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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Coulter's system is CW, the magnetron runs off smoothed DC.
Just replacing the capacitor in a microwave oven with a smaller one will not give you CW operation. The output will be a series of pulses as it was before, but they will be shorter and of lower peak power.
Low power CW operation of oven magnetrons is difficult and best avoided if you don't specifically need CW.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I'll have to study his article again in more detail, but I was under the impression that microwave ovens switch the power to the magnetron on and off at intervals of a few seconds, and that this varies with the power setting. I was also under the impression that if you run a microwave magnetron continuously using an otherwise 'standard' microwave oven circuit, the magnetron overheats and burns out. I was also under the impression that Doug's intention was to reduce the power supplied to the magnetron to a level where it would run continuously without burning out.
I still have a few more articles to re-read as well. I'll try to post links when I find them.
EDIT: Looks like you are correct, Steve. I need to add an extra diode and filter capacitor in order to obtain 'true' DC.
EDIT: This is one of the papers on microwave cavities:
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, and you probably don't want true DC either. It is hard to design a magnetron that runs stably at low power: they were originally designed as pulsed devices for radar.
So, oven magnetrons don't perform well at low power. That is why they feed them with pulsating DC: they actually perform better when generating 100/120 brief high-powered pulses per second, than running off smooth DC at the same average power level. I didn't actually know they worked at all at low power, until I saw Coulter's article.
Another reason is cost, the smoothing capacitor costs money.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Steve McConner wrote ...
Yes, and you probably don't want true DC either. It is hard to design a magnetron that runs stably at low power: they were originally designed as pulsed devices for radar.
So, oven magnetrons don't perform well at low power. That is why they feed them with pulsating DC: they actually perform better when generating 100/120 brief high-powered pulses per second, than running off smooth DC at the same average power level. I didn't actually know they worked at all at low power, until I saw Coulter's article.
Another reason is cost, the smoothing capacitor costs money.
For a plasma or ion source DC would generally be required. Although some applications may require pulsed sources, I imagine this would be better accomplished by pulsing the power to the extraction electrodes.
HV caps don't cost much if you roll your own.
Here is a simple drawing of the discharge cavity that Doug copied:
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, they are also quite cheap if you buy Soviet surplus ones from Ebay, or gather them up from old microwaves and connect in series/parallel.
I would be more scared of the HV smoothing capacitor than the microwaves. Coulter used one the size of a breeze block that looked pretty lethal. You never specified in your original post that you wanted to build a plasma ion source, so I recommended pulsed operation as the safest option.
Most of the microwave energy is supposed to stay inside the cavity. It's not supposed to be able to get out of any aperture that's small compared to a wavelength. This is what's meant by "waveguide beyond cutoff" in the paper - the tube is too narrow for the EM wave to fit inside, so it can't pass through it.
But, if the plasma gets big enough that it extends out of the cavity, maybe it could couple it out in much the same way that a metal probe would. Taking into account the low power that Coulter's ion source ran at, and the existence of a nice hot plasma as proof that most of that modest power is being absorbed, I think any remaining RF leakage would be absolutely harmless.
You can use a neon lamp as a microwave field detector.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Steve McConner wrote ...
Yes, they are also quite cheap if you buy Soviet surplus ones from Ebay, or gather them up from old microwaves and connect in series/parallel.
I would be more scared of the HV smoothing capacitor than the microwaves. Coulter used one the size of a breeze block that looked pretty lethal. You never specified in your original post that you wanted to build a plasma ion source, so I recommended pulsed operation as the safest option.
Most of the microwave energy is supposed to stay inside the cavity. It's not supposed to be able to get out of any aperture that's small compared to a wavelength. This is what's meant by "waveguide beyond cutoff" in the paper - the tube is too narrow for the EM wave to fit inside, so it can't pass through it.
But, if the plasma gets big enough that it extends out of the cavity, maybe it could couple it out in much the same way that a metal probe would. Taking into account the low power that Coulter's ion source ran at, and the existence of a nice hot plasma as proof that most of that modest power is being absorbed, I think any remaining RF leakage would be absolutely harmless.
You can use a neon lamp as a microwave field detector.
I think you may have mistaken the magnetron for a capacitor Steve (I can't see a capacitor that looks like a housebrick in the photos)
His diagram shows a smoothing cap of 2 microfarads. The largest HV ones I've made in the past were 50 nanofarad but I should be able to make larger ones and parallel them.
I may try pulsed operation first, I apologise for being vague in my first couple of posts. I suddenly realised what Doug had meant by 'series capacitor' during an alcohol fueled flash of inspiration.
I enclose this excerpt from the above mentioned paper on discharge cavities regarding leakage.
How sensitive would a neon lamp be? what would be the best type to use, or are they all basically the same?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Can you spot the brick-sized capacitor here?
I don't know how sensitive neon lamps are, but I imagine they are all about the same.
A LED with a RF Schottky diode in anti-parallel, and the legs sticking out like a half-wave dipole, works too. (This verified by carefully prising the door of the coffee room microwave at work. )
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
James wrote ...
I would power the filament from a separate transformer and use a variac to control the one providing the HV.
.
As I understand it, doing this would vary the voltage, whereas using a smaller capacitor/variable capacitor will vary the current.
I'd assumed you'd need a certain PD between anode and cathode in order for it to work, and that varying the current would control the number of electrons 'spiralling round' creating the microwave radiation.
Are you (or anyone else) able to 'throw any light' on this?
Robert, nice pic, all the ones I found were in black and white.
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Basic Magnetron Question
