How small can a magnetron be made?

[Guineafowl]

I was asked the other day about a miniature magnetron for a ‘thought-invention’ by a friend. Putting aside (for now) safety and power supply considerations, can they even be miniaturised? Say you wanted a 50- 100W one instead of the usual 1000W.

My intial thoughts, unqualified, are that much of the physical size is related to heat sinking, bulky magents and the resonant cavities, whose size are dictated by the wavelength of microwaves. So no, in other words.

A search online revealed many normal-size magnetrons and an awful lot of unrelated devices called ‘magnetron’ because it sounds cool. Exercise bikes, for example.

 

[.Scott]

Here is a link to some research:
http://www.dtic.mil/dtic/tr/fulltext/u2/a533571.pdf

The key to the miniaturization is the wavelength.
So I googled "Terahertz magnetron".

 

[Baluncore]

The answer depends on what you define as being a magnetron. Common cavity magnetrons are limited by the dimensions of the resonator cavities to a diameter of about half a wavelength. There are alternative constructions that could have smaller folded cavities or slower material resonators. You must decide why you need to make a smaller magnetron, when a gunn diode will do the job in a smaller space. If you want more power from less volume you should consider a gyrotron.

 

[tech99]

I was asked the other day about a miniature magnetron for a ‘thought-invention’ by a friend. Putting aside (for now) safety and power supply considerations, can they even be miniaturised? Say you wanted a 50- 100W one instead of the usual 1000W.

My intial thoughts, unqualified, are that much of the physical size is related to heat sinking, bulky magents and the resonant cavities, whose size are dictated by the wavelength of microwaves. So no, in other words.

A search online revealed many normal-size magnetrons and an awful lot of unrelated devices called ‘magnetron’ because it sounds cool. Exercise bikes, for example.

To go back to the invention of the magnetron, when people needed very short wavelengths, they had tried making smaller and smaller triodes etc.but it was of limited success. But the magnetron enabled a large structure to generate short waves, so it could handle a lot of power and the construction was relatively simple and rugged. So to pursue miniaturised magnetrons seems the wrong path to follow.

 

[sophiecentaur]

Common cavity magnetrons are limited by the dimensions of the resonator cavities to a diameter of about half a wavelength.

I would have thought that internal EM fields would not scale nicely and that could limit the possible power at higher frequencies due to arcing.

 

[Baluncore]

I would have thought that internal EM fields would not scale nicely and that could limit the possible power at higher frequencies due to arcing.

The fields scale as expected. Since the magnetron is a high vacuum device the breakdown voltage is going to be decided by electron emission and anode current. Smaller devices will have lower voltages for the same current. Magnetrons are very hard to tame because they have the negative-resistance needed to make them oscillate. Magnetrons have unstable anode currents, even when in series with their specially inductive power transformers. Magnetrons thrive on internal voltage breakdown.

The only way to control the generated frequency is by tuning the cavities. Each time anode current starts to flow in a microwave oven magnetron, thermal heating expands the metal anode cavities which typically results in a down chirp of about 5% of operating frequency. The down chirps can be seen with an S-band analyser, especially at meal times when they sweep across that ISM band, annoying the little blue-toothed hoppers that graze the spectrum grass. It is a jungle out there.

 

[Guineafowl]

Many thanks all. It doesn’t sound like a viable option, then.