Sparks jumping from 4' to 9' (was Re: 20 joules)

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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