Re: [TCML] Spark models, revisited

[Steve Ward <steve.ward@xxxxxxxxx>]

Hi Udo,

Still trying to catch up on the rest of this conversation, but wanted
to offer up some numbers from my QCW experiments.

The max straight spark length for the QCW gun is about 56".  The peak
inverter power is 350VDC*100Apk*.707 = 25kW at an operating frequency
of around 330khz with the biggest sparks.

I found that tuning makes a considerable difference in inverter power,
but im not sure how to explain it.  It looks like good tuning maybe
provided a significant efficiency increase.  For the gun, initial
tuning gave about 40" sparks at best with 120A inverter current.
Lowering the coupling some (due to flashover problems), and re-tuning
the primary improved spark length to 50" and dropped inverter current
to 100Apk.  Lowering secondary impedance (giving it more C) made the
required inverter power drop as well in various bench test
configurations for different coils.  I tried to keep Fres about the
same and see the effect of higher capacitance secondary coils.  With
the Cself of just 16pF or so (coil and toroid), detuning was
significant and it took 170A or so for 50" sparks.  Adding 16.7pF of
film caps for ~33pF total (and less Lsec to keep Fr ~350khz) gave the
same 50" sparks at just 100A.  Trying yet a 3rd configuration with
2x16.7pF caps added (nearly 50pF total) gave about the same
performance, maybe even slightly more inverter current for the same
spark length.  I figured it helped keep the coil in better tune as the
spark grew larger but excessive capacitance just was wasted energy
storage.

> The arc length grows with the square of Vinp. I believe Steve Ward
> has seen a rather sudden growth of spark length with voltage
> and my own observations confirm this.

My tests make it look more like a linear thing with a big offset.  It
took about 56kV to produce a 6" spark, and then fairly linearly the
voltage climbs to 65kV with a 50" spark.  Maybe im still at the base
of the curve so my crude numbers look linear?  i seem to recall 66"
sparks needing ~72-76kV, so it had looked to me like sparks were
actually going to require even more voltage to grow bigger.

Another observation is that splitting of sparks greatly reduces their
length.  As expected, making 2 sparks instead of 1, makes them shorter
with a given amount of power input.  When i "over-voltage" the coil by
ramping the inverter voltage to maximum early on in the spark
formation, it can only achieve about 36" sparks (same 25kW as before),
but it produces usually 3 main arc channels each with several little
branches.  Its only with a very slow and controlled increase in
inverter voltage over a 15mS period that i can make straight long
sparks.  This, i believe, is due to keeping the spark voltage low
enough that branching is not promoted, and only enough voltage is
supplied to overcome the existing channel resistance and keep the tip
propagation going.

Steve
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