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Hey, Ayush,
A spark that jumps from a sharp edge or point will go into corona first,
which leads to haphazard firing and longer current risetimes. The gap
will have to be wider for the same voltage, which will have a longer arc
channel with corresponding higher series resistance. Both of these
effects are detrimental to your coil's operation. You can easily round
the end with a grinder with a fine wheel, or even a belt sander, but
tungsten has high heat conductivity and the piece will get hot
surprisingly fast.
I've never tried aluminum, but I suspect it would erode rapidly. Brass
is a very good material--many high current spark-gap switches from
Maxwell and other manufacturers have brass electrodes and withstand
repeated firings of tens of thousands of amps. You can get brass cap
nuts or acorn nuts from hardware stores that work well and require no
machining.
As a general rule, if the hemispherical electrode radius is less than
1/10 of the gap spacing the gap will go into corona breakdown. It's best
to use the largest practical radius, since that will present more area
for the sparks to hit and will have a lower erosion rate and a shorter
gap length. You can use a brass bar with a flat end and radius the edge
so that the flat is maybe 60% of the electrode diameter for an even
bigger active area. If you want to design to a fare-thee-well, you can
put Rogowski curvature on the edge,
(http://home.earthlink.net/~jimlux/hv/rogowski.htm) If you do this, it
will give you the warm feeling that you are using the ultimate in
electrodes. In any case, look at your electrodes after a minute or so
of operation to be sure you are getting sparks where you want them.
---Carl
On 4/17/2015 5:45 AM, Ayush Sharma wrote:
Hey all
I have been working on a small size SGTC, and have posted a few doubts
regarding capacitors previously on the mailing list. I also have a few
doubts regarding the Spark Gap of my coil.
I am planning to use a Tungsten welding electrode (TIG) for the spark gap.
It comes in rods of dia 2.4mm and length 150mm. I am planning to cut the
rod into 4 pieces and use them for the spark gap.
I wanted to ask whether it is absolutely compulsory to smooth the edges of
the rod, and make them hemispherical at the end after i cut it into pieces,
or it will be ok to just use the flat sides in the spark gap.
Also, I wanted to ask if anybody has ever used Aluminium balls for making a
spark gap, and what are the pros/cons for the same?
Thanks
Ayush sharma
ᐧ
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