Safely Evacuate a CRT

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
Jeroen Vriesman
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Re: Safely Evacuate a CRT

Post by Jeroen Vriesman » Sun Jan 24, 2016 10:39 am

Bob, is it possible to reactivate thoriated tungsten after "wearing out"?

You need to heat them a white to create the thorium oxide layer, I don't exactly know what process causes the "wearing out" you mention. Is it contamination, diffusion of the thorium oxide back into the cathode? Or something else? Would reactivation help?

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Bob Reite
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Re: Safely Evacuate a CRT

Post by Bob Reite » Thu Jan 28, 2016 2:34 pm

Although thoriated tungsten filaments are first a uniform composition of tungsten powder and thorium oxide a subsequent step is to "carburize" the filament by heating in a low pressure atmosphere of hydrocarbon vapor. This process creates a thin surface layer of metallic thorium which is a better electron emitter. However this thin layer will evaporate though ion bombardment. A poor vacuum will hasten the process.

Incorrect filament voltage will also hasten the demise of a thoriated tungsten filament. The procedure that I follow to get the most out of a transmitting tube with a thoriated tungsten filament is to run the filament at the manufactures rated voltage (as measured right at the tube socket) for the first 200 hours. Then I drop the voltage until I note the output power dropping (or in the case of a modulator tube, the distortion going up). From that point turn it up 0.1 volts. This will typically be 7.2- 7.3 volts for a nominal 7.5 volt tube. Two to three years later, for a tube in 24/7/365 service, when the emission starts dropping I turn it up. When I'm back up to rated voltage, that's when I order a spare if there is not already one at hand. When I'm at this stage, I usually can get another 6 months out of the tube before it won't do rated power anymore.

The big transmitting tubes are rebuildable. The rebuilders don't try to "fix" the filament, they just install a new one.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.

Jeroen Vriesman
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Re: Safely Evacuate a CRT

Post by Jeroen Vriesman » Sun Jan 31, 2016 6:46 pm

Bob,

isn't the carburization process creating a tungsten-carbide layer to minimize evaporation of the filament?

This text: http://electrontubestore.com/thoriated-tungsten.pdf mentions the metallic thorium forming a mono-atomic dipole layer on the surface, the thorium coming from reduction (where is the oxidizing part? the tungsten? is it ThO2 + W -> Th + WO2? )
It also mentions "the right temperature for the diffusion speed of thorium to the surface to compensate the evaporation of the thorium", so the reduction of ThO seems to be an ongoing process.

Anyway, the idea seems to be creating a tungsten carbide layer, not to reduce the ThO to Th with carbon.

If the ThO2 is reduced by tungsten, maybe the creation of WO2 is the "wearing out", or (seems more likely to me) the depletion of ThO is the causing the performance degradation, or something else...

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