Pushing the IGBT Envelope Discussion
Edited/Updated: October 23, 2004
First Post
Date : Fri, 10 Sep 2004 07:01:18 -0600. Subject : RE: Pushing the IGBT Envelope
Original
poster: "Steve Conner"
>I'm going to start checking the dumpsters behind the building here at the
>laboratory where this work is being performed.
I think that's Greg Leyh's project. His work with IGBTs on those klystron
modulators indirectly inspired the current crop of big DRSSTCs. So what goes
around comes around I guess :). Steve C.
Date : Fri, 10 Sep 2004 18:08:43 -0600. Subject : RE: Pushing the IGBT Envelop
Original
poster: "Dave Leddon"
After reading that article yesterday I went over to the lab where the modulators
were developed and spent an hour with Ed Cook who was one of the design
engineers who worked on the project. He showed me some of the current batch of
modulators as well as some earlier pulse generators based on mosfets that were
developed for Los Alamos. Very impressive stuff. I particularly liked the
modulator transformer which consisted of a single
turn, oil-cooled primary. I showed him a copy of Dan McCauley's DRSSTC
construction notes, figuring that any pulse power engineer would be enthralled,
but he came back with the familiar refrain, "But what's it good for"? I then
mentioned Greg Leyh's work on Electrum and he said "Oh, I know Greg". Small
world. Dave. Pleasanton, CA.
Date : Fri, 10 Sep 2004 23:14:25 -0600. Subject : Re: Pushing the IGBT Envelop
Original
poster: "Eastern Voltage Research Corporation"
Yeah, gotta love that phrase. I hear it ALL the time! By the time you
hear it the 50th time, you just want to smack them!
> I showed him a copy of Dan McCauley's DRSSTC construction notes, figuring that any pulse power engineer would
>be enthralled, but he came back with the familiar refrain, "But what's it
good for"? I then mentioned Greg Leyh's >work on Electrum and he said "Oh, I
> know Greg". Small world. Dave, Pleasanton, CA.
Date : Sat, 11 Sep 2004 10:18:07 -0600. Subject : RE: Pushing the IGBT Envelop
Original
poster: "Greg Leyh"
Hi Dave,
I'm assuming that you work at LLNL? Next time you're over at Ed's lab you
should ask to see some of their Inductive Adder work. Beautiful stuff.
Unfortunately, the entire 'warm' design plan for the NLC has been abandoned, in
favor of the superconducting design. No more need for 500kV, 550A pulsed power
supplies. The 10MW 'cold' klystrons will require only 120kV at 140A, but for
long pulses... about 26kJ per pulse, and 5 pulses per second. Still lots of
interesting design issues, however.
BTW, a single-die IGBT package is now commercially available, using the 4500V
Mitsubishi silicon we've been playing with at SLAC/LLNL. Here's the preliminary
spec sheet:
http://www.pwrx.com/pwrx/docs/qis4506001.pdf
This single-die package appears to perform well, with minimal lead inductance
and a substantial heat spreader. The prop delays and rise/fall times are tight,
as well.
Another single-die package featuring 6500V silicon from Infineon is currently in
development through another company, although I've gained a fondness for the
4500V SPT silicon over the 6500V NPT... The 4500V material just seems to be
more robust, and well behaved. We've operated the 4500V 60A dies with fault
currents of >1000A per die, at >2kA/uS di/dt.
PS -- I tried to look up 'DRSSTC' but it's not in the pupman acronym list. What
does that stand for? It'd be nice to find a glossary of all the acronyms that
fly about on the list these days.
Date : Sat, 11 Sep 2004 18:12:58 -0600. Subject : Re: Pushing the IGBT Envelop
Original poster: "Steve
Ward"
Hi Greg,
Very fascinating stuff going on at SLAC. I've also read some of your papers
about IGBT module design (and the downfall in commercial modules layouts).
Anyway, DRSSTC is a Dual Resonance Solid State Tesla coil. We basically drive a resonant tank circuit
with a solid state driver (using IGBTs) and pulse its operation to achieve very
high peak powers. Jimmy Hynes was the first person to have a very successful
coil built under this idea. I think my coil is currently the largest:
http://hot-streamer.com/srward16/DRSSTC-2.htm
It uses 300A IGBT (dual modules) rated 1200V. We typically overdrive the gates
so that we can push greater currents (my system runs at about 1200A peak)
through smaller IGBT's, mainly since us amateurs cant afford to buy enough
silicon to actually run these things in their SOA... and after all, this is
tesla coiling!
