[TCML] QCW sparks

[Steve Ward <steve.ward@xxxxxxxxx>]

Greetings,

First, a bit of rambling: I recently obtained a Jennings vacuum capacitor
voltage divider.  While i could not find a manual for it, i did see mention
of it on the Jennings site as being rated for 50kV peak and 60hz to 30MHz.
 I did my own calibration with a 40khz AC source up to 5kVpk or so, and
found the division ratio to be right at 800:1 for this unit, i may look for
more ways to verify this later as its important, but for now i at least have
relative data to compare.  The Jennings website mentioned ~600:1 for this
unit, so i have some uncertainty.  I'll go with 800:1 since it gives what
should be "worst-case" voltages.

So what have i done?  Well ive been working on this "QCW"
(quasi-continuous-wave) DRSSTC.  The power supply is unique in that it can
ramp the bus voltage to the DRSSTC from ~30V (enough to reliably start
oscillation of the tesla coil) to 300V, basically a high power arbitrary
wave-form generator feeding the H-bridge drive of the tesla coil.  I
discovered that keeping the Fres of the system suitably high (> 300khz), and
providing a linear ramp in supply voltage over 10-15mS could grow extremely
long, straight, sparks compared to the size of the coil (best is 65" sparks
from a 9.25" length winding of 30awg).  The top voltage was suspected to be
low because of the lack of flash-over between primary and secondary, and
because of the small 2" minor diameter of the toroid.

http://www.flickr.com/photos/kickermagnet/4386617170/in/photostream

So finally i made some measurements!  So far what i see is the top voltage
ramps quickly to about 45kV, at which point the breakdown process begins.
The secondary base current is about 1.76A pk at this point.  From here it
takes relatively little extra voltage to grow sparks up to 4 feet long, i
measure just 56kV!  The base current is just over 3A at this point, which
means the secondary current is growing faster than the voltage, so the spark
impedance is going down over the 12mS pulse.  Waveform can be seen here:

http://www.flickr.com/photos/kickermagnet/5770183734/in/photostream

Further pictures of the setup can be browsed from my home page:

http://www.flickr.com/photos/kickermagnet/

Im not 100% sure what to make of it yet.  The explanation i like to give is
that its a "self-stretching arc".  If you consider the arc length you can
stretch from a 15kV transformer (many feet if there is enough current... say
2A), then the fact that this tesla coil functions at relatively low voltage,
yet still producing long sparks, makes sense.  The spark just takes a
loooooong time to get to that length.

Another interesting point is that the top voltage is a lot lower than V = IZ
would suggest for a lumped model of my secondary coil.  Basically, the
secondary is ~25mH, and with 3Apk current through 25mH at 325khz, id expect
a peak voltage of 153kV.  The only way i can account for this large
discrepancy in apparent impedance of the coil is that there must be
significant capacitance from the secondary to primary, so the base current
looks much larger compared to say, the toroid current.  I'd like to next
work on measuring the current between secondary and toroid and see if it is
consistent with my voltage measurement claims.  The fact that the 50kV probe
hasnt shown any signs of stress makes me think that the voltage really isnt
150kV, but really is more like the 56kV i claim.

The next step will be to measure the ramp voltage and work on a spice model
to determine the streamer impedance.

Thoughts?

Steve
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