I was contemplating the number of gas atoms inside a typical vacuum tube vs
the number of electron emitting atoms on the surface of an oxide Cathode of a
KT88, or other tube. Also the velocity of said ions.
As one can see quoted below, the difference between oxide atoms vs gaseous atoms, also considering
the velocity inside a vacuum tube.
So I have to question how can a cold cathode be seriously damaged by so few positive ions.
"Outer space is an even higher-quality vacuum than the best laboratory vacuum tubes — in
intergalactic space, there are fewer than one hydrogen atom per cubic meter,
compared to about 100 particles per cubic centimeter in ultra‑high vacuum chambers.
Particle density comparison."
Only 100 particles vs oxide cathode service.
"Calculating the Number of Ions in a Vacuum Tube
In a properly evacuated vacuum tube, the number of ions is essentially zero — the tube is a high vacuum, so there are no significant gas molecules to ionize.
Why there are no ions in a hard vacuum
A vacuum tube is designed to be a hard vacuum, meaning the internal pressure is extremely low — typically on the order of 10−6 to 10−9 torr. At these pressures, the number density of gas molecules is so small that the probability of ionization is negligible under normal operating conditions."
"Estimated Number of Atoms on the KT88 Oxide Cathode Surface
A reasonable guestimate is on the order of 10¹⁸–10¹⁹ atoms on the oxide cathode surface of a KT88 tube.
Notes
This is a surface atom count, not the total number of atoms in the entire cathode structure."
"Speed of Positive Ions to the Cathode in a Vacuum Tube
In a vacuum tube, positive ions can travel to the cathode at speeds on the order of tens of thousands of meters per second — roughly 10⁴–10⁵ m/s — under typical operating voltages."
10 to the 18th oxides vs 100 ions per cubic centimeter.
Any thoughts?
Cheers
steve