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Re: [Phys-l] Q of microwave oven "cavity"


On 2011, Apr 28, , at 04:26, brian whatcott wrote:


Magnetrons feature mode straps which are intended to keep the internal
cavities in mode lock.
Some specialized magnetrons can inject a signal to synchronize its output.


Inject signal to synch. other magnetrons?

I thought (hoped) one could use a bunch of magnetrons to make a DIY linear accelerator.


It has been customary to preheat indirectly heated magnetron cathodes
for microwave use.

Not directly heated? I haven't noticed any circuitry to delay application of the EHT, except, perhaps, it's charging of the filter cap.?

Magnetrons have internal resonant cavities. The external oven cavity,
even if its Q
were in the hundreds of thousands, would stabilize in milliseconds.


Phys. Rev. B 6, 4557 (1972): Impurity Ionization in n-Type Germanium
by JF Palmier - 1972 - Cited by 10 - Related articles
(iii) The rise time of the apparatus is basically limited by the microwave cavity itself: Q/lTFo 10 nsec at Fo = 35 GHz with Q 700. ...
link.aps.org/doi/10.1103/PhysRevB.6.4557




How do they "get" this?


I disagree w/ a "The PTP" article that claims the sparking of an optical disc is the high field due to thin (sharp) edges. I think it's local heating that vaporizes the Al. I accidentally found the sparking is not instantaneous as one wold expect by the high E field. My oven had very low power (<500W?). I further reduced the power by adding glasses of water. The delay became seconds. On removal of the disk before sparking, I found it was quite hot.

bc

By the way, the only "sharp" edges are at the peripheries. The Al is continuous otherwise, and I think of constant thickness**. The groves are in the plastic disk. I think the varying adjacent permittivity causes uneven heating, which results in the circular spark tracks. IIRC, the sparking was intermittent with very reduced available power; I had removed the mode changing fan.


I was fortunate to purchase a reel of defective discs, which I discovered after they deteriorated! First angry and then thankful! The final cover coat is missing so the top layer is the aluminium. W/difficulty I can remove small pieces (cm size).