The DRSSTCs are, as you might expect, slightly more efficient than even the best
SGTCs. My coils seem to follow the formula of: spark length (inches)=2*sqrt(power).
I was almost hoping that if the advanced lightning facility every became
reality, that it would be a DRSSTC. Mainly because you would not need extremely
high voltage silicon to replace a spark gap. With a DRSSTC, our bang energy is
not limited to the storage energy in the tank capacitor, it can be much greater
(usually its 2X as much). I think the fact that my large DRSSTC can make 11'
sparks from just 600VDC rail voltage and only 4-5kW shows promise for even
larger coils (silicon permitting). Steve.
> PS -- I tried to look up 'DRSSTC' but it's not in the pupman acronym
> list. What does that stand for? It'd be nice to find a glossary of all the acronyms that fly about on the list these days.
Date : Sat, 11 Sep 2004 18:13:50 -0600. Subject : Re: Pushing the IGBT Envelop
Original
poster: "Dan Strother"
DRSSTC stands for "Double/Dual Resonant Solid State Tesla Coil." It's a topology
initially pioneered by Jimmy Hynes (
http://www.hot-streamer.com/chunkyboy86 ) and then further advanced and
publicized by Steve Ward (
http://www.hot-streamer.com/srward16 ).
By using a bridge of IGBTs driving a tuned primary circuit, this variety of
SSTCs can match and, quite often, exceed the efficiency of a spark gap coil
(Steve's largest DRSSTC produces nearly 12' streamers from less than 5kW).
Dan Strother.
On Sat, 11 Sep 2004
10:18:07 -0600, Tesla list
<tesla@pupman.com> wrote:
> Original poster: "Greg Leyh"
>
> PS -- I tried to look up 'DRSSTC' but it's not in the pupman acronym
> list. What does that stand for? It'd be nice to find a glossary of all the acronyms that fly about on the list these days.
--
Dan Strother
http://www.dan.4hv.org/
"The major difference between a thing that might go wrong and a thing that
cannot possibly go wrong is that when a thing that cannot possibly go wrong goes
wrong it usually turns out to be impossible to get at or repair." - Douglas
Adams.
Date : Sun, 12 Sep 2004 18:08:37 -0600. Subject : Re: Pushing the IGBT Envelop
Original
poster: "Steve Ward"
Comments:
On Sun, 12 Sep 2004
16:33:26 -0600, Tesla list
<tesla@pupman.com> wrote:
> Original poster: "Greg Leyh"
>
> Interesting website, and coil work! I'm still unclear on the DRSSTC theory
> of operation, however. Near as I can figure, the system actually runs as
> an oscillator, but in a quasi-CW mode for relatively short bursts, at an
> unspecified dwell and repetition rate. Is this roughly correct?
Yes, thats correct. Some DRSSTCs use an oscillator to drive the H-bridge, but i
use feedback taken from the secondary coil's base current. Either way seems to
work well, though its not entirely that simple as you might imagine.
The coil operates for maybe 200-300uS and then is at rest for several mS
(depending on the desired BPS). The duty cycle on my system is only 3%.
>
> H-bridge drives are great for power converters to 100's of kW, and
> especially when operation from a unipolar DC source is required.
> However, an H-Bridge plan wouldn't be cost-effective for the ALF since it
> requires 4X the silicon for the same primary peak current and voltage swing
Hmm, what would be the alternative? I sort of had the idea that you were going
for an OLTC-like scheme, where you charged the tank cap and dumped them into the
primary like a normal TC, but replaced the spark gap with IGBTs or some other
thyristor. Its interesting to make a very crude comparison between my DRSSTC-2
and Steve Conners OLTC-2. We are both using about the same amount of
silicon (i use 4x 300A IGBTs, while he uses 2x 600A IGBTs). We both run just
about the same input power (around 4kW). His coil makes a 7' spark, while mine
does 11' ;-).
But another big question is, would an OLTC become more efficient at higher
voltages? Say if it was possible to get IGBT stacks up to 15-30kv range? On
the other hand, the DRSSTC seems to work well in all cases, even with much lower
input voltage. I run 600VDC on my coil while Conner uses 1kv if i remember
correctly.
> -- an important consideration when the material costs for the silicon alone is likely to be mid 6-figures.
