Magnetron limits What are the practical limits on generated wavelength in a Magnetron?
We know that Magnetrons could be used efficiently for generating microwaves for water heating, or for radar applications, but what are the achivable wavelength limits?
 A: To produce higher frequencies by a magnetron, one needs 
higher plate (anode) voltage and a stronger magnetic field, 
to keep the electrons within the central cavity.
So, first limit is the magnet, if we exclude expensive 
solutions like superconductors. 
On the other hand, one could construct a wider central cavity, 
but remember, the cathode surface is the inner wall of it! 
This in turn would ask for a a lot of power to heat 
this cathode surface to yellow heat. 
Georg 
A: I interpret the question as "which frequencies/wavelengths can be generated by cavity magnetrons?"
As far as the physics is concerned, I do not believe there are fundamental limits that constrain the frequencies generated. These frequencies are dictated by the cavity dimensions (see http://en.wikipedia.org/wiki/Cavity_magnetron). Now there are many practical engineering constraints (which brings the question outside the realm of physics) that render cavity magnetrons practical devices mostly for generating cm wavelength EM radiation. And of course as far as the heating of food is concerned, a magnetron needs to emit radiation that is in resonance with polar molecules. And that restricts magnetron wavelengths to cm scales.
