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Re: Optimal Quenching Tests

John Freau wrote:

> GL,
> 
> Thanks for the info.  Can you give a brief description of the internal and
> external design of the hollow anode hydrogen thryratron (how does it
> withstand polarity reversals?) and of the crossatron type switch?  Also some
> tube model numbers if possible, you never know what might be found at a
> hamfest or elsewhere--rare, but who knows?   I'm assuming here that the
> hollow anode thyratron is different from a run-of-the-mill hydrogen
> thyratron?     Thanks.
> 
> John Freau

The hollow anode thyratron employs a hollow metal envelope as an anode 
structure, with an aperture in the surface facing the cathode.  During
polarity reversal, ions are accelerated towards the anode and pass thru
the aperture, impacting the anode on its inside surface.  This protects
the smooth outer surface from pitting and ablation damage, which would 
cause premature breakdown.  Look out for tubes in the CX1500 and CX1700 
families, or 'double ended' tubes such as the CX1192B with two cathodes.

A crossatron is essentially a cold-cathode triggered sparkgap, with a 
huge cross-field magnet that quenches the arc.  A great idea that never
caught on, these beasts last retailed for about $37,000.

But the best emerging technology for replacing the rotary gap is a
solid state device called the Modified GTO.  Here's a partial re-post
that I wrote some months ago describing it:

----
In recent years solid state devices have improved dramatically in
their capacity for switching large peak currents, at high voltages.
But the primary reason that high peak capacity devices such as SCRs and GTOs
have not been applied to fast pulsed power applications (such as Tesla Coils)
is due to their di/dt limitation, which for a good inverter grade GTO is
about 500 amps/uS.  A typical TC has primary current risetimes on the order
of 1000 to 2000 amps/uS.

The 500 A/uS limitation is due to the finite rate of growth of the conduction
area away from the gate structure, when the GTO is fired. However, A new
gate design called the 'interdigitated gate structure' promises to raise this
di/dt limit significantly.  At work we are testing such a device, which is
called the Modified GTO.  At present, the device will hold off 4200V, and
switch pulsed currents of over 50,000 amps peak, at risetimes of 15,000 A/uS!!

People at work imagine replacing the 6000 amp thyratrons along the accelerator
with MGTOs, but I already have dreams of sticking one into my coil.  My present
rotary gap drops about 1200V at 2000A, and takes over 200uS to quench.  The
MGTO at work (stacked appropriately) would drop only 10V, and 'quench' in 20uS.
----

-GL