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Re: [TCML] Thyratron
G Hunter wrote:
The recent trigitron coil question has got me
wondering: Has anyone ever built, seen, or even heard
about a successful disruptive tesla coil using a
thyratron tube or tubes in place of the spark gap? If
so, kindly share what you know. A link will be most
appreciated.
Happy New Year,
Greg
Hi Greg,
As you may recall (if you were on the TCML at that time), Richard Hull
of the TCBOR made a thyratron-driven coil. Unfortunately it "fired" only
for one half of an RF cycle and then quenched (at the very first current
zero). It also used a comparatively low power H2 thyratron - the 3C45 (3
kV, 35A peak). When triggered, it conducted for one-half cycle, but when
the voltage across the tube began to reverse, the tube went into cutoff,
blocking any further current flow. Because "ring-up" was prematurely
aborted, very little primary energy was actually transfered to the
secondary. You can likely find information in the TCML archives circa
1997 - search on "thyratron".
BTW, Richard Hull performed some very interesting experiments with this
particular coil that seemed to demonstrate that disruptive Tesla Coils
generated a DC electrostatic field around them. Subsequent research
determined that this effect was an instrumentation artifact:
http://www.electricspacecraft.com/hull%20for%20web.pdf
There is at least one other Tesla Coil that uses a hydrogen thyratron
switch - a somewhat larger EEV 8503/CV6022 (16 kV, 325A peak - similar
to a 5C22, but ruggedized). This system, while larger, suffers from the
same premature quenching problem as Richard's earlier coil. However, by
using tighter coupling, a bit more energy can be transfered to the
secondary during the single primary pulse. The circuit operates like a
bell (the secondary) being struck by a hammer (a single pulse). Although
the site is in German, you can use Google language tools to help
translate it to English, and it contains a schematic:
http://www.rapp-instruments.de/tesla-coils/thyratron/thyratron.htm
In order to avoid the premature quenching problems seen in the above
systems, one could use an anti-parallel HV diode across the thyratron to
conduct in the reverse direction. Other options include an anti-parallel
hydrogen diode (a scarce device, similar to a hydrogen thyratron, but
with no accessible control grid), or a similar H2 thyratron connected in
anti-parallel. One would also need to provide a series of properly
phased trigger pulses to turn on the thyratron(s) on every appropriate
half cycle until ring-up (energy transfer from primary to secondary) had
completed.
It may also be possible to use a specially constructed "Hollow anode"
thyratron that can safely conduct in the reverse direction. Normally,
reverse conduction causes arcing that destroys cathode material. These
devices are often used as switches in excimer laser systems. Since
reverse current is usually limited to 50% of the previous forward
current, their use in a high Q Tesla Coil primary circuit may be
limited. The relatively high cost of hydrogen or deuterium thyratrons
and associated circuit complexity have most likely been the reasons why
thyratrons have not been actively pursued.
Bert
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