Yikes!
H-bridges are also susceptable to
> 'shoot-through' failure, where timing errors or cosmic-ray induced firing
> of IGBTs can hard-short the DC mains, resulting in catastrophic failure of the entire IGBT stack.
Yes, a full-bridge is definately more complicated and requires more IGBTs. But,
if you weigh in efficiency... or rather the fact that the DRSSTCs require less
peak current for a specified spark length, it may turn out to be the better
option.
For perspective, here's a photo of a catastrophic
> H-bridge drive failure at another lab, believed to have been caused by a
> cosmic ray induced false-triggering one of the IGBTs:
>
http://www-group.slac.stanford.edu/esd/HBridgeDebris.jpg
Impressive ;-). Ive seen plenty of my own IGBT failures with shattered IGBT
cases spread on the floor afterward.
>
> The IGBT casings and structures can be seen evenly distributed on the
> floor. This failure occurred on a system operating at several hundred
> kW... imagine scaling this up to a 5MW system! It's clear that
> partitioning the primary drive circuitry in order to isolate component
> failures will be essential for a 5MW primary drive system.
Yeah, *complete* a failure on such a system would be a huge financial set back!!
Steve Ward.
Date : Sun, 12 Sep 2004 16:33:26 -0600. Subject : Re: Pushing the IGBT Envelop
Original
poster: "Greg Leyh"
Interesting website, and coil work! I'm still unclear on the DRSSTC theory of
operation, however. Near as I can figure, the system actually runs as an
oscillator, but in a quasi-CW mode for relatively short bursts, at an
unspecified dwell and repetition rate. Is this roughly correct?
H-bridge drives are great for power converters to 100's of kW, and especially
when operation from a unipolar DC source is required. However, an H-Bridge plan
wouldn't be cost-effective for the ALF since it requires 4X the silicon for the
same primary peak current and voltage swing -- an important consideration when
the material costs for the silicon alone is likely to be mid
6-figures. H-bridges are also susceptible to 'shoot-through' failure, where
timing errors or cosmic-ray induced firing of IGBT's can hard-short the DC
mains, resulting in catastrophic failure of the entire IGBT stack. For
perspective, here's a photo of a catastrophic H-bridge drive failure at another
lab, believed to have been caused by a cosmic ray induced false-triggering one
of the IGBTs:
http://www-group.slac.stanford.edu/esd/HBridgeDebris.jpg
The IGBT casings and structures can be seen evenly distributed on the
floor. This failure occurred on a system operating at several hundred kW...
imagine scaling this up to a 5MW system! It's clear that partitioning the
primary drive circuitry in order to isolate component
failures will be essential for a 5MW primary drive system.
Date : Sun, 12 Sep 2004 18:08:53 -0600. Subject : Re: Pushing the IGBT Envelop
Original poster: "Antonio
Carlos M. de Queiroz"
Tesla list wrote:
>
> Original poster: "Greg Leyh"
>
> Interesting website, and coil work! I'm still unclear on the DRSSTC theory
> of operation, however. Near as I can figure, the system actually runs as
> an oscillator, but in a quasi-CW mode for relatively short bursts, at an
> unspecified dwell and repetition rate. Is this roughly correct?
My investigation, so far only theoretical, shows that a DRSSTC can be operated
almost exactly as a magnifier would (there are three simultaneous oscillations
involved), generating a voltage rise at the output while the driver is operating
and reaching an instant
where all the energy in the system is in the output capacitance. At this
point, if the driver is switched off, the energy is trapped in the secondary
system, as happens in a conventional Tesla coil after the quenching of the
primary spark gap.
The possible advantages over a capacitor-discharge system are:
- The voltage gain can be much higher.
- The primary capacitor must store only about 1/4 of the total "bang" energy. (Can be less, as much as you want, but with less than about 1/10 the required element values may be impractical)
It is possible to continue to operate the driver after breakout, but the input
current may grow excessively.
It is also possible to operate the driver at one of the (two) resonance
frequencies of the system, but this causes a relatively slow increase of the
output voltage, with the input current increasing too.
I wrote a program that implements my ideas about how to design these systems,
and that can simulate the structures too. The "sstcd" program is available at
http://www.coe.ufrj.br/~acmq/programs . Antonio Carlos M. de Queiroz.
