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Radical Flat Cap design
It has been recommended that "multiple" layers of dielectric be used in order
to minimize the chance of breakdown due to flaws in the dielectric. While
philosophically correct, this practice usually causes problem when
implemented. Most home brew cap builders build with what is available. What
is typically available at most home supply and hardware stores is roll
polyethylene with a maximum thickness of 6 mills. In order to build a robust
cap with a dielectric thickness of 90 to 120 mills, 15 to 20 layers of
dielectric material between plates is required. This implementation is
seriously flawed!
First, polyethylene sheets trap air bubbles between them, the more layers,
the more air is trapped. A 120 mil dielectric of twenty layers will trap ten
times the air of a 120 mil dielectric composed of two layers. There is
therefore a trade off between reducing the likelihood of failure due to flaws
by using multiple layers -and- using the minimum number of layers to reduce
the total quantity of trapped air pockets (which can also cause failure due
to heating effects). I propose that two or three layers of thicker
dielectric -two 60 mill sheets, for example- is sufficient to reduce the
potential flaw line-up problem significantly while at the same time *greatly*
reducing the amount of trapped air.
Second, in standard flat cap design, it is suggested that the cap be left
idle for at least a week after assembly to permit "off gassing" to occur. I
would suggest that in a traditional multi layered dielectric design, one week
is not nearly enough. Low density polyethylene has a soft waxy consistency
which tends to "weld" it together with other layers, particularly if the
plates a "clamped" together with some force as they should be in any good
flat cap design. This "welding" of the polyethylene creates air pockets that
the wicking action of the oil is not sufficiently strong to overcome. The
first build of my cap which used fewer thicker plates (as per the first
suggestion above) had this problem. I constructed the case out of 1/2 inch
clear acrylic and I observed that even one week after assembly, after all
off-gassing had ceased, a 3 or 4 inch diameter air pocket in the center of
each plate remained. The size of the air pocket stopped diminishing after
the fourth day, leading me to believe that the remaining trapped air at the
center of each layer of dielectric would only gradually be eliminated after
months, perhaps, if ever.
I propose that this problem may be solved by interleaving thin absorbent
paper sheets between each layer of dielectric to improve the wicking action
of the oil and more completely and quickly displace the air pockets while at
the same time having a negligible impact on the dielectric strength and
dielectric constant of the assembly. I implemented this approach on the
first -and only- rebuild of the cap. The result was a spectacular success.
*All* off-gassing stopped within 12 hours and *all* air pockets were gone by
24 hours. The day after assembly, I cranked the cap up to half power -which
is supplied by a 15Kv -at-60ma neon xformer- for a minute or two and observed
only one thin stream of about 10 bubbles. After this stream of bubbles had
stopped no more were apparent. I slowly cranked the neon up to full power
and ran it for several minutes with no heating and *no* further out-gassing.
I suggest that two to three thickness of dielectric (at most) with paper
wicks between each layer is a highly superior design approach.
Sticks and stones welcome......
I apologize for the poor quality of my last post on foil-tab arc over.
Composing at two AM is apparently not workable for me. I even forgot my
name! Although in that case it may have been fortunate ;)
Gas-less flat cap building to you.
- David F -