[TCML] DRSSTC tuning at high power

[Michael Twieg <mdt24@xxxxxxxx>]

Hello, for the last year or so I've been working with a large group on a
pair of large (>20KW) musical DRSSTCs.  So far we've been pretty successful,
but I'm starting to think that our tuning is far from optimal, resulting in
poor spark lengths for our given power draw.  At low DC bus voltages (like
under 100V), I'm seeing nice quenching of the primary current, but as the
voltage increases, the dip in the current envelope starts to disappear and
peak current rises very quickly.  At around 400V, the quench is completely
gone, and the current envelope doesn't dip at all (instead it just kind of
plateaus before increasing again).  Also of interest is that as repetition
rate goes up, the waveform becomes even worse (presumably because the plasma
becomes denser and gives the topload a lower resistance load).  But overall,
putting the bus voltage at 450V, and using a tone frequency of about 440Hz
gives consistent 7ft arcs, but the thing draws over 10KW and the peak Ip is
over 1500A, which seems like way too much.  Somehow we're not efficiently
transferring energy to the topload.

I've got several theories on what could be causing the issue:

   1. Simply bad tuning of k and Lp.  I don't think this is really the case
   because we've played with the tuning a ton and nothing seems to improve the
   Ip waveform or the power draw.  Even if the topload capacitance is changing
   with arc size, we should have been able to compensate with tuning changes.
   2. Losses due to coil resistance, eddy currents, etc.
   3. Bad topload design.  Right now we have a shperical topload
   approximated with five aluminum rings (about 54" diameter).  We seem to get
   a lot of corona at low power levels, but once we draw a solid arc most of
   the corona goes away.
   4. Bad breakout point.  We're using a 12" long breakout point on the side
   of the topload.  Maybe it's breaking out too early?

The main issue I can't figure out is where most of the power is going.  Our
silicon barely heats up, the primary coil and capacitor get warm but
probably dissipate no more than 1KW total.  I have trouble believing that
we're actually putting ~10,000W into arcs, otherwise we should be getting at
least ten feet.

Any ideas?  Or maybe I'm expecting too much?

-Mike
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