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Re: [TCML] Re: Slo-Mo Videos of Tesla Coil

Greg Leyh wrote:
Bert wrote:

 >> (Greg Leyh wrote:)

Hi Bert,
> > Interesting, that recovery time is fairly independent of prior arc > current or gap spacing. It makes sense that increasing topload capacity > and peak output voltage are important towards obtaining maximum spark > length. So, given a fixed energy per bang available to the secondary, > how might you imagine trading off voltage and topload C? GL >

Hi Greg,

I'm not sure there is a clear answer to this question. My 2 cents worth for large coils...

Let's assume that your topload is sized so that that it provides sufficient electrostatic shielding for your secondary, the minor ROC is large enough to prevent initial breakout for the highest expected secondary voltage, and the topload has an adjustable breakout bump/projecting rod to tweak initial breakout voltage and direction.

One could argue that a topload that satisfies the above criteria may provide the optimal trade off between topload C and output voltage for a large system. The "larger toroid is better" strategy that worked so well for smaller systems may reach a point of diminishing returns on large systems. Since large systems already use physically large toploads to satisfy shielding requirements, the resulting topload C already provides a sufficient reservoir of charge to support efficient leader/streamer propagation. Further oversizing the topload lowers peak voltage but may not significantly improve spark growth processes. Spark propagation will cease when the field at streamer tips drops below about 5 kV/cm. Spark length is (ultimately) constrained by maximum terminal voltage. The objective should be to maximize V (commensurate with a topload sized to meet shielding and breakout restrictions).

Your thoughts?


Guess I'm wondering why the "larger toroid is better" strategy worked for smaller systems in the first place? Does there need to be some minimum, low inductance, reservoir of charge supporting the base of the arc? GL

The short answer: Yes...  :^)

The longer Answer:
In SGTC, OLTC, and SSIG systems, the "larger is better" strategy only works up to a point. For a given bang size, there is a maximum topload capacitance that provides maximum spark length.

A toroid that's too small lacks sufficient capacitance to support the formation of a hot leader. High current pulses drawn by corona streamers cause rapid reductions in topload voltage. These voltage collapses temporarily choke off streamer growth and current, and prevent a single hot leader from forming. The results are a multitude of relatively short, bluish-purple, gas burner-like corona-streamer discharges. At higher break rates, multiple shorter leaders may form, but the resulting current division reduces individual leader temperatures (i.e.m conductivity). Higher voltage drops along the smaller, cooler (more resistive) leader channels reduce maximum spark length. The mechanism is discussed below.

A toroid that is properly sized supports smooth transition from streamer-corona to a single leader and subsequent streamer and leader growth within a bang and from bang to bang. For break rates of about 100 BPS and above, the previous leader channel remains sufficiently hot to leave a lower density path that is then preferentially broken down during the next bang. Since the former path is already partially heated, reignition is much easier and the next bang can extend the overall leader length a bit more. The toroid only has to be large enough to deliver ampere-level transient current pulses for small to medium coils) to low 10's of amperes (for large coils) that (re)break down and (re)heat leader channels without excessive voltage collapse. At amperes or 10's of amperes, leaders possess high electrical conductivity, characteristic of arcs.

In an optimally sized sized system, as you increase break rate, a point is reached where the spark length reaches its maximum. At this point, further increases in break rate (and input power) only serves to make channels thicker, brighter, and hotter... but not longer. At this limit, spark length is constrained by maximum output voltage. At maximum spark length, the coil's maximum output voltage (minus resistive voltage drop in the leaders) can not maintain an E-field at streamer tips above ~5 kV/cm. Further streamer and leader propagation cease, and the coil reaches its maximum spark length (for a given bang size, break rate, and input power).

As I mentioned in the previous post, sufficient topload C will usually occur when the topload provides good (but not excessive) electrostatic shielding of the secondary and primary.

If we add too much topload capacitance, we might think that this would allow more current to go into the leaders. However, this requires more demand from the corona streamers at the end of the leader(s). However, with a fixed bang size, maximum topload voltage now becomes lower, and the reduced E-field also reduces corona-streamer currents. With reduced peak leader currents, leader voltage drops increase and output spark length becomes shorter. When using an optimal break rate, the coil is again limited by (now lower) peak topload voltage, so this system will have a shorter spark length than the optimal case above. The additional topload capacitance cannot be used effectively - the topload is now too big for the bang size.

I've seen this situation occur on smaller systems with toroids on underpowered systems. Increasing the bang size (and using a more robust HV power source) usually fixes this problem.

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