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Mini Coils and Streamer Quality

Subject:       Mini Coils and Streamer Quality
       Date:   Mon, 26 May 1997 11:51:43 +1200
       From:   "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Organization:  Wellington Polytechnic, NZ
         To:   tesla-at-pupman-dot-com


Hi All,
         I played around with a couple of small coils over the 
weekend and came up with some results that might be useful to those 
trying to decide which way to go, more BPS-wise or more Vp-wise. The 
experiments were quite revealing. The important thing was that power 
input was constant over a wide range of BPS. I used the flyback 
supply published on the HVLIST as a power source and one 4.7nF mica 
transmitting cap as Cp. These caps are quite remarkable. They appear 
to be extremely robust. I ran the 6000VW-rated unit at 12kV at low rep 
rate several times and it never blinked. Anyhow, I digress.... 

     I used a single static gap to feed the coil. Tuning of primary 
and secondary was within 5%. I experimented with different coupling 
constants but no real improvement was noted for positioning the 
secondary deep in the primary or well above it. Best reults were 
obtained with the primary starting around the bottom of the secondary 
for the taller of the two secondaries.
      I simply set the gap to fire at 6kV then cranked the supply up 
to max and noted the discharges. I then progressively decreased the 
gap until it was firing at just under 2kV. With the supply at max 
power setting it must have been firing at several hundred BPS at 2kV.

Results: The decrease in output voltage was obvious as the gap was 
reduced. One could tell by the free-air extension of the streamers.
What was interesting however was the dramatic difference in the type 
of discharges. With a 6kV setting, the gap was not firing often 
enough to cause much more than a loose coagulation of streamers to a 
discharge rod. Length obtained was about 3". The sparks to the rod 
were sharp and crackly like those you'd get from a standard induction 
coil. With the gap set to 2kV, the attached streamers were just as 
long!!! However, the high BPS caused them to almost become a 
plasma-like streamer, much as you'd observe if you drew an arc from an 
old B&W TV EHT supply.
    Important point: this is direct evidence that streamer length 
depends heavily on power throughput. It was the same in both cases.
Output voltages were quite different. The higher voltage/low BPS 
streamers had by far the greatest reach but never really formed what 
you would call an "arc". The top of the coil was surrounded by wispy
showers of free-air streamers stretching out in all directions about 
5" long. The drawn "arc" was shorter which showed power had to be 
concentrated in the arc to support it.
     With the 2kV/high BPS gap setting, the free air discharges were 
much shorter and fewer but now concentrated in bright flaming 
streamers we normally see at higher power levels. But the drawn arc
was a fierce hissing plasma much longer than the free-air streamers,
again about 3" long.

     This was fun! :)  I am going to increase the supply capability 
to 100W (was around 40W) and try some more. The nice thing was seeing 
the full range of TC-type discharges in table-top miniature without
wondering what was going to be struck next.
     Based on this, I would say that if your BPS is still rather low
and the sparks assume something of a "crackling" quality, try upping 
the break rate before increasing input voltage. I was amazed at how 
little dependence there was between cap voltage/energy and attached 
streamer length.

Malcolm