Original poster: Steve Conner <steve@xxxxxxxxxxxx>
Hi all,
I was thinking about Steve Ward's observation that the sparks on his
coil suddenly grow in length from about 4ft to about 9ft as the
input voltage is increased.
Then I thought about an old post from Marco DeNicolai where he says
that discharges longer than about 1 meter grow by a different
mechanism that needs only 1kV/m electric field, rather than 5kV/m
for smaller gaps. (are these figures really right? they seem very small)
4ft is about 1 meter, and 9ft is a bit over twice 4ft, and 5kV
divided by 1kV is (a bit over twice) squared. So I'm thinking maybe
Steve is seeing this transition as he comes up on the voltage that
produces a 1 meter discharge?
If this hypothesis were true, it would imply:
The effect would only be seen on coils that give all bangs the same
size. It would not be seen on coils with async rotary gaps, for
instance. If the bangs are assorted sizes, some will be big enough
to push it beyond 1m and others won't so I expect the transition
will be a lot more gradual.
It would also only be seen on coils with a long enough energy
transfer time to allow growth of a >>1 meter discharge in the first
place. If the field is all depleted by the time the tip makes it
past 1 meter, nothing much is going to happen. Hence why I never saw
it on my OLTC2, which is rather too short at only 60us.
DRSSTCs tend to have lower peak power and longer energy transfer
times than spark gap coils so maybe this is why Steve sees the
effect so clearly.
Does anyone have any evidence that would confirm or deny this?
On a broader note, all this kind of suggests (to me anyway) that the
key to long sparks is filling a large enough volume of space with a
big enough electric field to drive growth, for long enough to let
the growth complete.
The way we talk of TC leaders suggests that growth at the tip is fed
by conduction from the toroid through the discharge channel. I am
beginning to think what really happens is that the tip feeds on the
electric field in its immediate neighbourhood. So you can pump all
the power in you like, but if your coil isn't tall/big-toploaded
enough to generate 5kV/m (or 1kV/m if over 4ft) in the furthest
places you want the sparks to go, it ain't gonna happen. You'll get
lots of action near the coil and nothing further out.
Steve Conner