Cyclotron emissions from electrons.

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Chris Bradley
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Cyclotron emissions from electrons.

Post by Chris Bradley » Wed Apr 14, 2010 11:33 am


In a magnetron we have electrons spinning around in phase and coupling with structures that conduct the microwave energy.

However, if you have a volume permeated by a magnetic field and in which a pile of electrons are randomly (by location and across an energy distribution) spinning around, then what magnitude of cyclotron emission might you expect, given that the phases of the emissions will not be in phase (unlike a magnetron)?

I presume there is some sort of Poisson distribution of emissions, but putting it in overly-simple terms, one might suggest that all such emissions would, on average, cancel out because all the emissions are phased randomly?

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Re: Cyclotron emissions from electrons.

Post by JohnCuthbert » Thu Apr 15, 2010 7:38 pm

I think the number of emissions would equal the number of absorptions.

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Re: Cyclotron emissions from electrons.

Post by DaveC » Thu Apr 15, 2010 10:08 pm

Chris -

One can consider the simple open Magnetron configuration ( that is without any resonant cavities) as a magnetically "inhibited" diode. By this I mean, electrons are emitted by the cathode into the cathode-anode space, but as soon as they begin to move outward, they experience the V [cross] B force which makes their path a spiral.

If the B field is strong enough, they reach a stable orbit for the anode -cathode voltage, and the "diode" current remains small. The input power of (V * I ) just equals the new power radiated out of the system as EM radiation.

Too high an A-K voltage, and the electrons acquire enough velocity that their radius of curvature exceeds the anode-cathode distance, and a standard vacuum diode results, albeit one with a slight transit time delay.

The issue about the random phases is handled by squaring the amplitudes of all the EM emanations of the circling electrons. This number is always positive, and thus the output is positive for all Voltages and B values.

As I see it.....

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Carl Willis
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Re: Cyclotron emissions from electrons.

Post by Carl Willis » Thu Apr 15, 2010 10:54 pm

Chris,

I'm inclined to regard this kind of radiative effect the same way I would any situation where individual emissions add in superposition: thermal radiation from a hot object for instance (the vibrations and rotations of atomic electrons constituting a bunch of sources with limited or no phase relationship). Cancellation of the fields everywhere externally is not implied by having such a source with poor spatial coherence, and indeed a lot of power loss can be expected under some conditions (e.g. you put a resistively-loaded electric dipole antenna in the volume, oriented perpendicular to the magnetic field, resonant at the ECR frequency).

A magnetron oscillator doesn't work on the same principle at all Its frequency is determined by the resonant structure built into it or connected to it. Changing the magnetic field has no impact on this frequency.

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Chris Bradley
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Re: Cyclotron emissions from electrons.

Post by Chris Bradley » Fri Apr 16, 2010 6:03 pm

Some interesting responses.

On Carl's argument that cancellation doesn't happen for thermal emissions, so why should it for cyclotronic, I felt that was very convincing when I first read it, but can this be said with certainty? Clearly there is an emission from the external surface of a hot body. But, for example, I would imagine a candle is transparent (just doesn't look like it because it is so bright) but if you had a lump of something solid and opaque but of the same size as the candle flame and just as hot, would it be as bright? If a candle flame is no brighter than an object just as hot, and it is transparent, then a hypothesis that cancellation goes on for the light emitting sources inside the flame would not be excluded. But I do not know if either supposition holds true.

It may well come down to the other emission/absorption/re-emission argument. Like the Sun, perhaps the EM energy bubbles up to the surface of the plasma?

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