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Re: Question



Tesla list wrote:
> 
> Original poster: "Jeffery Pederson" <kd4lyh-at-webtv-dot-net>
> 
> Has anyone on this list used a Hydrogen Thyratron (Hollow Anode or
> Double Cathode) in a Tesla application with success or are these devices
> not used in Tesla applications.
> Regards to all

Jeffery,

Richard Hull used standard thyratrons (4C35's and 5C22's) in his
experimental systems. To my knowledge, nobody has actually used modern
hollow-anode or double-cathode thyratrons in a Tesla Coil. A number of
us on this list have acquired these style tubes (as well as hydrogen
diodes) for potential Tesla Coil and pulsed power experiments. 

Normally, a thyratron is a unidirectional switch that cannot tolerate
the reverse current flow that's a necessary part of the "ring-up" during
energy transfer between the primary and secondary. If forced to conduct
in the reverse direction, arcing and permanent cathode damage are the
usual consequences. In Richard Hull's experiments, the thyratron he used
actually conducted only for a single positive half-cycle, then turned
off, allowing relatively little of the energy stored in the tank
capacitor to be transferred to the secondary. There was some discussion
of using a back-to-back pair or a thyratron and hydrogen diode but I am
not aware that this was actually done. There has been very little
on-list discussion about using hollow-anode thyratrons. So... let's talk
about these a bit! :^)

Hollow-anode tubes are comparatively new devices. They work by trapping
plasma in the hollow anode when turned on. This trapped plasma then acts
like a psuedo-cathode, supplying electrons to maintain ionization
between the cathode and anode so that the device can safely conduct
reverse current (anode to cathode) without damaging arcing that would
otherwise destroy the tube. However, the maximum reverse current is
limited to about 1/2 the amplitude of the previous peak positive
current. The capability to safely handle reverse current permits their
use in low-Q "ringing" discharges, and suggests that a hollow-anode tube
may also have the potential of replacing a spark gap in a disruptive
Tesla Coil. 

Hollow-anode tubes are typically 2" - 4.5" diameter ceramic-bodied
tubes. They are often used in pulsed excimer laser systems where they
efficiently handle high-amplitude underdamped ringing discharges. Other
applications include medical, pulsed power, and nuclear research. As you
indicate, another alternative could be a double-ended (dual-cathode)
thyratron, however, these devices are not nearly as prevalent as hollow
anode tubes in the surplus market. Coilers are forced to buy these as
surplus since they are prohibitively expensive as new tubes! Both EEV
and ITT make a number of hollow-anode thyratrons, but only EEV makes
double-ended thyratrons.  

By using a DC resonant charging source to charge the tank cap, and
substituting a hollow-anode thyratrons for the spark gap, [in theory]
one should be able to store a precise "bang" size in the tank cap,
trigger the system at controlled, variable rates, fully ring up the
secondary on each bang, and then optimally quench the primary circuit.
Sort of like having a syncronous, variable-speed gap. It could be almost
an ideal Tesla Coil research tool! 

However, because of the reduced number of charge carriers (the thyratron
operates with a reduced pressure hydrogen or deuterium atmosphere), the
"on" resistance of a conducting thyratron is substantially higher than
that of a spark gap at STP, so it's average power dissipation is
significantly higher than a spark gap. Careful design would be necessary
to insure that the tube's Average and RMS current and anode dissipation
ratings were not exceeded. Because of the higher dissipation, operation
under oil or water cooling may be necessary particularly at higher rep
rates. High voltage insulated filament and reservoir supplies are
another complication. 

However, it should work, and is on my "things to do" list, since I have
a number of tubes that SHOULD be up to the task...  :^)

Safe coilin' to you!

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-com
Web Site: http://www.teslamania-dot-com