Re: [TCML] Magnifiers and transients

[Paul Nicholson <tcml88@xxxxxxxxxxx>]

Dex wrote:
> I wonder if these transients ripples can trigger racing sparks.

It is not known.  But consider the following.

The size of the transient is related to the distribution of
primary-secondary mutual coupling.  Roughly speaking, the more
concentrated it is, the higher the transient.

Here is the mutual inductance distribution when the flat spiral
primary is level with the secondary base,

 http://abelian.org/tssp/tmp/c6a.png

The actual values are irrelevant, only the shape of the curve
is of interest.   Obviously the coupling is quite concentrated
on the lower part of the secondary.

The system looks like

 http://abelian.org/tssp/tmp/c6b.png

and has k=0.12 (actually this is one of John Freau's coils).

I then experimented with modelling a different kind of primary,
one which gives a much smoother distribution of coupling.  It
looks like

 http://abelian.org/tssp/tmp/c6c.png

and has k=0.24, twice that of the first example.  But the
coupling is much more evenly distributed along the secondary:

 http://abelian.org/tssp/tmp/c6d.png

The transient energy of the two systems is

flat pri k=0.12:  transient=0.18%
cone pri k=0.24:  transient=0.30%

and it looks at first glance that things are worse.

But although the transient energy is higher with the cone
primary, nearly all of that energy is in the 3/4 wave overtone
and virtually nothing in the HF modes, something like 0.0007%,
whereas the flat primary had 0.03% of bang energy in HF modes.

That means the transient is much lower in terms of
volts-per-turn,

   http://abelian.netcom.co.uk/tssp/tmp/c6.grad.gif

Compare that with the low k flat primary,

   http://abelian.netcom.co.uk/tssp/tmp/c1.grad.gif

Both systems are running with identical bang energy, firing
voltage, Lpri and Cpri.

We have obtained here a low transient voltage gradient and
high coupling, without resorting to a 3-coil system.

It these transients have anything to do with racing arcs,
then systems using wide, cone or parabolic shaped primary
coils would be less likely to produce racing arcs for a given
firing energy and ought to be able to run at higher coupling.
Probably the worst case is with a compact flat primary level
with the secondary base, especially when the outer diameter
of the primary is not much greater than the inner diameter.

Perhaps what I ought to do is model the full range of primary
shapes and tabulate the transient behaviour.
--
Paul Nicholson
--
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