Date : Mon, 13 Sep 2004 07:52:08 -0600. Subject : Re: Pushing the IGBT Envelop
Original
poster: "Greg Leyh"
Hi Antonio,
Nice program! I'll have to work with it some more to understand it more fully,
though.
For some reason it took my inputted values and changed them? I probably entered
them
in the wrong spot. Have you studied primary drive scenarios that attempt to
recover the unused energy from the secondary? I think this feature will be
essential for the ALF. I'm
presently designing an energy recovery scheme into the 1:12 scale ALF prototype,
and hope to test it soon.
>Original poster: "Antonio Carlos M. de Queiroz"
>Tesla list wrote:
> >
> > Original poster: "Greg Leyh"
> >
> > Interesting website, and coil work! I'm still unclear on the DRSSTC theory
> > of operation, however. Near as I can figure, the system actually runs as
> > an oscillator, but in a quasi-CW mode for relatively short bursts, at an
> > unspecified dwell and repetition rate. Is this roughly correct?
>
>My investigation, so far only theoretical, shows that a DRSSTC can be
>operated almost exactly as a magnifier would (there are three
>simultaneous oscillations involved), generating a voltage rise at
>the output while the driver is operating and reaching an instant
>where all the energy in the system is in the output capacitance.
>At this point, if the driver is switched off, the energy is trapped
>in the secondary system, as happens in a conventional Tesla coil
>after the quenching of the primary spark gap.
>The possible advantages over a capacitor-discharge system are:
>- The voltage gain can be much higher.
>- The primary capacitor must store only about 1/4 of the total
> "bang" energy. (Can be less, as much as you want, but with less
> than about 1/10 the required element values may be impractical)
>It ispossible to continue to operate the driver after breakout,
>but the input current may grow excessively.
>It is also possible to operate the driver at one of the (two) resonance
>frequencies of the system, but this causes a relatively slow increase
>of the output voltage, with the input current increasing too.
>
>I wrote a program that implements my ideas about how to design
>these systems, and that can simulate the structures too. The "sstcd"
>program is available at
http://www.coe.ufrj.br/~acmq/programs
>
>Antonio Carlos M. de Queiroz.
Date : Mon, 13 Sep 2004 07:52:26 -0600. Subject : Re: Pushing the IGBT Envelop
Original poster: "Greg
Leyh"
>
>Original poster: "Steve Ward"
>Comments:
>
>
>On Sun, 12 Sep 2004 16:33:26 -0600, Tesla list
<tesla@pupman.com> wrote:
> > Original poster: "Greg Leyh"
>[snip]
> > H-bridge drives are great for power converters to 100's of kW, and
> > especially when operation from a unipolar DC source is required.
> > However, an H-Bridge plan wouldn't be cost-effective for the ALF since it
> > requires 4X the silicon for the same primary peak current and voltage swing
>
>Hmm, what would be the alternative? I sort of had the idea that you
>were going for an OLTC-like scheme, where you charged the tank cap and
>dumped them into the primary like a normal TC, but replaced the spark
>gap with IGBTs or some other thyristor. Its interesting to make a
>very crude comparison between my DRSSTC-2 and Steve Conners OLTC-2.
>We are both using about the same amount of silicon (i use 4x 300A
>IGBTs, while he uses 2x 600A IGBTs). We both run just about the same
>input power (around 4kW). His coil makes a 7' spark, while mine does
>11' .
It's rather impressive just on general principles to produce an 11ft arc using
600V mains!
It implies either a lot of voltage multiplication or very precise arc creation
and maintenance. I'm not exactly certain what an 'OLTC' entails, but
essentially the ALF
would employ a rather straightforward capacitor -> series-switch -> primary
arrangement, which would exhibit primarily I2R conduction losses since all
switching events would be at
zero current. The trickier issues lie in charging the capacitors from the
mains, and
managing the energy properly as it bounces back and forth between primary and
secondary.
>But another big question is, would an OLTC become more efficient at higher voltages? Say if it was possible to get
>IGBT stacks up to 15-30kv range? On the other hand, the DRSSTC seems to work well in all cases, even with much
>lower input voltage. I run 600VDC on my coil while Conner uses 1kv if i remember correctly.
True, the efficiency will increase with rising input voltage, due to the quickly
falling I2R losses. However at some point the loss improvements become
marginal, and the high
voltage issues start to rear their ugly head. The final choice must be
dependent on efficiency, corona issues, switch capabilities, total cost,
etc. For the 1:12 ALF prototype I've settled on ~2400V @ 2800A peak,
representing a compromise between I2R losses and device ratings.
