Re: Triggered spark gaps for coils

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-uswest-dot-net>" <bert.hickman-at-aquila-dot-net>

Marc,

Xenon flashlamps will most likely not make good replacements for Tesla Coil
spark gaps. Flashlamps tend to be lower pressure devices, and they thus
have fewer available charge carriers.  And, if they are longer length
linear or helical style, the cool walls of the lamp restricts the outward
growth of the plasma channel, thus introducing a marked positive resistance
characteristic. The combination results in a comparatively lossy discharge
(when compared to a free spark in air), with most of the pulse capacitor's
energy generating light (and heat) in the flashlamp instead of generating
sparks from your coil. In fact, most flashlamp circuits are overdamped by
design to prevent the flashlamp from turning off and then reigniting during
zero crossings. While this is desirable in a light emitting flashlamp, it's
not desirable in its function as a potential spark gap replacement. 

Another class of xenon lamp uses a continuous arc discharge instead of a
pulsed discharge. These "arc lamps" may be constructed as either "short
arc" or linear tubes, and are filled with xenon, krypton, mercury vapor or
a mix of the above depending upon the application. The arc length of a
"short arc" is typically measured in millimeters, while an arc lamp may
have a longer gap of 5-15 mm. Both types are typically filled with gas
under a pressure of 1-5 atmospheres, and high power short arc tubes may
reach internal pressures of over 20 atmospheres. This makes them behave
like little bombs if they accidentally burst! They can also explode during
careless handling at room temperature, so be very careful when working with
them. Since the latter devices are made to handle a large amounts of power
(multiple kilowatts), and since they use beefy tungsten electrodes, it
should be possible to use one of these as a spark gap if you can find a
surplus one cheaply enough. Unfortunately, the inability to rapidly remove
the ionized gas from between the electrodes, and the greater number of ions
(higher gas pressure, denser plasma, and shorter mean free path) will all
probably conspire to make these tubes very poor at quenching. 

BTW, I'm including some URL's for some excellent sources of material for
both flashlamps and arc lamps. One is material from a Laser and Electro
Optics training course created by the Center for Occupational Research and
Development's (CORD) - see http://www.cord-dot-org/index.cfm and then "Projects
and Activities", and then "Optics and Photonics Resources" and "LEOT Course
Materials" for details). A subset of these includes:
Pulsed Laser Flashlamps and Power Supplies (excellent theory and practical
treatment!):
  http://cord-dot-org/cm/leot/course03_mod02/mod03_02.htm

Flashlamps for Pulsed Lasers and Flashlamp Power Supplies:
  http://cord-dot-org/cm/leot/course04_mod04/mod04_04.htm 

Arc Lamps and Power Supplies:
  http://cord-dot-org/cm/leot/course04_mod05/mod04_05.htm

Perkin-Elmer also has an excellent site for both product and technical
information. One excellent paper is "Design Considerations for Triggering
of Flashlamps" at:
  http://opto.perkinelmer-dot-com/library/papers/tp10.htm

If you happen to get some more of these devices, give one a try on a low
power coil and let us know how well they work - or don't work... :^)

Best regards,

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "Metlicka Marc by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <mystuffs-at-orwell-dot-net>
> 
> bert, Gary, all
> i to have been giving this some thought. i was actually looking at the
> triggered xenon flash lamp as the source of the gap.
> if you've ever noticed them, they have the trigger electrode dead center
> between two tungsten points and are filled with the gas. it is built of
> heavy pyrex or something like quartz so cooling could be overcome.
> the one i had here met a sorrowful death under my cordless drill, but i
> see them come up on ebay once in a while. i don't think one would quench
> because the gap is only about an inch, but maybe three or four in series
> with only the first one as the triggered unit?
> or am i not understanding the xenon properties correctly? kind of a real
> poor mans triggered gas spark gap?
> marc
> 
<SNIP>