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



Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Steve,

Sorry I'm just getting to your post. One thought that has been crossing my mind is that the higher the field is at a chosen distance from the toroid, the less energy will be required when the streamer tip reaches that point. The implications would be favorable for larger toriods where the field strength drops off less rapidly as one moves away from the surface and the charge reserve to support streamer growth is better. Maybe I will find my answers in "Spark Discharges"

Gerry R.


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