Solid Toroid Beneficial?

To: tesla-at-pupman-dot-com
Subject: Solid Toroid Beneficial?
From: "Gary Johnson" <gjohnson-at-ksu.edu> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Sun, 12 Mar 2000 16:11:15 -0700
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme
I have been working on a solid state drive for a Tesla coil for many years.
Standard wisdom has been that sparks will be inferior to those produced by
conventional Tesla coils (with a spark gap), but explanations given always
seemed a bit vague to me.  I have been trying to combine theory and
experiment in such a way that they will be readily understood by electrical
engineers, even if my sparks are not as long as those of conventional TCs.
I think I am getting close to understanding what is going on, but hit
something the other day that surprised me.  Perhaps someone can help me past
the mental block.

I am testing a variety of coils and toroids, but for purposes of discussion,
I will describe one coil and two toroids.  The coil is 14 ga magnet wire
spacewound on a polyethylene form, diameter 39.64 cm, winding length 116.62
cm, 387 turns.  One toroid is half spun aluminum, 6 inches by 24 inches. I
assume it was made by starting with a flat disc of aluminum perhaps 40
inches in diameter, and turning the edge down and back in until the outer
portion looks like a toroid.  I set it on the coil so the bottom lip is
level with the top turn and only an inch or so away.  It would definitely
form a shorted turn at the top of the coil.  The other toroid is made from
about 15 lengths of quarter inch copper tubing spaced about a quarter inch
apart.  The center is open, so there is no place for eddy currents to flow.
Size and capacitance are almost identical to the half spun toroid, so
resonant frequency is basically the same.

The coil is driven directly at the base by a square wave from an IGBT
inverter. Think of an extra coil being driven by a function generator.  The
power rating is on the order of 50 kW for short periods.  I have actually
seen a peak power of 33 kW at the point of spark initiation.  It has the
capability of CW operation, but because of thermal and circuit breaker
constraints I mostly use disruptive operation.  Power is applied for say 4
to 10 ms for 1 to 10 times per second.  Sparks look like standard disruptive
TC sparks, white, thick, up to 3 ft long in air.  A coil this size with a
properly sized top load should produce sparks at least 10 ft long.  The key
is the peak power applied.  A conventional TC driven by a pig with an
average power of 10 kW would be supplying peak powers to the coil of at
least 100 kW during discharge.  Other things being equal, a peak power of
100 kW will always produce a longer spark than 33 kW.  It might be possible
to build solid state drives capable of pushing hundreds of kW into a TC, but
it would be a challenge!  So my inverter is not going to win any spark
length competition, but it is nice for making measurements on coils.

I measure the top voltage in two ways.  One is a fiber optic system (good
for any voltage) and the other is a capacitive voltage divider good up to
100 kV.  The latter is used to calibrate the former.

Finally, I am ready to ask my question.  If I am not mistaken, the standard
wisdom among Tesla coilers is that shorted turns are bad.  Eddy current
losses will lower the efficiency of the TC.  However, the current and
magnetic field decrease as one goes to the top of a TC, so a shorted turn at
the top does not decrease performance a noticeable amount.  The 'big dawgs'
do not cut their beautiful toroids with the hope of getting a few more
inches of spark.  But I am measuring the opposite effect.  The shorted turn
toroid actually improves performance.  I am testing well below spark
breakout, with input power in the range of 100 to 800 Watts.  At fixed input
voltage, the shorted turn toroid will have a top voltage about 10 to 15
percent higher than that of the open turn toroid.  The input current will be
10 to 15 percent higher as well, for the shorted turn toroid.  The trends
are consistent: more current = more input power = higher top voltage =
longer spark (if voltage is high enough for breakout).  The surprise is that
this occurs for the shorted turn toroid rather than the open turn toroid.
Has anyone else seen this effect?  Does the effect show up in PSPICE models?
Is there a 'simple' explanation?

I have noticed my name on the list lately in regard to some early results I
published about input impedance.  Anything I have written about Tesla coils
is in the public domain.

Gary Johnson