How Critical is Magnet Precision in Magnetron Functionality?

[Locrian]

I'm in the process of refurbishing a 915mhz 30kw Microwave power supply. The problem I'm having is that the electromagnet coil's water tubing is busted, and so we are having to replace it, and the standard replacement part is somewhat expensive ($2000+) and will take weeks to arrive. I've found a local shop that specializes in electromagnets and say they can duplicate my old one for less money and quicker return.

Anyways, it's the theory I'm interested in.

There are lots of salespeople in this industry, and a few people who know details about magnetrons, but I'm not finding anyone who can tell me how specific the magnet has to be. Obviously I hope wherever I get a replacement it will be the same as the original, but I want to have my bases covered if the magnetic field produced by the replacement is off by a few %.

So does anyone have any information and/or thoughts about how the magnet affects the magnetron? I understand the theory, but knowing how it will react in practice is a very different thing...

 

[Locrian]

I've talked to a lot of people at many different companies and a couple of universities about this.

It's a simple question, that I haven't found an answer to.

I've come to the conclusion that all the physicists who understand magnetrons have long died, and all we're left with are a compliment of EE who know how to wire them up, and a mass of drooling salespeople determined to hock you another one.

 

[turin]

I will venture a guess.

It doesn't seem as though the magnet is a very critical component. For one thing, they seem to be using simple permanent magnets in microwave ovens. For the frequency output, it seems like the resonant cavities within the anodes vanes are the most important feature. For the power output, it seems like the anode voltage (the potential difference between anode and cathode) is the most important feature. As far as I can tell, the magnet just needs to be strong enough to significantly circulate the electron beam.

Keep in mind that I know almost nothing about magnetrons. In fact, I hadn't heard of them until I read your post, so I am posing wild conjecture based on some brief internet research. I am rather surprised that someone is trying to charge you $2,000 for a cooling system.

 

[Locrian]

Appreciate it turin! I agree with your advice and my company is going to have one wound for us instead of buying the one from the company that made the original. We recently grabbed an osciloscope and determined it's inductance (with a little help from a sophmore physics lab sheet that my parter used to teach ) and are going to have a local specialty place wind us a new one. That's the trick; the cooling coil isn't separable from the magnet so if you bust your coil you have to replace the whole thing.

If something melts down and a 12kV 50 amp arc decimates our labspace I'll let you know, if I'm around to do it!

 

[Njorl]

I never worked on a magnetron, but I did work on an orotron. The most important factor was field continuity, not the exact field strength. The condition of the pole pieces was the most significant factor in field continuity. Can you remove the pole pieces before shipping out the magnet? This assumes you'll be more careful removing and replacing them than the repairman though!

Njorl