Multi-gap Sync RSG's and quenching

From:  Gary Lau  07-May-1998 0849 [SMTP:lau-at-hdecad.ENET.dec-dot-com]
Sent:  Thursday, May 07, 1998 8:13 AM
To:  tesla-at-pupman-dot-com; futuret-at-aol-dot-com
Cc:  lau-at-hdecad.ENET.dec-dot-com
Subject:  Multi-gap Sync RSG's and quenching

To John Freau (and anyone else w/ $.02 to share):

After reviewing some of your posts in the archives, I have some questions
as I embark upon improving my sync RSG. 

Briefly, my coil specs are:
15KV/60mA NST
Secondary: 4.25" x 23.5" x #22AWG
Primary: .25" dia Cu tubing, 15 degree conical, ~6.25" ID, tap #13
.01 uF 100KV cap, FCI brand (surplus from Fair Radio Sales)
Topload: 4.5" x 15.5" toroid plus 6" x 24" toroid on top
Tank wiring: very short, using .04" x .50" copper strap.
Fres = 212 KHz
Measured k = .14, lowermost pri & sec turns are even with each other.

Using a 10 x .03" gap RQ static gap w/ fan, performance was 42" sparks.

I made a 120 BPS sync rotary gap using a motor modified to 1800 RPM and a
9" G10 disk with 4 rotating electrodes, 8-32 x 1.25" brass screws
perpendicular to the disk (all connected together), and two stationary
electrodes.  The stationary electrodes arc to the rotating screw
electrodes on a .50" long segment of the screw's length.  The coil's
performance with the RSG fell to about 30" sparks, and the sparks in the
RSG appear to drag about 1/2" from each gap, I assume due to poor
quenching.  I also tried putting a 5 x .03" RQ static gap w/ fan in
series with the RSG, but no significant difference was seen.  Also, just
with the RSG, I sometimes get sparks racing along the secondary.

I'm about to make one last attempt to improve the RSG performance.  I'm
about to rebuild it as an 8 series quench RSG configuration, with each
gap arcing along the length of a pair of 1" long .25" dia copper tubes.
I'm hoping that the copper tubes's smoother surface, as compared to the
8-32 screw's threads, might quench better.

I've been scanning the archives and see where on March 30, 1997, you
describe a "sync 12 point super-series quench rotary".  When you tried
shorting out 6 of the gaps, "quenching went from 1st to 3rd notch with 20%
weaker sparks, running poorly and unsteadily".

This contrasts to a Sept 21 1997 post where you compared a 2 gap sync RSG
with an 8 gap sync RSG, concluding:  "Bottom line is, there wasn't too
much difference;  sparks still hit the 42" mark on a regular basis, but
the sparks may have lacked some pizzazz using the 2 gap series quench
sync rotary."

I can see a way to construct my sync RSG in such a way that there would
be 16 series gaps (shortable to any number less), and am wondering
whether this might offer any benefits over 8 gaps.  Do you think so?  Has
anyone else experimented with rotary gaps that arc along the edge, rather
than the end of a cylinder, to improve quenching, or am I wasting my time?

