Famulus fusion
Some considerations. Teflon is a tough high temperature plastic, but it will probably sputter ferociously when hit by plasma. To mitigate this some you might sand or otherwise add some curvature on the sharp edges of the Teflon disks. This would decrease the exposure to hot plasma some- much like WB6 compared to WB4 coil casings. Also, if possible lining the inner face (and inner and outer sides) with aluminum or stainless steel would possibly decrease the outgassing/ sputtering. I have used Teflon tape on the stalk supporting the cathode grid in a fusor. It quickly browns and disintegrates, no doubt coating the entire chamber with a film of vapor deposited Teflon products- admittedly the vacuum levels were not as good as you anticipate.FAMULUS wrote:supports are ceramic, coil formers are teflon. brackets are aluminum, the screws will all be stainless in the end.
Perhaps you could cut and mold thin aluminum flushing sheet metal to the exposed surfaces, even using aluminum wire to secure it. Just make sure your windings are insulated from the presumed high voltages on the magrid structure (assuming you plan to have high voltage on the magrid, as opposed to high voltage electron guns). trying to glue the aluminum plates (or using aluminum tape) to the Teflon may not be a good idea as it may not stick well and the glue may outgass a lot. Just wrapping the Teflon with heavy aluminum foil might work, if arcing concerns do not become a problem. If you completely enclose the teflon in aluminum foil, make sure there are some holes so that the wire wrapping grove can quickly reach low vacuums and not be a slow leak that impedes reaching final low chamber pressures in a reasonable amount of time.
I wonder how much outgassing teflon does? It may take a prolonged pump down time (several days or more) to condition the the chamber and grid.
Dan Tibbets
To error is human... and I'm very human.
They say nothing, probably because power/time levels were not so high to become worried by Teflon ablation.
Some interesting links for FAMULUS, maybe not for this phase experiments but for the next one:
Teflon Ablation modelling and literature:
http://www.wpi.edu/Pubs/ETD/Available/e ... chmann.pdf
Reflection of an incident wave in a Teflon medium:
http://www.pupr.edu/Plasma/download/Imp ... ormato.pdf
Some interesting links for FAMULUS, maybe not for this phase experiments but for the next one:
Teflon Ablation modelling and literature:
http://www.wpi.edu/Pubs/ETD/Available/e ... chmann.pdf
Reflection of an incident wave in a Teflon medium:
http://www.pupr.edu/Plasma/download/Imp ... ormato.pdf
I also have reservations about the stainless fastners. Maybe it would be better to use teflon.
But, Giorgio, as you said, the time scale would seem to help minimize material issues.
I also have concerns about wire type. From his photos, it looks like he is using THHN, and given he is now in the realm of high voltage, that will not be enough. Hate to see that cap bank explode when a THHN grounding wire gets a pinhole and direct shorts.
Famulus, you need to take care about air gap spacing, sharp edges, and wires. When you fire the cap bank, as you know it induces some physical shock in the rig. That in turn over a number of shots can translate to unseen rubbing as well as insulation breakdown on any sub-rated components. Put together can mean a failure. In my past I have typically gone at least twice the rating on pulse fired components. Just food for thought dude. Hate to see your Sydney rig go the way of WB-6.
But, Giorgio, as you said, the time scale would seem to help minimize material issues.
I also have concerns about wire type. From his photos, it looks like he is using THHN, and given he is now in the realm of high voltage, that will not be enough. Hate to see that cap bank explode when a THHN grounding wire gets a pinhole and direct shorts.
Famulus, you need to take care about air gap spacing, sharp edges, and wires. When you fire the cap bank, as you know it induces some physical shock in the rig. That in turn over a number of shots can translate to unseen rubbing as well as insulation breakdown on any sub-rated components. Put together can mean a failure. In my past I have typically gone at least twice the rating on pulse fired components. Just food for thought dude. Hate to see your Sydney rig go the way of WB-6.
Famulus, there is a lot of armchair advice of dubious value here.
As someone who is currently running a magnetic plasma experiment with copious amounts of teflon, peek and PE in the chamber, I can tell you this; whatever you expect to happen - something else will. Just run it and be prepared to change it. No amount of advice will actually help very much.
These comments may help or hinder your design thoughts, but are not meant to be 'design-advice':
1) Teflon goes brown in chambers because electrons that are whizzing around collect carbon and dump it on the teflon (and also on any other surfaces). Carbon is ubiquitous - don't ask me where it comes from, but I have such effects, I have put the materials into EDS analysis, and it is all carbon! You see this effect with SEMs as well. Put a sample in for a bit too long and it comes out all covered in carbon. The electron beam picks up carbon that is floating around, and carries it along. Like I say, don't ask me about how or why, it just happens! Some claim it is 'metallic sputtering', and there probably is some of that going on in some chambers, but I'd wager that whenever someone sees 'brown' they've looking at carbon. I rub it off with a fine abrasive polish. No chemicals needed.
2) Initially, any charged beams making their way to teflon or any other materials will build up a charge. The voltage on that surface will likely build up. It will then reduce the flow of those charged particles until some equilibrium condition is reached, or there is a discharge. Usually, it's a discharge! Charge will likely build up well above the actual acceleration voltage you are using. Then, at some point, you'll see a direct discharge from it to some other point with a different potential.
3) As the mechanisms of 1 and 2 go on, you will find plastics will become coated with carbon and become more and more conductive, attract more and more energy, which causes some sublimation. This will further accelerate carbon coating and further accelerate discharges.
