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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?
Bert
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.
Bert
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