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Re: Recent s.s.t.c work

Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>

Finn (& all)-

Those are excellent and no doubt correct observations, which I had not thought of. Thank you very much for passing them along. I will keep a close eye (and ear and nose!) on my coil when I get to firing it up. As to a field-shaping shield, any gaps in it for the purpose of preventing circulating currents would of course, as would the entire shield, have to have rounded edges.

It happens that my primary has no sharp curvatures, within the secondary, smaller than the 1/4"-diameter smooth copper tubing, so I may be OK. The curvature at the top where the top end returns down axially to the external connection is much larger, and smooth with no discontinuities.

Should not a shield be floating rather than grounded so as not to introduce additional voltage stress? If so, perhaps, if necessary in my two-bucket design, I could interpose an array of small metal balls or rounded button-shapes between the buckets, perhaps glued onto or pressed into holes in a 3rd bucket. That would cause the secondary to have to be raised up a few inches (the buckets being tapered), but since I presently have a measured k of around 0.3, that likely would not cause a problem.

Are you saying that your entire secondary shorted out because of tracking--starting, presumably, from the initial point near the bottom? What a bummer!

Are there electric-field experts among us who can offer opinions on this?

Ken Herrick

Tesla list wrote: 
Original poster: Finn Hammer <mailto:f-h@xxxx><f-h@xxxx>

Ken,

Before you get to firing this coil up at full power, It is my duty to report my recent experience with internal primary`s.
I was amongst the crowd that reported troublefree operation of internal primary`s.
The coils that I based my recommendation on, were a couple of OLTC`s and a couple of SSTC`s, both running in short bursts of up to 20 discharges at a time. The SSTC`s were pulsed at 200µS and the OLTC`s had the usual ring down caracteristic with one noch quench at K=0.2 anf Fres=70kHz.

I later built a DR-SSTC prototype to run about 1.2 meter streamers. K=0.23, Fres=55kHz and internal primary.
This did not work out for very long. I did not understand the fault mode very well to start with (wonder if I do now!) but this is what happened:
Running the coil, I soon noticed a lot of smoke from the inside of the sec. former, as well as light from it.(running in the dark)
I inspected the primary coil, which was hevy PVC insulated wire, and found burned insulation on the top turn. Just a point where it is bent back inside the primary to go back to the bridge.
On the sec. coil there was also a zoot mark.
Running the coil at length in this condition caused the sec. to form a long zooty track on the inside, the whole length of the winding, and the output got progressively shorter in the process.

Stripping the wire off the sec. former showed that there had not been any puncture in the sec. former, so the transfer from the sec. to pri. has been capacitive.

I started to understand this at the same time when Stephen Ward showed, that it is not possible to insulate one`s way out of a corona problem: If you stuff dielectric into the gap, and the gradient in the remaining air gets higher.
Another poster noted that at RF, there are no insulators, just better or worse capacitors.

I now think that to make an internal, or any primary in physical close proximity to the sec. work, the only solution is field controll. Since the pri. windings are inherently pointy in an electrostatic sence, I think the solution lies in introducing a smooth grounded shield btwn pri and sec. this shield has to be slotted, perhaps in multiple pieces, to avoid 1, shorted turn, 2, loop current heating.

The purpose of this shield is to shape the field btwn sec. and this shield, so that no corona is formed.

I hope this all makes sence, and that it will be of use to you. In the meanwhile, the coils that I ended up making got to aquire a rather dull, conventional look:
<http://home5.inet.tele.dk/f-hammer/DSC00135.JPG>http://home5.inet.tele.dk/f-hammer/DSC00135.JPG



Cheers, Finn Hammer