Re: TSG Configuration

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

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

Greg,

There are many different styles of triggered gaps. The type you saw on the
EG&G site is a style called a trigatron. The HV trigger pulse can cause
initial spark breakdown between either the trigger electrode and the
adjacent electrode or from the trigger electrode to the opposite electrode
(preferred). In either case, a trigger pulse of sufficient energy will
cause breakdown of the main gap through either direct streamer injection
into the main gap, or via the creation of large numbers of free electrons
through UV photoionization combined with a bit of field distortion, which
is then followed by avalanche breakdown of the main gap. At least one
theorist proposes that shock ionization from the triggering spark may also
occur. Trigatrons operate best when the main electrode voltage is 50-80% of
that required for self breakdwon.

A true field distortion gap uses a trigger electrode that is usually
centered halfway between the main electrodes. The trigger electrode has a
rather wide hole in its center, often with a sharp chisel-like profile. The
trigger electrode is typically biased to a DC voltage that's halfway
between the voltage applied across the main electrodes. A pair of equal
value HV resistors is used to form a voltage divider which DC biases the
trigger electrode. The HV trigger pulse is AC coupled to the trigger source
via a HV (typically a doorknob) capacitor. The trigger pulse alters the
electric field in the gap, overvolting one half of the gap, causing it to
break down between a main electrode and the trigger electrode. This causes
breakdown from the center electrode to the other main electrode, completing
the path between main electrodes. Erosion of the trigger electrode is a
wearout mechanism for both trigatron or field distortion gaps.

There are many other styles of triggered gaps - the specific designs vary
depending upon the desired holdoff voltage, peak current, gap inductance,
coulombs/shot, and repetition rate required. Some gaps are designed to use
special electron-attaching (electronegative) dielectric gases such as
sulfur hexafluoride (SF6), oxygen (O2), or Freon (CF2CL2), sometimes at
high pressure to further increase holdoff voltage and reduce corona. Other
gaps use forced gas flow to cool and purge ions from the gap region, and
gaps used in repetitive applications may require the use of hydrogen gas to
speed recovery after firing. Sometimes field distortion and trigatron
principles are used (as in a "swinging cascade" gap). Field distortion is
also used for special low inductance rail gaps, where the conductive path
is actually occurs on the surface of a solid insulator. 

Spark gap designs combine a degree of art with science. Triggered spark gap
design is not a science that most coilers are familiar with... so its not
surprising that there are significant differences between commercial gaps
and those currently being designed by coilers. There are many ways to skin
a cat, some of which may still remain to be discovered... :^)

BTW, some excellent resources for spark gap design include the following
books (most are out of print but available in large technical libraries):
Frungel, "High Speed Pulse Technology", Vol 1, Academic Press, 1960, 620pp

Frungel, "High Speed Pulse Technology", Vol 3, Academic Press, 1976, 498pp

Glasoe, G. N. & J. V. Lebacqz, "Pulse Generators", McGraw-Hill, 1948, 741pp 

Sarjeant, W. J. and Dollinger, Richard E., "High Power Electronics", Tab
Professional and Reference Books, 1989, 392pp 

Schaefer, Gerhard, "Gas Discharge Closing Switches (Advances in Pulsed
Power Technology, V2)", Plenum, 1991, 569pp

Vitkovitsky, Ihor, "High Power Switching", Van Nostrand Reinhold, 1987,
304pp

Some excellent information on triggered gaps is also available on the web
from:
http://www.clare-dot-com/home/PDFs.nsf/www/trig_an.pdf/$File/trig_an.pdf
http://opto.perkinelmer-dot-com/Downloads/switchtb.pdf
http://www.marconitech-dot-com/sparkgap/3elec_sg.pdf

Good luck!

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

Tesla list wrote:
> 
> Original poster: "Gregory Hunter by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <ghunter31014-at-yahoo-dot-com>
> 
> Dear List,
> 
> I did a little research on the TSG (couch potato
> style, on the internet). I came across some commercial
> sites including EG&G and others. The most common
> drawing I saw used two main electrodes facing each
> other across an air (or other gas) gap, with a slender
> trigger electrode located coaxially inside one of the
> main electrodes. The trigger electrode appeared to be
> inside an insulated sheath, with the business end of
> it brought flush with a hole in the face of the main
> electrode. This suggests that the trigger spark would
> only need jump a very short distance to the edge of
> the hole in the main electrode, and this somehow
> triggers the main gap. Is this field distortion, or
> some other mechanism such as ion flooding? How come
> home brew spark gaps and commercially made spark gaps
> are so different from each other? The physical
> arrangement doesn't even seem related.
> 
> Greg
> http://hot-streamer-dot-com/greg
> 
> __________________________________________________
> Do You Yahoo!?
> Get email alerts & NEW webcam video instant messaging with Yahoo! Messenger
> http://im.yahoo-dot-com