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Re: The Electrum Project
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- Subject: Re: The Electrum Project
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- Date: Wed, 24 Aug 2005 14:23:42 -0600
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Original poster: boris petkovic <petkovic7@xxxxxxxxx>
> > >"dart leaders" - what`s it? usual leader or
> something else?
> > A good explanation can be found here:
> > http://en.wikipedia.org/wiki/Lightning
> > Dart and stepped leaders are covered near the
> bottom of the page.
> no - there`s no explanation of "dart leader" there,
> but it can be found
> "The "return stroke" produces more than 99% of a
> lightning bolt's luminosity
> and is what we see as lightning.
> After the return stroke ceases flowing up the
> channel, there is a pause of
> about 20 to 50 milliseconds. After that, if enough
> charge is still available
> within the cloud, another leader can propagate down
> to the ground. This
> leader is called a "dart leader" because it uses the
> channel already
> established by the stepped leader and therefore has
> a continuous path."
Pretty much so.
A dart leader follows the path heathed by the
preceeding return stroke discharge in a multiple
component lightning flash event.
Simply put,you may take that still hot channel serves
as a kind of 'waveguide' to a dart leader.
It might be surprising at first glance,but dart
leaders are more like the oldest of the known types of
propagating plasma channels:streamers!
Firstly,the velocity of a dart leader is typicaly
around 10 000 km/sec which is comparable with a
velocity of a long HV streamer.
Secondly,a dart leader hasn't got so called streamer
zone ahead of the tip (like the stepwise leader),but
it has well defined and fast propagating front 10
meter long or so ,which is identifiable by the
intense optical radiation as it travels to earth.
It clearly stands out against channel background, and
such appearance reminds of dart.Hence the name.
The difference between dart leaders and long streamers
are in conductivities averged over discharges'
The reason for HV streamer being practically nonviable
in a virgin air is fast loss of conductivity by the
'cold' plasma.On the hand,a dart leader has the
possibility (due to preheated channel track help) for
the region behind the tip to be heated to arc
temperatures since high gas temperarture significantly
reduces electron losses.
>From the foregoing,it is my understanding that the
dart leader,contrary to its name,is principaly not
leader at all.
It is sort of a superlong HV streamer in a 'waveguide'
> then i can`t understand - why do you talk about
> those dart leaders? the
> stepped leader influences the spark length - isn`t
The stepped leader influences the spark lenght but old
portions of channel might be fastly conductively
revived by "dart leaders" however.
Speaking of big top electrodes advantages in coiling
,I would rather think that the production of dart
leaders itself is not the key reason for longer arc
development.Superimposed current spikes feeding the
channel occur also in systems with comparatively
smaller electrodes and juding by scope trace images
the associated charge carried away is not too
In this sense "dart leaders" are sort of a byproduct
and are more or less expressed at the begging of the
breakdown. Besides,I can't even tell wether these are
true dart leaders or just some forms of a gas track
breakdowns frequency dependent on EM couplings in the
Anycase,the benefit of using larger isotropic
capacites I find in:
a)The larger the electrode-more charge available for
arc and its propagation by consequtive flashes.
b)Concentration of the resonator charge energy in one
place ,in the reservoir of low impedance (i.e. in top
electrode offering this way bypassing the high surge
impedance of the helix)
c)Change in volume of a space charge cloud around the
electrode and associated E-fields changes around the
d)As arc grows,its loading effect detunes 2-coil
system.Tunning point shift should be generally less
problematic for systems having resonators with big top
e)Lower resonating frequency and higher Q .
My opinion,for what is worth,