Regards, Gary Lau
Waltham, MA USA

>        Optimal Quench Tests
>  Date:
>        Sun, 30 Mar 1997 19:56:25 -0500 (EST)
>  From:
>        FutureT-at-aol-dot-com
>    To:
>        tesla-at-pupman-dot-com
>I set up a small 360 VA (12kV, 30ma neon) coil for quench testing.  Steady first
>notch quenching was achieved at k = .11, k = .12, and at k = .18, spark length
>was 36".
>Specs are as folllows;     
>Primary is #12 standed pvc ins, 15 degree inverted cone, tapped at 33 turns
>(448uH).  C = 0.007uF, polypropylene.
>Secondary is 6- 1/2" by 23" plastic form wound with #28 formvar, L = 108mH.
>Resonant frequency without toroid is 130 kHz, with toroid is probably about 100
>kHz or so.  
>Toroid is 4" by 17" aluminum dryer duct.  
>Gap is a 120 BPS synchronous 12 point super-series quench rotary (the spark
>snakes through 12 gaps during each firing).  There are 12 spinning and 12
>stationary electrodes, all electrodes are 1/8" dia. stainless steel.  Motor is
>3600 rpm, 1/10 HP salient pole design with ~7" rotor.  Six of the gaps are
>Maximum voltage applied to the Tesla coil transformer is 125 volts.
>First test:       Secondary was set at 2 1/4" above center of primary, k = .11,
>spark was 36"  and quench occured at first notch.  Quench was monitored using a
>solid state o-scope in the next room with an antenna type pickup.  To obtain the
>36" spark, it was necessary to attach a small aluminum foil "bump" to the toroid
>to force it to emit from that point only.  Without the bump, multiple streamers
>formed but were shorter.  Ringdown was near-linear, and took about 40uS.  First
>notch quenching was achieved for 2 to 3 foot spark.  At lower power levels (1
>foot spark length), the gap quenched intermittently on 1st notch since the coil
>was not loaded enough, and too much energy reflected back from the sec. to the
>Second test:       Sec. was set at 1 3/4" above pri, k = about .12, results were
>the same as in first test.
>Third test:      Sec. was set at  1 1/2" above pri, k = about .13, results were
>similar, but gap quenches mostly on the first notch but occasionally on the
>second.  Spark may have appeared slightly stronger, but still just reached 36"
>Forth test:     Sec. was set at 1 1/8" above pri, k = about .15, still quenched
>on first notch most of the time, but a second toroid streamer formed, so sparks
>were shorter.   Probably a slightly larger toroid would be helpful.
>Fifth test:      Sec. was set at 1/4" above pri, k = about .18, quenched at
>second notch, and the multiple streamers were strong, but not quite 36".  Racing
>sparks started to appear on the secondary at this k value.
>Sixth test:     Sec was set at 9/16" above pri, k = about .18, quenched steadily
>at first notch.  Here I no doubt "hit" the exact (.18) needed magic k value.  In
>some of the above tests, I no doubt "passed by" some of the exact magic k
>values, I'll have to go back and check in smaller sec.  height adjustment
>increments of 1/4".    No racing sparks occured.  Multiple streamers were strong
>but did not hit 36" but hit 35- 1/2" one time.  A slightly larger toroid would
>probably reduce the number of streamers and give a longer spark.
>The next test will be to try for k = .22, but to accomplish this, I will have to
>lower the secondary 1/2" or so below the primary.  Failure to quench here on the
>first notch will cause my secondary coil to be destroyed by racing sparks.  The
>primary/secondary as presently physically configured, cannot couple tighter than
>k = .22.  I may also try using a smaller capacitor with a higher rotary break
>rate for comparison, but this may destroy the neon transformer.     Comments
>Towards optimal coiling,
>John Freau
>Bert, All,
>Yes, I was surprised (but glad) to see such a rapid ringdown, seemed to show
>good power transfer, maybe I just happened upon a good impedance match.
>I did a few more tests which I'll describe:
>First I went back and found the the magic k points that I had missed before, and
>as expected, I was able to obtain first notch quench at these other k values.
>Next, I jumpered across 6 of the 12 gaps to test the system under "poor"
>quenching conditions.  This caused quenching on the 3rd notch, and sparks were
>about 20 % weaker, but coil ran poorly and unsteadily.
>Back to normal operation; one time at k = .15, with input voltage at 130 volts,
>I thought I saw the spark hit 42", but this happened only one time.  In general,
>spark is longer at k = .12 because only one streamer is formed, the spark
>regularly reaches 38" with variac set at 130 volts.
>Next, I tried to achieve first notch quenching at k = .22, but the attempt
>failed, and 2nd notch quenching was obtained, along with racing sparks which
>caused some damage to the secondary.   Perhaps a slightly larger toroid would
>help the quenching by increasing the loading, and might also cause only one
>(longer) streamer to be formed.  I have also noticed that quenching is easier
>with multiple streamers than with one long one.  At k = .22, the system is a
>real "Behr"!   : )
>Towards optimal coiling,
>John Freau