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RE: Some new DRSSTC numbers (long)

Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>

Hi Steve, Paul, all,

I've been following Steve's new DRSSTC with great interest 8) as I'm in the
middle of building one quite like it.

Mine looks like this at the moment-
4" x 13" John Freau toroid (very nice thanks John)
6" x 14" secondary wound with 0.4mm wire (I can try 0.315 too)
(this gives a resonant frequency of ~180 kHz and a Zo of ~50k. Those metric
sizes are roughly equal to 26 and 28 AWG)
9 turn pancake primary coil, L=19uH, k=0.2
52nF 10kV Hivotronics capacitor
(system will operate on lower pole at 140kHz)
H-bridge of the same IGBTs Steve uses, powered off 377V DC
Burst length 100us
Rep rate 100 bps

My simulation predicts a peak primary current of 400A, tank cap voltage of
9kV peak, bang energy around 6 joules and a spark length of about 50", but I
have no idea how accurate these are (especially the spark length- 50" from a
14" tall secondary is optimistic) It also predicts a power dissipation of 10
watts per IGBT (40 total) and a peak die temperature 24oc above case

I have included some comments on what Steve Ward and Paul Benham wrote-

> > So is the best efficiency is seen when the burst is just long enough to > > ensure breakout? >This is a good question.

It is indeed, and when we answer it I hope it will tell us something "deep"
about how streamer loads behave. So far Steve's data is the only stuff I
know of. I think as Steve said, the burst length on a DRSSTC is a good deal
longer than needed for breakout even of the bare toroid. And I believe this
is the secret of producing long streamers from a small resonator.

>less than 100uS burst length is in fact MORE efficient, >at the expense of more bus voltage needed on the IGBTs to get the same >spark lengths. You also have the benefit of reduced current stress >(the peak current is less

This can't be true without breaking some law of physics or other. If the
bang energy is a constant, then the peak current must go up as the duration
goes down.

>> So does the toroid size then set the length of spark >> achieved if the burst length is adjusted accordingly? >yeah, trend suggests that bigger toroids = better >sparks, but just how big can they get? dunno yet.

Because the burst (and hence streamer growth) can continue for a relatively
long time after breakout, I expect the toroid size won't have quite the same
effect as it does in classical TC theory, where breakout is assumed to
happen only after all the primary energy has transferred to the secondary.
For instance, the coil I am simulating right now needs 0.9 joules to break
out the bare toroid at 300kV. But it should then go on to deliver another 5
joules straight to the streamer load, with the peak toroid voltage staying
at 300kV.

So I expect toroid size to be less important than it was with spark gap
coils. The energy for streamer growth is not so much coming from the toroid,
as passing through it.

These are the rules I have used for toroid and secondary sizing. First, if
you want to operate with no breakout point, the toroid minor diameter should
be chosen so that it breaks out before the resonator flashes over. Antonio
de Queiroz has a table of breakout voltages for various sized toroids on his
website, and I take the strength of the resonator as about 30kV per inch of
winding. But it could be more than this.

Second, the toroid capacitance and secondary inductance should be
proportioned such that the resonator Q is more than 6 for the streamer
length you want to produce. Otherwise it will be detuned too much and the
primary current will "run away". A large toroid increases the Q and so can
produce a longer spark before it self-limits. I have posted before on how to
calculate the loaded Q for a given coil frequency and streamer length from
Terry's streamer model.

>Im using about 14 cycles with this setup, and >breakout is achieved (at full input voltage) within about 2 cyces!!

I take it you're using a breakout point :-o

> > It would be interesting to > > see if the primary current ringup is faster with higher voltages or lower > > primary resistance.

>Seems as simple as manipulating ohms law?

Well everything is interrelated due to the resonator "reflecting back" on
the primary. I don't think there is a simple answer. I use simulation to
fumble my way towards a solution, Antonio de Queiroz suggests aligning the
system as if it was a Butterworth or Chebyshev filter or whatever. I think I
have discovered some "hybrid" alignments around k=0.2 that give some of the
advantages of Antonio's tuning methods but with a self-resonant system.


note in the example coil I simulated a 4" x 16" toroid by mistake :(

Steve C.