From: boris petkovic
Date: Sat, 12 Oct 2002 12:09:03 -0700 (PDT)
Subject: Re: [TSSP] Top Voltage
--- Paul wrote:
>
> Terry gave the size and position of the sphere in a
> post a couple
> of days ago. Lower than the earlier tests because
> it didn't have
> the current probe beneath the supporting rod.
>
---
I have found that message.I missed the info during
first briefing tru the posts (my appologizes to
Terry).
---
I wrote:
> 190 kv (Emax~42 kv/cm)
> > looks like we start blowing away breakout
> mysteries pretty well.
---
I think that reported values in order of 100 kV/cm and
such for earlier OLTC setting (longer rod +
sphere)were simply impossible to happen on 38 kHz coil
without breakout.
IMHO there was something wrong:
a)serious error somewhere in field modelling
b)the rod itself gave away uniform corona sheet
earlier that decreased total field around sphere(i
doubt but can it be possible?)
I'm aware that breakout is a touchy event. I remember
one reading.. there were quite impressive standard
deviations given for smaller electrode objects under
short lasting voltage pulses.However,never that large.
---
>
> Well I suppose we're somewhere in the right
> ballpark. Given the
> low frequency, the DC value of 30kV/cm may be a
> better starting
> point than 26kV/cm. With this lower sphere, we are
> slightly
> better with our predictions, ie less than a factor
> of two out!
---
Agreed.Problem in modelling for earlier setup anyway ?
--
> Maybe the (unmodeled) effect of the current probe
> (its distortion
> of the field) makes the difference, by lowering the
> surface field
> strength of the sphere.
---
I have seen the picture of earlier setup.The probe
wasn't large enough to lower the field of the sphere
2x or more.
---
>
> Referring to
>
> http://www.abelian.demon.co.uk/tssp/cmod/
>
> we see John Freau's two setups just breaking out at
> a surface
> field of 42kV/cm and 35kV/cm, so same ballpark as
> Terry just
> measured. Marc gets multiple streamers when his
> coil tries to
> set the surface field to 53kV/cm.
---
While I'm ready to trust data for Freau's setups,I'm
suspicious about Marc's coil max field.
Did the both coilers measured secondary base currents
during these tests?This way ,from geometry of system
one can simulate everything else.
If they didn't ,how did you know how much of primary
orginal energy went into secondary?
Another thing why I'm suspicious:33 khz is enough long
for corona start to form but ,on other hand,may be
less visible on big smooth electrodes.
I think someone on pupman reported in past of heaving
big smooth electrode on higher powered system without
any signs of observable breakout.Nearly allways, as
the rule, the coil run in such mode for a few first
seconds that shoot single most powerful arc.The
explanation was 'that coil needs more time to build up
charge on topload'.There could be other reasons to
explain that (Irregularities in bang size comes first
to mind).
---
No breakout from
> Malcolm's
> at 13kV/cm, and Bart's is breaking out with short
> streamers at
> 26kV/cm.
---
Well, Bart doesn't live high in the mountains.Neither
Marc doesn't live deep in a coal mine.One doesn't live
in Siberia ,nor does another one in a Jungle.
Having said that,along the lines of the latest tesla
list discussions on atmosphere influence to TC
performance,the differences can't be so large.If you
know what I mean..Someting is "fishy" about all that.
Both coils are big, have well defined geometries,ks
etc.There are some differences in Fr ,but this is of
minor importance here.
Suggestion would be to run same systems
for the same observations but in Winter,and under
other air temperature/humidities variations.
Hard to beleive someting like 100% critical field
change can be ever observed for anyone.
---
>
> Between them, they all seem to bracket a range of
> values, which
> I would like to try to refine with base current
> measurements
> where possible.
---.
YES.
---
>
> I guess we should keep on collecting breakout data
> for a
> variety of situations, just to get a good picture of
> the
> terrain. We're bound to see a spread, and we might
> be able
> to discern a correlation with Fres and k.
> --
What else is left to us to do about it?
Nobody else have ever tried to find regularity in
discharge phenomena of TCs (as far as I know).
TCs are unpredictable in many aspects when they
discharge.For me,the essence of their beauty is
partialy buried in this fact.
But we should try ,at least,to find out when the
breakout from topload defined geometry is about to
start.
regards,
Boris
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