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Spark time duration

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LAN_403

Sun Dec 03 2006, 09:46PM

Banned on April 7, 2007
Registered Member #277 Joined: Fri Mar 03 2006, 10:15AM
Location: Florida
Posts: 157

I'll bet someone has discussed this before but I can't find it in the archives. I'd like to know the time duration of a spark jumping a 1cm spark gap, ie: how many nanoseconds does the spark exist? I realize there are 100 and 1 variables such as humidity, oxygen content of the air, the diameter of the spark gap spheres, and the question is broad, but anybody venture to make an educated guess or point me to a chart somewhere that shows this info? Yes, I googled it too, but nothing definative. Thanks in advance. CM

Avalanche

Sun Dec 03 2006, 09:54PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

I would think it is simply 'as long as the power supply can sustain it'.
You need to reach the breakdown voltage for the spark to initially strike, that's determined by a number of factors as you mentioned, but to sustain it you need to keep a current flowing...

Sun Dec 03 2006, 10:34PM

Banned on April 7, 2007
Registered Member #277 Joined: Fri Mar 03 2006, 10:15AM
Location: Florida
Posts: 157

Sorry, I should have been clearer. Assuming the spark events are seperated in time, ie: one spark jumps a 1 cm gap (two spheres compose the gap) about every 5 seconds, I am wondering how many nanoseconds the spark exists. Odd question I know, but maybe someone might have this info. The U.S. goverment pays millions to study the mating habits of the purple peckered termite during full moonlit nights, I assume somebody somewhere has looked into spark durations at sometime, just wondering if anyone knows where I might find that info

Thanks. CM

Avalanche

Sun Dec 03 2006, 10:49PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Hmm, unless I don't quite understand what you're asking, I'll stick by my original answer.
The only reason a spark would extinguish in the first place would be because the power is removed or reduced, which is usually a tank capacitor discharging through the spark gap. When that happens, the spark cannot re-initiate until the breakdown voltage is reached once again, so you get the rhythmic sparking. In this case, the duration of each spark would depend mostly on the capacitance of the capacitor bank.

Simon

Mon Dec 04 2006, 12:13AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

Yes, CM, this is kind of a "how long is a piece of string?" question.

You'd only be able to look at special cases, like lightning strikes, Marx generators, etc, and find rough typical values.

Maybe you could look at different aspects of sparks, like formation.

Colin 99

Mon Dec 04 2006, 01:20AM

Registered Member #192 Joined: Fri Feb 17 2006, 03:08AM
Location: Canada
Posts: 44

What is the source of the high voltage?

Tesladownunder

Mon Dec 04 2006, 11:17AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Here is my take on what happens. Take it with a grain of salt as I made most of it up.

The spheres have a capacitance of a few pF which will discharge rapidly limited by the resistance of the arc and the inductance of the 1cm path giving a discharge at a guess in 10's of ns.
What follows should be some ringing between the inductance of the path and capacitance of the spheres. with the ring down dependent on the resistance of the arc channel which is dependent on temperature and presumably total current that passes in that short time. Probably a few cycles of the same period ie another 20 - 30ns.

After that current flow will depend on the ability of the supply to give a current limited output. Consider DC first.
If the supply is terminated in a capacitor then current is delivered in a burst as the capacitor is discharged. This is slower and more likely to take microseconds rather than nanoseconds but after that there is no current flow while the cap recharges. The stored energy will give a bright single blue spark. It will also ring for a bit in the following microseconds or more.
If the supply is terminated in an inductor the current comes in slower again, in milliseconds and persists longer, also with ringing. It will resist breaking as this would interrupt the magnetic field of the inductor. The arc is soft, quiet and purple (or orange if sodium is present).
If the supply is terminated with a resistor the current can continue to be supplied slowly continuously. Average current is low. The arc is softer and more easily extinguished.

An AC supply is similar provided things happen in a timeframe less than the period of the AC. 50Hz AC will be different to 100kHz of a TC. The spark channel remains ionised for perhaps hundreds of nanoseconds after the spark passes but remains hot and a preferred path for a new spark for some milliseconds.

Peter
(Tesla-never-answer-a-simple- yes-or-no-downunder)
Pic shows a ringing spark of a TC at 100kHz viewed with a streak camera. The ringing originates from the TC rather than the gap and is a bit different to what I am talking about but helps give you the idea.

1165231029 10 FT18670 Hvrotmirrorteslabigspark1singel

Fusion

Mon Dec 11 2006, 09:28PM

Registered Member #354 Joined: Thu Mar 30 2006, 10:12AM
Location:
Posts: 55

I read that discharge through a spark gap happens until discharge of capacitors to an about 40% of breakdown voltage. Also I read that arc impedance goes to 0 as more current came in, so the spark duration may be driven by overall impedance. If you use an inductor, tha capacitor bank plus inductor make an oscillation (w^2=1/LC): it can be good in order to avoid to electrode be electron empty.
The spark gap also stores a lot of energy because low induction but lots of amps E=0.5*L*I^2.
If you set a serial inductor L1 to a spark gap: L2 then the energy stored (and released) at spark gap will be reduced to a factor= L2/(L1+L2)

I hope it is useful

Sulaiman

Tue Dec 12 2006, 12:51PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Maybe the question you're really asking is - how brief can a spark be?
Using special spark gaps and co-axial capacitors - tens of nanoseconds is the answer.
Small value capacitance with minimal series inductance in the discharge loop is required.

psychop

Mon Jan 08 2007, 01:35PM

Registered Member #83 Joined: Thu Feb 09 2006, 12:15PM
Location: Warsaw, Poland
Posts: 5

Look at TEA Laser keyword on the internet. Transverse Electrical discharge in Air - this is basically to get as short
duration sparks as possible. Involves minimum inductance, etc.

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