Electronics > RF, Microwave, Ham Radio
span of typical microwave oven magnetron (power level). mtron physics discussion
CopperCone:
How much can a output level of a typical magnetron be controlled via voltage regulation (i.e. putting a VARIAC infront of the high voltage transformer)?
I understand their frequency span is around 40 MHz , tunable by bias current (say from 2400 to 2440 MHz), but how about their power output?
I'm guessing they will stop working when the voltage gets low enough.. but there must be some sort of span associated with their output?
Can they also be temporarily over driven (by increasing voltage)? (i am however interested in increasing their longevity due to load mismatch).
Are there 'sharp' points to where the voltage will cause the device to immediately stop operating or having some kind of thermal overload caused by electron beam formation? Or can you do 'tricks' like send it a single pulse that's highly over driven to get massive power output levels from them, so long average power level is low? (interesting for the purposes of radar etc).
Does anyone have any bounds for these magnetron devices? (i.e. something like, 1000% of rated power output is OK so long the signal is 'chirped' with a duty cycle of 50us/1 second. Or does device geometry prevent this? (assuming you potted the connectors to prevent arc discharge. I.e. turn the 50% duty cycle 600W device into a multi kilowatt (or better?) device with a low duty cycle of like, 0.001%?
not that this type of knowledge is well defined, even with transistors...
or how about a one time 'destructive' pulse rating? How many (megawatts) can you get out of it for how many (nanoseconds) for a one time event? (say you got it in a sulfur hexoflouride environment, waveguide included). Connected to some kind of capacitor bank/marx genrator/vandegraf/flux compression generator thing
i did notice that TWT just enter a 'power saturation' stage, and you just get a little 'noise hill' under your signal of interest (rather then increasing signal of interest power further past the power rating of the tube).. I only have a spectrum analyzer, it would be interesting to see what a total power meter would say about the energy content of the noise hill however... (literary looks like your signal is a mole that popped out of a mole hole on a spectrum analyzer), about 40db down from the peak (within limits of the amplifier I have),... but I still wonder.. would that noise hill be massive broad band garbage if 'lighting struck' the tube grid or whatever?
Ian.M:
The Variac idea is dead in the water. Read https://www.repairfaq.org/sam/micfaq.htm
The Magnetron needs a filament supply which comes from a winding on the EHT transformer. As the body of the magnetron is grounded, and it is impractical to have 5KV isolation, a separate cathode is not possible, so the whole filament circuit runs at the negative EHT potential. If you increased the EHT, the filament would be overrun and if you decreased it it would be underrun, which would risk cathode stripping.
Also, the EHT transformer is run on the ragged edge of saturation and has magnetic shunts to give it essentially a constant current characteristic. Increasing the supply voltage would take it into full saturation and overheat the primary. Decreasing the voltage would reduce its ability to maintain a constant magnetron current, interfering with normal magnetron operation.
While it is probably possible to use a microwave oven magnetron for low duty cycle high power pulsed operation, similar to a RADAR magnetron, to do so you'd have to basically build a RADAR style EHT PSU and isolated filament supply for it, and use a fast high voltage switching device to gate the cathode supply. The limitation for short pulses would, most likely, be the filament's ability to emit enough electrons without disintegrating.
CopperCone:
well its not hard to get a separate transformer.
Thermal to me implies SLOW. Like 0.1 Hz would be fast for thermal. And you can parallel transformers or use a different voltage source if required.
I am interested in the tube itself. It's only like 30$.
ALSO, you just cut the HV wire from the transformer body and its isolated.
The limitation for short pulses would, most likely, be the filament's ability to emit enough electrons without disintegrating. Any idea what this is? Sounds rather high. Wish I had a busted magnetron to look at, perhaps its determinable by first principles.
I'm guessing what would actually happen is the special coating used on the material would be vaporized till the emission voltage requirement is changed... so some kind of nonlinear behavior would manifest itself. This would be a differential equation with a heat gradient across the material big enough to cause surface boiling? Does the electron emission cause some kind of change similar to 'pressure change' that effects the phase change of matter that changes calculations based on vapor pressure and melting point etc? Is there some kind of accepted fudge factor for this to allow for conventional calculation? Or is in the realm of requiring simulation? I imagine pulsed gross electrical heating capable of destroying the transmission medium requires special consideration. And once significant amount of matter is stripped... i imagine the nature of the electron beam changes right (since significant quantity of metal ions is being carried by the beam at this point, so it turns into a 'heavy' particle beam? (relativistic heavy ion particle beam physics come into play with enough voltage.. perhaps that requires a long accelerator though, the voltage requirements would be like >1 gigavolt to cause this in a short range of the tube I guess?
easy enough to calculate this ion acceleration.. but at that point the thing is fucked, guess its pointless to calculate,. perhaps later for the sake of completeness
Does the device still function as the thing is disintegrating and non-electron matter is being transfered via the beam? (and just randomly offgassing I guess)... it would contaminate the grid etc. how does the output signal look in this operating regime? does it depend at all on the density of non electron matter in the particle or just how contaminated things get? but these things use a surface coating to decrease heat requirement of electron emission.. but that layer would slowly decrease in thickness... so for some period of time you are operating with a intact coating but a contaminated electron beam (is this detectable? or only when the layer is fully stripped that increases electron emission density of certain points on the surface of the emitter (where the full current is emitted from a decreased surface area)? Is a monoatomic layer thick enough.. or do the coatings need to be multiple atoms thick? knowing the answers to these questions would make modeling more easy (to avoid calculating irrelevant regimes) .
