Filament lifetime

[Julien]

Recently it came to my ear that someone (I will not reveal whom - not allowed stated that the filament lifetime for W can be increased if a lower acceleration voltage is used... This sounded crazy to me, but I would like your opinion:

What does control the filament lifetime?

It is obvious that the filament lifetime is normally (chiefly?) controlled by the saturation (how much current is running through the filament), and then a certain amount of current is emitted out of the filament (the emission current or "filament load"). The hotter the filament is, the faster it will be used and get thinner. That being said, I cannot understand how the acceleration voltage would influence the lifetime of the filament... Would a higher acceleration voltage pull out more electron and thus put more "stress" on the filament?

I am not speaking of other factors that could be influencing the filament lifetime, such as the quality of the vacuum, the material use (W vs. LaB6 with a lower work function), the quality of the filament itself (how thick, irregular thickness with a point of weakness, coated with some material such as for a Schottky emitter...). However, I would love to hear your opinion on any other factor not mentioned here that would influence the lifetime of a filament...

Best,

Julien

[BenjaminWade]

I am not sure, but the only way I can think that might have an affect is if the bias or emission control on the filament circuit isn't being changed as the kV is being altered. Thus potentially what is ok for a lower kV might be overdriving it if the kV is raised? No idea...just a thought.

[Julien]

Received through the SX-50 email list --- from Jonathan E. Snow

Mass is lost from the filament by vaporization ionization and
acceleration of the W in the filament. This is well known from mass
spectrometry, which uses W filaments as well. As a hypothesis, at lower
accelerating voltage a lower fraction of the W evaporated from the
hottest point on the filament is redeposited.

[Julien]

And of course from Probemaster J. Donovan, a good thought...

A TEM run at much higher voltage, and the W filament lasts six months to a year just like the probe.