Date : Mon, 13 Sep 2004 07:53:44 -0600. Subject : RE: Pushing the IGBT Envelop
Original
poster: "Steve Conner"
>However, an H-Bridge plan wouldn't be cost-effective for the ALF since it
>requires 4X the silicon for the same primary peak current and voltage swing
I appreciate this, but so far (as Steve Ward already said) the findings are that
the DRSSTC needs about 1/8 of the peak primary current that an OLTC does, for a
given spark length. So it all comes out at least even, if not in favour of the
DRSSTC.
Steve, Jimmy H. and I have been engaged in a "Battle of the Coil Topologies" for
the last year or so, and the DRSSTC has come out the clear winner in terms of
"Feet of spark per buck spent on silicon".
I am running my OLTC primary circuit at 1kV and 5000A peak, and getting a bang
energy around 8J and sparks just over 6ft. Now compare that with Steve who runs
his H-bridge at 700V and 1200A peak, and gets 11ft sparks.
In fairness I must mention that you don't get something for nothing. The DRSSTC
has a lower peak current but it goes on for a longer time. Hence, Steve's IGBT
bricks see a much higher I2t than mine. Mine are on something like 90 A2s
(amp-squared-seconds) out of an available 2800. Steve's run about 220 and they
are rated for 360 at a case temperature of 25 degrees.
>timing errors or cosmic-ray induced firing of IGBTs can hard-short the DC mains
Indeed. The only thing the OLTC has in its favour is that (at the present state
of the art) it's less likely to explode violently. Steve C.
Date : Mon, 13 Sep 2004 17:13:14 -0600. Subject : RE: Pushing the IGBT Envelop
Original
poster: "Steve Conner"
>It's rather impressive just on general principles to produce an 11ft arc using 600V mains!
It sure is. I think Steve's setup just plugs into a standard 240V outlet and
uses a voltage doubler rectifier to give around 600V dc.
>I'm not exactly certain what an 'OLTC' entails, but essentially the ALF would employ a rather
>straightforward capacitor -> series-switch -> primary arrangement
Well, that's exactly what an OLTC is- a DC resonant charging Tesla coil with the
spark gap replaced by an IGBT switch. Terry Fritz did the first "amateur" one,
he called it the "OLTC" and the name stuck. There have been about nine or ten
built since.
>For the 1:12 ALF prototype I've settled on
>~2400V @ 2800A peak,
>representing a compromise between I2R losses and device ratings.
Yay! That means my OLTC II is a 1:2 scale model of your 1:12 scale model. I used 1kV @
5000A peak, so if your system performs similarly to mine you'll get
73" x (2400/1000) x sqrt (2800/5000)= 131"
See the tech spec of the OLTC II coil here
http://www.scopeboy.com/tesla/t4spec.html
There is also a pic (by James Pawson) of it producing a 6'1" arc at Derby 2004
http://www.scopeboy.com/tesla/oltc2_6foot1.jpg
>The trickier issues lie in charging the capacitors from the mains, and managing the energy properly
>as it bounces back and forth between primary and secondary.
Again, I just used DC resonant charging from a 600V DC supply, which I got from
240V AC with a voltage doubler. As for managing the energy, I opened the IGBT
switch at the first "Notch" to trap it all in the secondary. I reckoned it would
do less harm there, than sloshing around the primary heating up the IGBTs. In
practice though, my giant IGBT blocks seem to run cool no matter what I do.
Steve C.
Date : Tue, 14 Sep 2004 08:00:22 -0600. Subject : Re: Pushing the IGBT Envelop
Original
poster: "Antonio Carlos M. de Queiroz"
Tesla list wrote:
> Original poster: "Greg Leyh"
> Nice program! I'll have to work with it some more to understand it more
> fully, though.
> For some reason it took my inputted values and changed them? I probably
> entered them
> in the wrong spot.
The program really has some unclear parts... When you choose one of the two
desigh buttons, what you have entered after pressing the "edit elements" button
is replaced.
The "lossy design" is the impedance matching idea using a Butterworth filter. It
optimizes the input impedance for a certain amount of load. The "lossless
design" is similar to the design of a magnifier, with the operation mode defined
by three integers k:l:m, that are the ratio of the three frequencies that appear
in the waveforms. Note that both designs are almost identical if the load is
removed. The other parameters are for the built-in simulator, that always works
over the element values that appear when the "edit elements" button is pressed.
