Following is a simple and valid explanation of gas in vacuum power tubes.
It came from "The Application of Graphite as an Anode Material to High
Vacuum Transmitting Tubes", by EE Spitzer of General Electric, and later
RCA Radiotron. Yes, I was at the technical library again... This one is
dated 1933, August, Proc. IRE, page 1075-1081.
These days, graphite is exceptional for use in grids of large power tubes.
The anodes are usually copper, and pyrolytic graphic is a good way to
handle the grid construction.
John
K5PRO
----------------------
"The amount and kind of gases ADSORBED in the anode material and the ease
with which they can be DESORBED are two of the most important factors in
determining the suitability of the material. Gases are desorbed when
materials are heated in vacuum. It is very essential that no appreciable
amounts of gases be liberated by the anode during operation at the normal
temperature. Liberated gases become ionized under impact by electrons and
these ions partly neutralize the electron space charge which limits the
flow of electron current between the cathode and anode. Increased current
then flows through the tube, resulting in greater anode heating and more
rapid gas liberation. This process may quickly become cumulative and lead
to an arc discharge between anode and cathode. If no overload protective
device is used, such a discharge generally results in an abrupt termination
of the life of the tube by melting of the cathode, grid, and anode...."
"The kind of gases liberated by the anode is important in the case of
thoriated tungsten cathode types. The electron emission of such cathodes is
due to a monatomic layer of thorium adsorbed on the surface of the
tungsten. Very small amounts of oxygen will oxidize the thorium layer and
thereby destroy the emission of the cathode. This layer can also be
destroyed by sputtering of positive ions. The lighter gases, such as
hydrogen and helium, are relative ineffective in this sputtering compared
with heavier gases, such as mercury vapor."
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