Magnetron Working Principle

Question

I was reading this simple description of magnetron working principle.

Can you explain me why when electrons go near the cavities, they start to resonate at microwave frequency? I do not see the link between these two phenomena.

Answer 1

Can you explain me why when electrons go near the cavities, they start to resonate at microwave frequency?

It's not the electron that resonates. Or the cavity.

It's the electric field produced by the moving electron, as constrained by the geometry of the cavity.

Answer 2

I have just read the Wikipedia article as I also know very little about cavity magnetrons. It is much better. What I read from the wiki page the following is wrong/missing from the text above:

• The electrons do NOT travel in a circle but in a spiral finally hitting the anode. (Yes @DKNguyen you where right)

• This causes local charge which has to be equalized an thus a current starts flowing in the anode. (The local charge also makes that the next electrons want to land elsewhere)

• Due to the cavities that current has to go "around" the holes.

• This causes microwave radiofrequency energy.

Please do not upvote this. I know nothing about magnetrons, I only repeated the wike data.

Answer 3

Due to the combination of the magnetic field and the cathode-anode potential, the electron stream naturally wants to orbit the cathode. A magnetron with no resonating cavities at all will still generate energy at microwave frequencies (see the Wikipedia article or maybe search on "Hull Magnetron" or "Single-anode Magnetron").

I believe (but am not totally sure) that the cavities just amplify the effect of the interaction of the electrons with the magnetic field.

Answer 4

The static magnetic field forces the electrons to make a spiral around the cathode as the anode voltage pulls them to the anode. That's easy to believe.

The electrons form a cloud where electrons have numerous random motion possibilities due the repulsion forces between each other. Random movements mean random velocity and electron cloud density changes and both of these mean random electromagnetic field - noise. The resonators catch a part of that noise. It surely contains something also in resonator's bandwidth and at least some field directions match with possible oscillation modes of the resonators. The resonators start to weakly oscillate.

The anode sections are connected together by binding rings which force the oscillations of the resonators to be in sync with each other. This enforces the oscillation remarkably because the fields of the oscillating resonators generate bunches to the electron cloud and the fields of bunched electrons bypassing the resonators increase the oscillation amplitude in the resonators much more effectively than fully random noise. Quite soon after the anode voltage is applied all resonators oscillate in sync and the amplitude grows as high as the increasing losses allow. There's a hole in one resonator which leaks the power out for useful purposes like heating or radar operation.

Another way to justify the oscillation of the magnetron is to think linear amplifying tube klystron. Magnetron becomes believable (if you know how the klystron work) when you thing it as a klystron which is bent circular to generate feedback.

Answer 5

Firstly, when a capacitor is charging or discharging, there is a charge current between the plates, which has its own magnetic field like a conductor, even if it's a vacuum. The magnetron anode is a copper ring either with keyhole slots, or vanes on the inside, surrounding the cathode. These vanes and the tube between form a ring of 1 turn centre tapped coils, with the tips of the vanes or the sides of the keyhole slots acting as capacitor plates, forming a ring of tuned circuits. Connecting rings join every second tip together as well as the shared inductance of the vanes coupling them, so the output only has to be connected to one. A strong magnetic field is passed vertically through the tube causing the electrons to spiral out toward the tips. They are formed into a spoked cloud by the magnetic fields from the charge current between the tips & act as a high speed rotary switch, driving each side of the center tapped tuned circuits negative alternately. As the tips become more negative, the cloud is attracted to the other ones, which are now more positive, but the charge current magnetic fields keep them there until near the negative peak, when the charge current decreases. The cloud tips are then allowed to switch to the positive tips & drive them negative until the charge current again decreases & allows them to switch to the next ones, which are now positive. The wires to the cathode are wrapped around ferrite slugs. In one Toshiba magnetron, they appeared to be tuned, as some of the turns were spread apart, leading me to believe that may have a second purpose, to increase the efficiency, by smoothing the cathode current, preventing current surges as the cloud switches from negative tips to positive ones. The inductors would absorb the almost 8 kV change in potential, but, at that frequency, everything is a capacitor including the cathode, so initially the potential change would be forced across the resistance of the switch (which is the electron cloud), causing switching losses. By tuning the inductors, the cathode will be resonated up in time with the switching. Now I come to think of it, it is a very good solution. The cathode can be resonated up to a positive peak almost equal to the potential of the positive vanes in time for the cloud to switch to them, so there will be minimal switching losses. The efficiency of Toshiba magnetrons is about 70%. The aerial on microwave oven magnetrons is a top cap like a large valve top cap. The magnetron does not conduct until the voltage across it reaches the designed voltage (normally 4KV), acting like a 4KV zener. This is because, below that voltage, the axial magnetic field that makes the electrons spiral out steers them back towards the cathode.

Answer 6

Supplement to the other fine explanations

The slot to the circular cavity pushes and pulls spiralling circular accelerated electron charge clouds with a hub distance called the interactive zone. The slots are caps and in between is inductance. It opposite to a wire where the magnetic force is radial and the current is axial. Here the magnetic DC force is axial and the negative HV electric field is radial. They tap off free electrons on one of the peaks between cavities as the hub creates a circular electric wave of moving charges with the conservation of momentum e/m charge to mass ratio working in this principle in balance with the magnetic and electric field energy.

When two strong bipolar magnets on both ends of the circulating resonant microwave cavity with a -10kV hot centre cathode and 0V anode, the magnetic force becomes so great that the spiral oscillating bipolar wave currents move into an out of phase position being attracted to the + cap end then repelled by the - cap end that the electrons wobble around the hub gap where the electrons never reach the anode.

A scientist by the name Hull discovered this over 80 years ago where this magnetic field to orthogonal charge ratio to magnetic field ratio with resonant cavities produces a threshold of no current which they called the Hull Threshold. He did not have a pressing need for it until the US Military was in need of RADAR with lots of money.

Unfortunately, the RADAR guys used to get random noise so when they confirmed aircraft near Pearl Harbour, the big chiefs denied it. But there's a big difference between random noise and incoming birdies, and the Aussies also had working RADAR confirmed it, but they needed a disaster to united the passion of the nation to war. Same thing with nine-eleven and a planned implosion to coincide after a known attack. Meanwhile, a little Jewish Physicist proved the possibility to detonate massive atomic energy could be used to trump the enemy. History repeats itself as Da Vinci paid his bills designing weapons.

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