toroids (selecting the best size)

From:  FutureT [SMTP:FutureT-at-aol-dot-com]
Sent:  Saturday, April 18, 1998 1:26 PM
To:  tesla-at-pupman-dot-com
Subject:  Re: toroids (selecting the best size)

In a message dated 98-04-18 11:12:40 EDT, you write:

 >what factor(s) detrmine the size of the toroid a system should have?  i am
 >running a 6" secondary with 9kV 60-90mA.
 >kevin >>

Kevin, all,

As the size of the toroid is increased, the spark will generally get
longer, but at some point the sparks will become shorter again as
the toroid becomes too large for the sparks to easily break out.  The
main determinant of toroid size seems to be the power input to the
system, but bang-size, rep-rate, inductance ratios, cap ratios, voltage,
TC efficiency, quenching, etc, all have some effect on the optimal
toroid size.  

In general, smaller toroids will result in easier spark gap quenching,
so the best toroid size is somewhat of a compromise.  Often, good
results are obtained if the output streamers are 2 to 3 times the dia.
of the toroid.  Sparks that are more than 3 times the toroid dia. can
be obtained, but the efficiency may be lower.  This is just a guide.

I use a 6" x 26" toroid for a 15kV, 60ma NST powered coil.  The coil
draws about 2100 watts due to resonant charging with the .014uF cap.
Spark length is about 65".  Using a potential transformer, the same
spark length is acheived using only 1570 watts.  These measurements
are true watts as measured with a precision wattmeter.

There's a "middle ground" for toroid size where it doesn't make much
difference, i.e. the exact size is not too critical as long as it's in the
ballpark.  If the toroid within 15% of the correct size it should be ok.

Another way the toroid can effect the spark length is by
controlling the number of simultaneous streamers that are formed.
Larger toroids will tend to produce fewer streamers for a given system.
Often spark length can be maximized by selecting a toroid size that
produces only one streamer.  The number of streamers formed is also
affected by the coupling.  Often, looser coupling has the effect of
reducing the number of streamers and improving the quenching, both
of which can help to increase the spark length (assuming a properly
sized toroid is used).  

I think that if the TC power input is doubled, the toroid size should
be increased by 40%.  If the power input is increased by four times,
then the toroid size should be doubled (ballpark).

John Freau