The power of a magnetron declines over time\$^1\$. While not noticable for most consumer applications (A 2000-hour lifetime would last over 20 years if the device is used only 15 minutes a day), this is a problem in industrial situations.
What causes this loss of power? The best I have been able to find is "cathode degradation" but I still do not understand what this is supposed to mean (what mechanisms are actually going on to ''degrade'' the cathode?) and if this really is the only cause of loss.
I am familiar with semiconductor devices, and in those cases decline in performance can be attributed to factors such as electron-migration, hot-carrier injection, diffusion of dopants over time, etc… But a magnetron just seems to be a very ''simple'' mechanical construction, and large with respect to semiconductors, hence I can't imagine those effects causing issues here…
\$^1\$Leaders in Microwaves magazine, Microwaves & RF, 2018, page 13-14
Best Answer
A Magnetron is a "vacuum tube".
One limit on vacuum tube lifetime is cathode emissivity - the ability to provide electrons for the 'tube' to 'modulate'. Decay mechanisms can be complex but a first approximation relates to the availability of materials which liberate electrons and the action of trace gases on the cathode surface. [The material is usually not "used up" over the tube's lifetime but its efficacy may decline].
Wikipedia - Hot Cathode Includes:
Related:
Two related SE EE answers
here and here
ADDED:
Related:
Long ago you could buy "boosters" for TV tubes which contained an autotransformer to slightly raise the filament voltage and get a bit more 'oomph' out of fading tubes.
A similar "trick" may even work on a Magnetron, for a while. 2
A number here
And an example:
Image from here
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