The limitation for short pulses would, most likely, be the filament's ability to emit enough electrons without disintegrating. Any idea what this is? Sounds rather high. Wish I had a busted magnetron to look at, perhaps its determinable by first principles.
A space charge cathode is normally capable of pulsing at 10-20x normal (continuous) current levels for a few microseconds.
Beyond that, damage can occur (cathode stripping, saturation, ion bombardment?).
Classic hard modulator tubes, magnetrons were used with, are rated in the 10kV and 20A peak range, so this sounds reasonable even for a commercial magnetron.
I don't know what differentiates a pulsed magnetron from a CW one, if anything. I'm sure the pulsed lifetime will be more uncertain than with a proper radar magnetron.
I'm guessing what would actually happen is the special coating used on the material would be vaporized till the emission voltage requirement is changed... so some kind of nonlinear behavior would manifest itself. This would be a differential equation with a heat gradient across the material big enough to cause surface boiling?
It doesn't heat up further.
Nonlinear behavior is saturation, where the current can't go any higher because the surface emission has been exhausted (depleting the space charge).
Mechanical failure can occur due to the strong electric field pulling the cathode apart. No plasma is involved.
Some sparking may occur, but that's more a function of flying bits touching other high voltage bits and internal arcing occuring.
Does the electron emission cause some kind of change similar to 'pressure change' that effects the phase change of matter that changes calculations based on vapor pressure and melting point etc?
Hmm, I hadn't thought about electron flow in terms of pressure.
It should be comparable to the Maxwell pressure (e_0 * E^2 / 2), since that's what's pulling the electrons along, after all.
i imagine the nature of the electron beam changes right (since significant quantity of metal ions is being carried by the beam at this point, so it turns into a 'heavy' particle beam? (relativistic heavy ion particle beam physics come into play with enough voltage.. perhaps that requires a long accelerator though, the voltage requirements would be like >1 gigavolt to cause this in a short range of the tube I guess?
Right, that ain't gonna happen.
Electron relativistic effects start at a few 100kV, but in such a small tube, it will flash over first (field emission from points and surfaces, ion bombardment, arc discharge).
Note that negative ions flow towards the anode, while positive ions flow towards the cathode. Being very massive (ions like oxygen and copper), they move very slowly, and do not spiral appreciably in the magnetic field. An electron-ion cascade discharge (an arc) occurs very quickly once started.
it would contaminate the grid etc.
Grid?
Magnetron?
Grid contamination is another player in the lifetime of pulsed hard vacuum tubes (like modulator tetrodes), however.
Anyway, field emission of the microwave resonators (the corners of the slots, the antenna element) would also be a limiting factor. Low VSWR will certainly be needed.
I don't see offhand any amateur pulsed magnetron experiments, but there may be a good legal reason for that.
Tim