In my experience, no matter how pure the water, it'll eventually start conducting. Trace amounts of metals will solvate and you'll get breakdown under the water.

Go with dried (water free) oil, and pull a good vacuum on the oil too. It'll save you problems in both the short and long term.

Due to electrolyte process, liberate oxygen will form the oxides, and will pollute water, hence will not provide the insulation needed, as compare to oil for long run.

I see. A question for both: the use of a pump with filtering, sacrifice electrodes, and remove air from water making some vacuum would help?

In sandia z-pinch lab they used water:

http://www.compadre.org/informal/images/features/zpinch-small-4-5-07.jpg

Deionized water may be used as insulator only in pulsed mode. At longer pulses or DC, water will conduct.

There is a time constant limit, epsilon*resistivity. At room temperature, you cannot have water resistivity higher than 20 Mohm.cm (limited by natural ionization H2O = H+ plus OH-), i.e., the time constance cannot be larger than 150 us.

One can submerge his equipment in a SF6 gas filled tank or a high pressure tank, to avoid oil. But the gas pressure needs to be large enough so that no breakdown occurs.

Thank you very much, Hanwu.

I remember that my brother had to be care to avoid that any current cariying line in his boat can touch the water to avoid corrosion.

I supose I must be sure that there is no voltage or current in the circuit when the equipment is off and then use it in pulse mode.

Hanwu, if AC voltage is used how many time can I set it?

In DC, one says epsilon and resistivity. In AC, one often refers to epsilon and tan(delta). One thing said in two ways. Water has large tan(delta), so that large loss, due to current conduction.

So the answer is the same as for DC, the time voltage applied should be as short as the loss is acceptable, for AC frequency up to ~1 GHz.

Thank you all. I decided that the best option is SF6 with gas recovery system. I read its breakdown is 150kV for 6 bars at 5mm. It is not much pressure, I hope all the electronics will resist it.