Bill:
My explanation is quite simple, and believable. Like many other issues
confronted here, the actual analysis requires crunching some real-world
numbers. But I don't think it is an atomic physics issue.
You are assuming a constant E-field in the tube, which is not a good
assumption. At any contours, edges, or radii the electric field can grow
considerably. This is basic electrostatics in operation. Just like the corona
that happens at the end of antenna elements, where sharp edges occur, and is
traditionally counteracted by increasing the radius of curvature (i.e.
putting balls on the end).
There are no doubt areas in the tube, possibly around the filament if
it is a wound helix, which exactly fit the bill.
Eric von Valtier K8LV
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