I am kind of imaging the filament (or a portion of it breaking the circuit) would explode into plasma and fragmentation in short order, rather then slowly vaporize, however if the energy is high enough then it would fully 'detonate' due to plasma conduction and the plasmas position in space during that infinitesimally small time period. What happens to the electron beam in this scenario? This could be imagined as a massive matter flow between the filament and the grid right? highest peak power level?
You can pulse it with a thyrotron btw, pretty easy.
T3sl4co1l:
--- Quote from: CopperCone on August 15, 2017, 06:36:05 am --- The limitation for short pulses would, most likely, be the filament's ability to emit enough electrons without disintegrating. Any idea what this is? Sounds rather high. Wish I had a busted magnetron to look at, perhaps its determinable by first principles.
--- End quote ---
A space charge cathode is normally capable of pulsing at 10-20x normal (continuous) current levels for a few microseconds.
Beyond that, damage can occur (cathode stripping, saturation, ion bombardment?).
Classic hard modulator tubes, magnetrons were used with, are rated in the 10kV and 20A peak range, so this sounds reasonable even for a commercial magnetron.
I don't know what differentiates a pulsed magnetron from a CW one, if anything. I'm sure the pulsed lifetime will be more uncertain than with a proper radar magnetron.
--- Quote ---I'm guessing what would actually happen is the special coating used on the material would be vaporized till the emission voltage requirement is changed... so some kind of nonlinear behavior would manifest itself. This would be a differential equation with a heat gradient across the material big enough to cause surface boiling?
--- End quote ---
It doesn't heat up further.
Nonlinear behavior is saturation, where the current can't go any higher because the surface emission has been exhausted (depleting the space charge).
Mechanical failure can occur due to the strong electric field pulling the cathode apart. No plasma is involved.
Some sparking may occur, but that's more a function of flying bits touching other high voltage bits and internal arcing occuring.
--- Quote ---Does the electron emission cause some kind of change similar to 'pressure change' that effects the phase change of matter that changes calculations based on vapor pressure and melting point etc?
--- End quote ---
Hmm, I hadn't thought about electron flow in terms of pressure.
It should be comparable to the Maxwell pressure (e_0 * E^2 / 2), since that's what's pulling the electrons along, after all.
--- Quote ---i imagine the nature of the electron beam changes right (since significant quantity of metal ions is being carried by the beam at this point, so it turns into a 'heavy' particle beam? (relativistic heavy ion particle beam physics come into play with enough voltage.. perhaps that requires a long accelerator though, the voltage requirements would be like >1 gigavolt to cause this in a short range of the tube I guess?
--- End quote ---
Right, that ain't gonna happen.
Electron relativistic effects start at a few 100kV, but in such a small tube, it will flash over first (field emission from points and surfaces, ion bombardment, arc discharge).
Note that negative ions flow towards the anode, while positive ions flow towards the cathode. Being very massive (ions like oxygen and copper), they move very slowly, and do not spiral appreciably in the magnetic field. An electron-ion cascade discharge (an arc) occurs very quickly once started.
--- Quote ---it would contaminate the grid etc.
--- End quote ---
Grid?
Magnetron?
Grid contamination is another player in the lifetime of pulsed hard vacuum tubes (like modulator tetrodes), however.
Anyway, field emission of the microwave resonators (the corners of the slots, the antenna element) would also be a limiting factor. Low VSWR will certainly be needed.
I don't see offhand any amateur pulsed magnetron experiments, but there may be a good legal reason for that.
Tim
CopperCone:
ah that rules out alot of mystery.
If you don't care about integrity, and you only want one last hurrah from the thing, how much can you get out of it (with fatal tube damage)?
and right, I started confusing my tubes.
The arcing can only occur if metal is vaporized right? Since its in a vacuum. Normally it happens because of ionized atmosphere I think.
for the sake of discussion, say its hooked up to a explosive flux compression generator or something obscene like that capable of massive voltages/currents (not sure if EFCG does high voltage? think its just a massive current)
can the saturation effect be (temporarily) over come with a truely massive energy spike? Can you get a megawatt spike out of it or something like this for one time (like a ship borne radar).
your math says that it can possibly do 10kv@20A, or 200kW. Thats fair for a radar set. Sounds like IGBT territory, if you only wanna do it once ( I don't think they can turn off fast enough). You would either need to use a timed shunt to redirect the energy away from the tube or use something faster (SCR? large thyratron? )
In terms of damaging electronic devices, I imagine that the regular pulsed 50% duty cycle CW mode of operation would be worse right? But it seems to me that it kind of depends on circuit impedance, and that some stuff might actually be damaged more by a single high energy pulse then CW....
Not that I intend to do this, but I would not want to damage my laboratory.
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