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Re: What it takes to get big sparks?
At 03:31 AM 1/4/99 -0700, you wrote:
>Original Poster: "Jim Lux" <jimlux-at-jpl.nasa.gov>
>Starting with some assumptions:
>
>1) The actual spark develops very quickly if there is a suitable low
>impedance source. The leader moves very very fast, essentially running its
>entire length in a microsecond or thereabouts.
What length? The length of the arc in question? I ask because I am
considering the mean thermal distance a positive ion can travel before a
collision. So the leader is a conduction process, rather than a ballistic
particle beam? What about electrons & there distance before collision?
>3) As the spark develops, it is essentially a capacitor that has increasing
>capacitance (because it is getting bigger) and is charging at the same
>time, as well as heating the air up. This means that current is flowing
>through this "spark capacitor".
Any ideas on the ionization energy requirements for the arc channel? What
are the arc channel dimensions? No doubt this is incremental, so the energy
price is paid on each RF cycle rather than once as a solid-liquid or
liquid-gas phase change. What about radiation loss?
I don't mean to be meticulous; Just know what the numbers are within 20% for
a typical kilo-watt Tesla Coil.
>I asked myself, where is the other connection of this capacitor?
The potential drop of the electric field; a large concentration of positive
or negative charges does not require a complementary or mirror image to
sustain a field. Unlike the magnetic poles; which are not observed isolated
like electric ones are.
>And, because we are talking fast time scales here (microseconds, again),
>the inductance of any sort of wire connection from the bottom of the coil
>to the "ground" means that the coil can't really supply any appreciable
>current while the spark is forming.
If you have a big shorted inductor, with a big magnetic field, you will be
unpleasantly suprised by the voltage it puts across an opening in the
interupted current.
>So then, what we have is an inductor (the secondary) which charges a
>capacitor (the top load working against the ground). That capacitor then
>breaks down (i.e. a leader forms) and discharges into the spark.
In that order? Why?
> The actual spark formation is almost independent of the secondary inductor
because the
>current required to develop the spark is much higher than that coming from
>the inductor to the capacitor.
That would explain some of those spikes in Terry's oscillographs. What are
those peak currents? Do they correlate to a leader burst?
>and assuming you want your capacitor
>to be able to discharge in something on the order of 10-100 nSec, making it
>bigger than 20-30 ft isn't going to buy you much.
What's so special about 10-100 nSec? What are those spike width's in Terry's
oscillographs?
And today:
>1) Formation of the leader (slightly luminous), very fast, requires enough
>charge and a low inductance source (i.e. the top load)
>
Sounds like the high-voltage (4 KV), low current ionizing pulse for a Xe
strobe light.
>2) Development of the visible arc: requires enough energy to make the air
>hot enough to conduct, and luminous as a result, occurs much slower, and
>can take power through a relatively high inductance source (e.g. the
>secondary coil).
Sounds like the low voltage (300 V) high current (100A?) pulse that flows
after the Xenon's ionized by the high voltage. Remember my suggestion to
research a dual-mode coil; one that gets ionized by infrequent ( < 10
per/second) 10 Joules impulses, and heated/sustained by a sub-kilowatt CW
coil?