually filtered, degassed, and deionized so that it has a resistivity of ~5-7 megohm-cm in order to work reliably as a dielectric in high energy pulsed power pulse systems. For short pulses, water combines high dielectric strength, high dielectric constant (~80), and is "self healing" in the event of an electrical breakdown. These properties allow pulsed power engineers to create compact, high-energy storage and transmission systems using water as the dielectric. For example, water is used as the dielectric in low impedance, high current, high voltage transmission lines that feed 20 million ampere pulses into the center of the huge "Z Machine" at Sandia Laboratory - the world's largest pulse generator - see: http://www.sandia.gov/media/z290.htm and The main challenge is to keep the water sufficiently pure and keep gas bubbles from forming on the electrodes. Since water is the "universal solvent", it easily becomes contaminated by impurities (dust and ions leaching from the container that increase its conductivity). These impurities must be continually removed since their presence always degrades the water's performance as a high voltage dielectric. For short pulses, J. C. Martin developed an empirical breakdown scaling relation for water and mineral oil under a uniform E-field over a range of voltages, pulse times, and electrode area based upon his work at Sandia. The relationship is as follows: F = k*(t^(-1/3))*(A^(-1/10)) where: For example, solving for the positive streamer breakdown field (F) for 1 square cm electrodes in water, stressed by a 1 microsecond pulse in water, we get F = 300 kV/cm. If we used a 100 nsec pulse, this would be expected to increases to 646 kV/cm, and almost 1.4 million volts for a 10 nsec pulse. Breakdown behavior changes with longer (>10 microsecond) pulses, since ionic conduction may begin to alter the E-field distribution within the gap. Considerably more detail can be found in "High Power Switching" by Ihor M. Vitkovitsky, ISBN 0442290675 and Introduction to High Power Pulse Technology by S. T. Pai and Qi Zhang, ISBN 9810217145. Breakdown within water begins as streamers that initiate from points of field enhancements (bubbles, small projections, or particles on the electrodes). As noted above, streamers will form and propagate more easily from the positive electrode in a uniform field. I am not aware of explicit data relating breakdown strength to water temperature. However, increasing the waters temperature will reduce the water's density and increase ion mobility these factors may tend to decrease the dielectric strength. Increasing the applied pressure will significantly increase the breakdown voltage, possibly because it makes initial bubble formation (which seems to be necessary for slow streamer formation) more difficult. Best regards, -- Bert --
: What is the dielectric strength of water? How is it affected by
: temperature and contaminants?
:
http://www.sandia.gov/pulspowr/facilities/zaccelerator.html
F = the peak breakdown field (in megavolts/cm)
t = duration of applied voltage (in microseconds)
A = area (in square cm)
k = 0.3 for water (positive streamers the normal case)
k = 0.6 for water (a special case where field enhancement is purposely adjusted to cause streamers to form preferentially from the negative electrode instead of the positive electrode)
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