Re: Lower secondary cself => better performance?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Duncan,
            As you suggest, isolating the various factors is required 
to test some of the ideas floating around. Typically, one adds 
topload, then adds Lp which affects two parameters straight away - 
the primary gap loss as well as the Ctop/Cs ratio in the secondary.
The gain in the primary can be quite significant and can easily be 
seen on the scope if breakout is suppressed, especially if the 
primary had a poor L/C ratio to begin with. Additionally, k is most 
likely altered as well.
     This suggests that a cleaner approach is called for - holding 
the primary losses constant as one changes the topload. Both Lp and 
Cp should be increased proportionally to maintain tune and Vp backed 
off to maintain the same Ep. Increasing Ctop and Lp added 25% length 
to the output of one of my coils with no other change. Caveat though -
the quench characteristics changed markedly and required some 
airblast through the gap where previously there had been none. In 
talking about quench I referring to the power-arc type quench.
      
On 14 Feb 01, at 16:28, Tesla list wrote:

> Original poster: "Dr. Duncan Cadd by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>
> 
> Hi All!
> 
> 
> >Thus, selecting the optimum h/d ratio and using the smallest possible
> >coil length, along with the smallest possible topload, is the advice
> >I would give to achieve the maximum output voltage for a given input
> >energy.
> 
> I agree.  It's classical intermediate frequency transformer theory.
> However, I have done much thinking over this last year and am no
> longer persuaded that the potential difference across the secondary
> coil is the bit which is important.  This simply is a measure of the
> amount of energy needed to stick a given charge (or number of
> electrons) on a given top load or capacitance.
> 
> >However, the experienced coilers on this list all seem to advocate
> >using the largest possible topload, reporting that performance
> >improves as the toroid size is increased.
> 
> This is what got me thinking in the first place, because to start with
> I simply could not reconcile the idea of IFT theory with larger
> capacitors apparently giving higher voltage outputs.  It seemed on the
> face of it impossible.  I started off on the track that a larger load
> helps quenching and thus efficiency, and doubtless there is some truth
> in that, but I still couldn't satisfy myself that this was the real
> reason why the sparks got bigger with the topload size.

See my note above regarding quench. (for at least one particular 
coil). 

> >Why should this be?  There must be some other factor(s) which in
> >practice are more important than energy storage considerations.
> 
> I began to wonder about the physics of charging isolated electrodes
> and concluded that the basic process has to be the same for Tesla
> coils as for Van de Graaf generators.  The volts there in a VDG accrue
> through an increase in charge, physically transported as it were and
> almost mechanically dumped on the topload.
> 
> When it comes down to it, what are sparks made of?  At the lowest
> level, it's charge i.e. electrons added or electrons taken away.  In
> air at STP it's either positive or negative ions (no free electrons in
> air, leastways not for long).  No charge = no spark.
> 
> >The conditions for obtaining efficient primary circuit operation and
> >an optimum coupling to the secondary may demand a larger topload. The
> >effective inductance Les of the secondary coil does increase as
> >topload is applied, and it could be that an over-sized (from the
> point
> >of view of energy storage) toroid is of benefit by enabling use of a
> >higher primary L/C ratio and a lower f1, both of which may lead to
> >greater primary efficiency. Also there is the matter of obtaining an
> >optimum power-transferring impedance match between the toroid and the
> >breakout loading, and it may be the case that a large toroid provides
> >the appropriate shunt matching - the optimisation is then for power
> >transfer rather than top voltage.
> 
> That's what I started to think but after considering VDGs began to
> reason that, whilst this does indeed matter, there is something far
> more important going on.
> 
> I concluded that since 0,5 CpriVpri^2 = 0,5 CsecVsec^2 = 0,5 qsec^2
> /Csec that as the secondary capacitance increases, so does the free
> charge qsec on the top electrode for a given bang size and that it is
> the electric field established by the total charge on the topload
> which is important, rather than the work done to put it there - if the
> capacitance is increased, the same work puts more electrons on the
> electrode (or removes them).  As the number of electrons increases
> (decreases), so does the field due to them.
> 
> Of course there is a certain amount of scientific semantics here
> because the charge, capacitance and voltage are all inter-related.
> Maybe it ought to be expressed in terms of "wave equations" of the
> Tesla secondary ;-)  Some kind of expression of its characteristics as
> a whole, rather than a lumped system.  Eigenvalues, Green functions or
> I-know-not-what.  I rather like what Antonio wrote:
> 
> "Streamers, maybe except for an initial forming transient, become part
> of the whole system."
> 
> It seems that the charge on the secondary topload can easily establish
> a larger electric field than the potential difference across the coil
> if the top capacitance is increased.
> 
> Example.  One joule bang size.  Looking at the field at one metre
> distance and at the topload surface:
> 
> Top capacitance 30pF.  Pd across coil 258kV.  Charge 7,7uC.  Electric
> field at one metre = q / 4.pi.Eo.r^2 = 69kV /m.  If topload is
> spherical, diameter = 54cm.  Field at the surface = 950kV/m.  No
> spark. If topload is toroidal with a minor radius of 7,5cm, field at
> the surface = 12,3MV/m.  Spark!
> 
> Top capacitance 100pF.  Pd across coil 141kV.  Charge 14,1uC.
> Electric field at one metre = 127kV /m.  If topload is spherical,
> diameter = 1,80m.  Field at the surface = 157kV/m.  No spark.  If
> topload is toroidal with a minor radius of 15cm, field at the surface
> = 5,6MV/m.  Spark!
> 
> Top capacitance 250pF.  Pd across coil 89kV.  Charge 22,4uC.  Electric
> field at one metre = 201kV /m.  (No point in calculating for a
> spherical electrode.)  If topload is toroidal with a minor radius of
> 30cm, field at the surface = 2,2MV/m.  Borderline.  A breakout point
> may be needed.
> 
> These calcs assume 100% efficiency: 50% is probably more like it, in
> which case they are for a 2J bang size.

You can typically do a lot better - I have seen transfer efficiency 
for a single pri-sec energy transfer approaching 90% on a small coil.
Whether that figure is degraded then depends on whether the secondary 
dumps its load in an (attached) output discharge at that moment ro 
whether pri-sec transfers continue which is the case for air 
streamers.
 
> Depending on the physical size, curvature of the topload, a field of
> 2-3MV/metre can easily be produced at the topload surface and hence
> dielectric breakdown of air and hence a spark.  What I currently have
> no means of doing is estimating the spark length from a knowledge of
> the charge and radius of curvature of the electrode (and whatever else
> might be relevant).  I suspect it will be some hideous integral
> describing the energy required to move each electron/ion however far
> it travels down the ion channel.  The further it travels, the less
> energy remains, the sparks get thinner and peter out.
> 
> >Anyway, for what it's worth, those are my speculations on the
> subject.
> 
> FWIW, my speculations are at:
> http://home.freeuk-dot-net/dunckx/wireless/scotty/scotty.html
> and in other pages to a lesser extent.
> 
> I am not totally convinced by everything I have written there - it was
> simply the best I could do at the time.  Especially Prof. Cotton's
> derivation I may have misapplied as the RC time constant vs
> oscillation frequency may well come into it.  But I would be
> appreciative of your thoughts, you lot out there!

That's my lot FWIW.

Regards,
malcolm
 
> I'm currently trying to (mis)apply NEC2 to Skip's coil.  It has
> sufficiently few turns to be barely modellable on my system, but first
> results don't look too encouraging.  Think I need a Sun ;-)
> 
> 
> Dunckx
> 
> 
> 
>