Re: [TCML] DRSSTC tuning at high power

[Michael Twieg <mdt24@xxxxxxxx>]

I sent this email a few days ago, but got no replies and it didn't show up
in the pupman archives... very odd, trying again.

This whole frequency increasing thing is really starting to bug me.  We're
observing it very consistently as we change the bus voltage from 340V to
450V.  The difference is almost 1KHz most of the time...  I'm starting to
suspect the zero crossing detection on our gate drive board has some issues
with it, maybe some lag or lead on the phase, or the timing is dependent on
primary current.   It's been a while since I've looked at the zero crossing,
maybe it's drifted off...

On a more positive note, we switched to our toroidal toploads and they
seemed to have helped somewhat.  Less corona than before, and maybe longer
streamers.  Still getting the occasional arc to ground or the strike rail
though (but those arcs to ground are at least 10 feet so that's nice I
guess...).
http://i.imgur.com/9hs4f.jpg
http://i.imgur.com/gaJyo.jpg

Also I've got several more waveforms with secondary and primary current.  In
all of these, green is the primary current (2.5mV per amp) and yellow is the
secondary current (0.1V per amp).
First is with the bus voltage low, around 100V.  We see pretty textbook
waveforms.  Primary current notches nicely just as the secondary current
peaks.  Notch and peak happen about 8 cycles in, as expected.
http://i.imgur.com/041D5.png

Now the bus voltage is raised to 450V, and the notch mostly goes away.  You
can see where the primary current starts to ramp up quickly towards the end,
so we shorten the drive time to 7 cycles in order to avoid getting out of
hand.  Now the secondary current peaks in about 6 cycles, which is strange.

http://i.imgur.com/EfJLP.png

Another one, but at a higher BPS (440Hz I think).  Detuning becomes worse,
presumable due to denser plasma:
http://i.imgur.com/X82Rw.png

Here's another waveform at 450V, but this is one that broke out into an arc,
and you can see pretty clearly when.  The detuning is even worse and the
peak current goes up quite a bit.  I'm not sure what's up with the secondary
current after the arc.  Looks like the fo is still there but there's some
much higher frequency crap on top of it.  It even persists after the primary
current (and presumably the streamer) has died out, which is strange.  Any
idea what that could be?
http://i.imgur.com/QXenB.png

-Mike
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