"Ruud Broens" wrote

...Even lower, say below 10 V or
so, the cathode will no longer be uniformly heated, resulting in
increased
noise and the risk of cathode sputtering - this *does* decrease
lifespan!

Aha, right, cathode sputtering, er, what's that?

Searched and got a zillion hits. Seems sputtering involves
disintegration of the cathode coating by ion bombardment (what we
call stripping) and subsequent deposition elsewhere. You got me
interested because I thought of it as a deposition process and
hadn't considered the stripping part.

So where does the stripped material get deposited? I thought it
ended up as crumbs and powder that just fall to the bottom of the
envelope. My chemistry is hopeless.

If part of the cathode is not protected by space charge, but the
valve is operating, then I suppose that part could get bombarded?
But where does the continuing supply of ions come from?

Also, the most common cathode material replenishes its surface by
leaching, er, I thought, dimly. I had in mind some kind of dynamic
equilibrium.

Cathode poisoning has the opposite cause: leaving heaters on with no
HT. Is that sputtering, with the heater acting as cathode?
Presumably poisoning stops the leaching.

cheers, Ian