Stretching the spark (was RE: Official air breakdown voltage?)

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Marco Denicolai" <marco.denicolai@xxxxxxxxxx>

Hello all,

Just to clear a couple of things...
1. The initial ionization required to start a (possible) breakdown is about
35-45 kV/cm.
2. When 1. is verified plus a suitable electron is found, the discharge
starts.
3. For distances over 1 meter, the leader starts growing as a "snake". On
the leader tip there is a corona "brush" made of streamers. The length of
the streamer part can be relevant (e.g. 80 cm). That's why shorter gaps are
bridged already by streamers (no leader involved).
4. The E field along the leader is about 1 kV/cm.
5. The E field on the leader tip is about 40-60 kV/cm.
6. If the E field at the tip of the streamers is over 4.5 kV/cm (for
positive polarity, for negative 5.5 kV/cm) then the leader grows length and
the discharge advances.

More, space charge and induced charge are produced in the gap. The applied
voltage must "win" them and achieve a net increment in the net gap charge.
This constraints the profile of the applied potential (crest time i.e. rise
time). Whenever the net charge starts decreasing, the gap current gets to
zero and the discharge stops.

So, inside of the gap you'll find 1 kV/cm, 30 kV/cm and whatever E field you
can imagine :)
Just going on with the discharge model design.... It ain't easy :-)

Regards

> -----Original Message-----
> From: Tesla list [mailto:tesla@xxxxxxxxxx]
> Sent: Tuesday, November 29, 2005 3:15
> To: tesla@xxxxxxxxxx
> Subject: Re: Official air breakdown voltage?
>
> Original poster: Aaron Banerjee <spam_proof@xxxxxxxxxxxxxxxx>
>
>
> I've seen anywhere from about 4Kv/cm for a point gap to 30Kv/cm for a
> spherical gap.  Once you get the spark going, I'm not sure how far it
> could be stretched -- I guess so long as you keep the "conductive" path,
> you could concievably go quite a ways (depending on amperage I suppose).
>
> If you're trying to calculate voltage from a spark, I'd suggest starting
> so far away that you don't get a spark, and then moving in until you first
> get breakdown.  Before doing this, of course, you will have taken
> appropriate safety precautions.
>
> I'm thinking that the length you get will be somewhat closer to the
> "breakdown voltage" you'll see in a table.  Maybe that could be used to
> estimate output voltage?
>
> On Mon, 28 Nov 2005, Tesla list wrote:
>
>  > Original poster: "JT Bowles" <jasotb@xxxxxxxxxxx>
>  >
>  > Amperage determines how far an arc may be drawn. Cannot it be true
>  > that at 20KV, one inch may be stretched to 10 feet? (perhaps if the
>  > amperage is INCREDIBLY HIGH, millions of amps)
>  > Thank you mike- 20Kv is a common measurement
>  >
>  > >From: "Tesla list" <tesla@xxxxxxxxxx>
>  > >To: tesla@xxxxxxxxxx
>  > >Subject: Re: Official air breakdown voltage?
>  > >Date: Sun, 27 Nov 2005 19:32:23 -0700
>  > >
>  > >Original poster: "Mike" <mike.marcum@xxxxxxxxxxxx>
>  > >
>  > >Well, 60Hz  is weird and definitely not linear. Haven't seen a
>  > >breakdown table like DC,  but it's around 20kV for 1", 100kV for
>  > >10-12", and 11 feet for 500kV (from a lineman that works on 'em hot
>  > >and needs to know that distance). It's roughly following the rule of
>  > >thumb 5x the voltage=10x the distance with current controlling how
>  > >far it grows after that.
>  > >
>  > >Mike
>  > >----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
>  > >To: <tesla@xxxxxxxxxx>
>  > >Sent: Sunday, November 27, 2005 11:54 AM
>  > >Subject: Re: Official air breakdown voltage?
>  > >
>  > >
>  > >>Original poster: "JT Bowles" <jasotb@xxxxxxxxxxx>
>  > >>
>  > >>I say you are correct. Mostly. Voltage requirements for dielectric
>  > >>beakdown, in this case atmosphere, are linear, at slow-pulse DC
>  > > From what I have been reading lately, AC voltage requirements for the
> same
>  > >>dielectric are often much higher. Thus, we can conclude ac requires
>  > >>a much higher voltage to generate, say 1' arcs, than dc does.
>  > >>
>  > >>
>



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