Re: Aluminum electrodes in TCBOR static gap

Hi Don, all,

I read this post with GREAT interest, because it is the exact
OPPOSITE of what I am experiencing.

> Original Poster: Don Allen <dona-at-amigo-dot-net>
> I tried an experiment the other day using aluminum pipe on every other
> gap electrode in my 11 gap static unit. It worked great the first 30
>seconds, and then the streamer length went down to nothing until
>the only sound coming from the coil was just the noise from the gap
>unit. I'm not quite sure what happened.

My gap is a very crude 18mm copper pipe flat gap. One day, while
experimenting, I ran out of Cu pipe (I wanted to see the effects of
using more gaps, but with the same total gap width) and I had a
few aluminum stand-offs handy, so I used these. The results were
spectacular. I posted these results quite a while ago and I donīt
remember if you were a member at the time. Experimentally, I found
that one or two pieces of Al were enough. Adding more didnīt change
the results at all. To make comparisons, I kept the total (!) gap width
the same (==> ~ same breakdown voltage). My resulting sparks
were constantly longer. Of course, not twice as long, but a continual
3-5" longer than with a pure Cu static gap (same total width, same
number of gaps). My gap uses absolutely no cooling, other than
convection cooling (i.e.: no fan). The erosion factor is also very
Seconds after first run, the Al looks like it has been sand blasted, but
after that there is no (real) further erosion. In other words, the Al
NOT erode faster than the Cu pipes. In fact, the appearance of the Al
is MUCH cleaner than that of the Cu pipes, which usually blacken (due
to CuO2) and need periodic cleaning. I have never cleaned the Al. Run
times are usually around 3-5 minutes and I have never seen the sparks
degrade. I did one run of 15 minutes and noticed a slight degrading of
my sparks during the end of the run. However, the gap was so hot, that
it had charred the mount (a simple piece of particle board with mica
between the board and the pipes). The heat and arcing had even
destroyed the insulating mica! At one time I ran the coil for about 105
minutes, only stopping to make adjustments or changing strike targets,
etc. The only cool down time any of the components had were the
adjustment times. No gap problems at all.

My 1st question: Did you notice an improvement in spark length during
that 30 sec run (as compared to your pure Cu gap).

2nd question:
What (total and single) gap clearance are you using.  I have tried
several different combos (see the archives), but found that setting
the Al gaps slightly wider (and closing up the Cu gap for same total
width) seems to give best performance.

> The gap unit WAS firing and making a huge glow, but no spark. I

What do you mean by this? Did the gap have a different sound than
usual?. Usually, I do rough tuning, etc "by gap hearing". The sound of
the gap is a good story teller. It will tell you if you are in tune,
you (very) shortly before something goes south. You can even tell if
the coil is arcing (i.e: streamers hitting something grounded) or just
spitting streamers into free air, all by just listening to the gap.

> I removed the gap unit and dis-assembled the unit to do a post-mortem.
> The spaces between the electrodes were clean. Absolutely no
>carbonization at all between copper and aluminum except a fine
>"spray" like pattern on the aluminum electrode,

Snip. Sounds very similar to my findings.

> unit to just the NST. I could not get a spark to go all the way around
>the electrodes, like with all-copper electrodes. The spark would
>actually stop at the first series gap unit that contained aluminum.

I have always wondered (and not liked) something about the TCBOR
gap. Due to itīs construction (round) there must be gigantic
in the gap (E- & magnetic field) during a discharge. What I mean is
picture the field lines of the gap. Now visualize that each gap (as they
are housed in a cylindrical pipe) has another gap on the opposite side.
The field lines of each individual gap must create a great deal of havoc
in the gap straight across. In a flat gap, this effect is (should be?)
less pronounced.

The only other (real) differences in your setup compared to mine are:

1.) I am using a flat primary.
2.) I run at much lower voltages (7500Vrms)
3.) My coupling is higher (my primary is 4.5-5" ABOVE the lowest
     secondary wire.
4.) My primary current is much higher, due to the lower voltage, higher
     primary capacitance and resulting lower primary inductance.

Points 1 & 3 might cancel each other out. However, points 2 & 4 should
actually make it harder for the gap to quench.

Coiler greets from Germany,