The simulator assumes a linear resistive load, that is on of off all the time,
and leaves the driver operating all the time (to be improved later).
> Have you studied primary drive scenarios that attempt to recover the unused energy from
> the secondary? I think this feature will be essential for the ALF. I'm presently designing
> an energy recovery scheme into the 1:12 scale ALF prototype, and hope to test it soon.
The lossless design ideally recovers all the energy, at least if some of it is
removed from the output capacitance at the instant when the maximum voltage is
reached. It would also recover most of the energy left unused by a load
connected to the output when the output voltage is close to the maximum. But the
driver would have to operate with reverse current for this, and be active for
the full output "beat".
What is ALF? Antonio Carlos M. de Queiroz.
Date : Tue, 14 Sep 2004 21:03:09 -0600. Subject : Re: Pushing the IGBT Envelop
Original
poster: "Dr. Resonance"
That's Greg's Advanced Lightning Facility (ALF).
Dr. Resonance
> > Original poster: "Greg Leyh"
>
> > Nice program! I'll have to work with it some more to understand it
more fully, though.
> > For some reason it took my inputted values and changed them? I
probably entered them
> > in the wrong spot.
>
> The program really has some unclear parts... When you choose one of the
> two design buttons, what you have entered after pressing the "edit elements" button is replaced.
> The "lossy design" is the impedance matching idea using
> a Butterworth filter. It optimizes the input impedance for a certain
> amount of load. The "lossless design" is similar to the design of a
> magnifier, with the operation mode defined by three integers k:l:m,
> that are the ratio of the three frequencies that appear in the
> waveforms. Note that both designs are almost identical if the load
> is removed. The other parameters are for the built-in simulator, that
> always works over the element values that appear when the "edit
> elements" button is pressed. The simulator assumes a linear resistive load, that
> is on of off all the time, and leaves the driver operating all the time (to be improved
>later).
>
> > Have you studied primary drive scenarios that attempt to recover the
unused energy from
> > the secondary? I think this feature will be essential for the ALF.
I'm
> > presently designing an energy recovery scheme into the 1:12 scale ALF prototype, and
> >hope
to test it soon.
>
> The lossless design ideally recovers all the energy, at least if some
> of it is removed from the output capacitance at the instant when the
> maximum voltage is reached. It would also recover most of the energy
> left unused by a load connected to the output when the output voltage
> is close to the maximum. But the driver would have to operate with
> reverse current for this, and be active for the full output "beat".
>
> What is ALF? Antonio Carlos M. de Queiroz.
Date : Wed, 15 Sep 2004 08:15:33 -0600. Subject : Re: Pushing the IGBT Envelop
Original poster: "Greg
Leyh"
>Original poster: "Antonio Carlos M. de Queiroz"
>[snip]
>The lossless design ideally recovers all the energy, at least if some
>of it is removed from the output capacitance at the instant when the
>maximum voltage is reached. It would also recover most of the energy
>left unused by a load connected to the output when the output voltage
>is close to the maximum. But the driver would have to operate with
>reverse current for this, and be active for the full output "beat".
That's a reasonable scenario, as non-quenching sparkgaps almost exclusively
operate in that fashion. Solid-state switches can, as well.
>What is ALF?
It's a theoretical coil that I would like to see built. The physical scale of
the coil is set at the maximum size that still allows shot-to-shot buildup of
the arc while retaining a relatively short operating duty factor. Both of these
are necessary for a good arc-length
to power ratio. I'm estimating that a break rate of at least 250PPS is needed
for good
shot-to-shot buildup, yielding a shot period of 4mS. Here I'm also defining a
short duty factor as <10%, in order to maintain a reasonable power
multiplication. This allows about 400uS for the beat envelope to couple the
power to the secondary. The largest coil design I've come up with that can
couple the power in 400uS has a secondary that's approximately 96ft high and
22ft in diameter. This coil has a Fo of about 7500Hz. Another factor of two in
scale can be had by operating two identical coils, in antiphase.
I would imagine the finished twin coil system appearing something like this:
http://www.lod.org/720L5M.jpg
Here's some preliminary specs, although they're terribly dated at this point:
http://www.lod.org/alftech.html
