Re: A Puzzle
From: Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: bert.hickman-at-aquila-dot-com
Sent: Thursday, September 04, 1997 2:24 AM
To: Tesla List
Subject: Re: A Puzzle
Tesla List wrote:
> From: Richard Wayne Wall[SMTP:rwall-at-ix-dot-netcom-dot-com]
> Sent: Wednesday, September 03, 1997 12:15 PM
> To: Tesla List
> Subject: Re: A Puzzle
> Bert wrote:
> >The current levels required to support streamer growth are in the
> >multi-ampere range.
> Bert, have these current levels been experimentally measured and
> validated in the multi-ampere range?
Yes. There's actually quite a large body of research that's been done in
the area of long-spark "arrested streamers" over the years. These
experiments typically use a high voltage impulse generator (usually a
triggered Marx Generator) connected to a rod-plane (non-uniform field)
test gap. In parallel with the test gap is another triggered spark gap
which can be fired at an adjustable interval after the start of the
applied HV impulse. A HV current transformer (typically a Rogowski coil)
and oscilloscope are used to monitor the streamer current flowing to the
rod, and high speed cameras or image converters are used to record the
growth of the streamers in the test gap. The optical and electrical
results are then correlated, giving a profile of streamer growth versus
current flow. Some references for this research are presented below:
1. "Physical Models of Long Air Gap Breakdown Processes" by I.
Gallimberti, in "Electrical Breakdown and Discharges in Gases", Volume
B, edited by Kunhardt and Luessen, Plenum Press, 1983. Pages 288 and 289
show examples of streamer propagation currents that are in the 12 Amp
range for large radius of curvature electrodes, and in the 2.5 Amp range
for small radius electrodes.
2. "Spark-Over Characteristics of Long Gaps" by G. Baldo in "Electrical
Breakdown and Discharges in Gases", Volume B, edited by Kunhardt and
Luessen, Plenum Press, 1983. Page 301 shows a direct correlation between
streamer (leader) velocity and current "spikes" in the 3 Amp range.
Interestingly, current spikes for positively polarized electrodes tend
to be significantly higher than for negatively polarized electrodes. On
page 306 a picture of an arrested streamer looks like a dead ringer for
a Tesal Coil streamer that's just below the point of connecting to a
3. "Electrical Breakdown of Gases" by J. M. Meek and J. D. Craggs,
Oxford Press, 1953, Chapter IV ("Experimental Studies of Spark
Discharges"). Meek presents several studies, involving long length gaps
with various values of series resistance and paralleled capacitance. In
one study with 20,000 Ohms in series with the gap (pages 203 and 204),
leader currents were typically in the range of 3.5 A for a 10 cm gap, up
to 15 Amps for a 55 cm gap.
Closer to home, it turns out that a Tesla Coil throwing off streamers
into the air (not ground strikes) appears to behave in a similar
fashion. In our case, the streamers are apparently "arrested" by
reaching a balance of input energy versus dissipative losses. However,
streamer appearance is very similar to "stills" taken by the above
high-priced equipment used by funded researchers. Furthermore, you can
get a feel for the magnitudes of the currents involved by forcing the
streamers to emerge from a projecting wire which is connected through a
small wattage 115 Volt light bulb to the top of the toroid. I happen to
use a 25 watt tubular bulb, but a regular style light bulb will also
work as well. When the coil is running at full power, the bulb is lit by
the streamer currents forced to flow thru the bulb and "into" the air.
By comparing the degree of brightness with an identical lamp driven from
a DC source, I found that this corresponds to about 110 mA of "average"
current. At full bore, my coil is firing at an "average" of 420 BPS.
Now, from oscilloscope measurements, under heavy streamer production,
the amount of time taken to "ring down" the secondary is about 150-200
uSec per bang. The maximum "on-time" every second is thus about 200
usec/Bang times 420 Bangs/Second or about 84 mSec. This corresponding to
a maximum on-time duty cycle of about 8.4%. This implies an "average"
streamer current of about 0.110/0.084 = 1.3 Amps. In reality, the
streamer currents are NOT constant during the entire discharge and
ringdown period, and peak streamer currents are significantly higher -
probably at least by a 3-5X factor. Not precise results by any
stretch... but certainly of a similar magnitude measured by the
researchers above. More definitive measurements will come when Greg
steps inside the toroid of his new coil and actually measures these
currents in real-time! :^)
Safe coilin' to you!
-- Bert --