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Re: electric strength for "x" cm of "y" material

Original poster: "Antonio Carlos M. de Queiroz" <acmdq@xxxxxxxxxx>

Tesla list wrote: 
Original poster: "Dmitry (father dest)" <dest@xxxxxxxxxxx>
Hello Antonio.
 > Original poster: "Antonio Carlos M. de Queiroz" <acmdq@xxxxxxxxxx>
 > So, if there is some corona
 > between two conductors, placing an insulating plate between them
 > can only increase the corona. In other hand, a possible solution
 > is to place dielectric plates behind the conductors, not between
 > them.
solution of what?


To avoid corona. But doesn't work. I was imagining that by placing
a dielectric plate behind a charged conductor, it would "attract" the
electric field to it and reduce the electric field at the "front"
side, where corona to another conductor would appear. I ran some
simulations, however, and noticed insignificant effect only.

imagine the voltage between the primary & secondary is 100kv, the
distance - 2.5cm, field is homogeneous. at the beginning there`s no
corona - the field strength makes the air breakdown at once.
so i replace (the bigger) part of the air to the LDPE - 2cm of plastic.
if designate dielectric constant as "e", relative length of the insulating
interval (relation of the interval lenght to the distance  between the
electrodes) as "L", voltage on the electrodes as "U", then:
Eair = e*U/(e*Lair + Lldpe)
so - in my case the field stress in the air gap will increase in 1.825
times and would be equal to 73kv/cm - right?


Didn't chech the number, but it increases.

now i would have a corona - it would take all these 5mm of air volume,
or it would be concentrated near the surface of my copper primary?


If the field is uniform, all the 5 mm.

if near the copper pipe - no problem, if only the polyethylene is not
gets too hot.


It would not last much with RF corona over its surface.

and what about replacing the whole LDPE sheet with roll,
as D. C. Cox said - "wrap the inside of the lower part of the tube
with 10-12 layers of 20 mil polypropylene"? then the corona would
occur between every layer and the neighbor one, so - all this roll
would shine from inside? :-)))


Possibly. In short runs, the insulating layer may impede arcs, but
it will heat and be damaged by corona over it.

and if we`d take a dc source, 2 electrodes, tight 2 sheets of plastic
to the each electrode, having left the air gap between them - would
the air be shining? it must be cool - air breakdown without dielectric
breakdown, what - dc current through the dielectric? :-)


There is no problem with DC. The plate will just be charged at both sides
as the dielectric plate of a capacitor. Once this happens, there is no
more corona. But any sudden change in the voltage at the electrodes
produces a lot of corona again.

Antonio Carlos M. de Queiroz