4) Teflon is 'vacuum compatible' if kept fairly cool, but it outgasses for a while. Once assembled, pump the chamber down and keep it sealed, pump it down some more again each day for a few hours, till all the adsorbed gasses have been sucked out of the teflon. Teflon itself doesn't outgas, but it lets gasses in, and through it, so it isn't much good as a seal.
5) Where neutral plasmas beam into, or come into, contact with plastics or metals, then they may sputter. Try aluminum at those points, it sputters least of all the readily available materials.
As someone who is currently running a magnetic plasma experiment with copious amounts of teflon, peek and PE in the chamber, I can tell you this; whatever you expect to happen - something else will. Just run it and be prepared to change it. No amount of advice will actually help very much.
These comments may help or hinder your design thoughts, but are not meant to be 'design-advice':
1) Teflon goes brown in chambers because electrons that are whizzing around collect carbon and dump it on the teflon (and also on any other surfaces). Carbon is ubiquitous - don't ask me where it comes from, but I have such effects, I have put the materials into EDS analysis, and it is all carbon! You see this effect with SEMs as well. Put a sample in for a bit too long and it comes out all covered in carbon. The electron beam picks up carbon that is floating around, and carries it along. Like I say, don't ask me about how or why, it just happens! Some claim it is 'metallic sputtering', and there probably is some of that going on in some chambers, but I'd wager that whenever someone sees 'brown' they've looking at carbon. I rub it off with a fine abrasive polish. No chemicals needed.
2) Initially, any charged beams making their way to teflon or any other materials will build up a charge. The voltage on that surface will likely build up. It will then reduce the flow of those charged particles until some equilibrium condition is reached, or there is a discharge. Usually, it's a discharge! Charge will likely build up well above the actual acceleration voltage you are using. Then, at some point, you'll see a direct discharge from it to some other point with a different potential.
3) As the mechanisms of 1 and 2 go on, you will find plastics will become coated with carbon and become more and more conductive, attract more and more energy, which causes some sublimation. This will further accelerate carbon coating and further accelerate discharges.
4) Teflon is 'vacuum compatible' if kept fairly cool, but it outgasses for a while. Once assembled, pump the chamber down and keep it sealed, pump it down some more again each day for a few hours, till all the adsorbed gasses have been sucked out of the teflon. Teflon itself doesn't outgas, but it lets gasses in, and through it, so it isn't much good as a seal.
5) Where neutral plasmas beam into, or come into, contact with plastics or metals, then they may sputter. Try aluminum at those points, it sputters least of all the readily available materials.
I'm inclined to agree with Chris here. While he is a bit mystified with exactly what is going on with the deposits, my experience is that any claim of certainty is an overstatement, and confessing that the exact processes are mysterious is honest.
Insulators do indeed charge up in this sort of experiment. You can actually watch it happen in a scanning electron microscope. When it collects a charge, it will start doing things you don't think an insulator should do, like attracting species of opposite charge. I believe they act as recombination surfaces, and I've seen them throw fireworks.
Carbon, eh? The chamber used for WB-6 was electropolished when new, an absolutely beautiful bright mirror surface. The photos you guys have seen are distinctly brownish, maybe hints of purple. The system had two mechanical forepumps, each with a roots blower, and six turbopumps running in parallel (probably rarely all on at once). It could pull to below 1e-7 torr with no effort at all, and probably much lower when really clean. Even so, some forepump oil probably made it back in the chamber.
Hydrogen comes off in these experiments enough to make experimenters read claims of hydrogen appearing from zero point with just a pinch less skepticism than theorists have. Undoubtedly ... I believe ... it comes from hydrides in all the metal parts.
There is a pottery shop here in Manassas that teaches pottery classes. I've had them fire ceramics for vacuum projects, but pretty low-grade stuff with more porosity than I would like. I've been tempted to take a class in porcelean ... that's how insulators were made in the old days when lab equipment was all hand made. McMaster Carr does have some "green" ceramic blanks that can be machined and filed, then fired in a kiln.
Insulators do indeed charge up in this sort of experiment. You can actually watch it happen in a scanning electron microscope. When it collects a charge, it will start doing things you don't think an insulator should do, like attracting species of opposite charge. I believe they act as recombination surfaces, and I've seen them throw fireworks.
Carbon, eh? The chamber used for WB-6 was electropolished when new, an absolutely beautiful bright mirror surface. The photos you guys have seen are distinctly brownish, maybe hints of purple. The system had two mechanical forepumps, each with a roots blower, and six turbopumps running in parallel (probably rarely all on at once). It could pull to below 1e-7 torr with no effort at all, and probably much lower when really clean. Even so, some forepump oil probably made it back in the chamber.
Hydrogen comes off in these experiments enough to make experimenters read claims of hydrogen appearing from zero point with just a pinch less skepticism than theorists have. Undoubtedly ... I believe ... it comes from hydrides in all the metal parts.
There is a pottery shop here in Manassas that teaches pottery classes. I've had them fire ceramics for vacuum projects, but pretty low-grade stuff with more porosity than I would like. I've been tempted to take a class in porcelean ... that's how insulators were made in the old days when lab equipment was all hand made. McMaster Carr does have some "green" ceramic blanks that can be machined and filed, then fired in a kiln.
What caught my eye the most was the photo posted for the meter shunt. A THHN wire riding around two sharp edges. By color, would appear to be grounding (but that is a guess on my part).
The other construction point I see which could fail with multiple shots is the reliance on crimp to lug connections. Professional crimping creates a solder typ connection, these crimps all look hand tool made that in turn does not garuantee the fusing. For this type of application in my past I used non-insulated lugs, hand crimped, then hand soldered, or just straight-up soldered on lugs.
All in all, good fun. I do miss un-intended blowing of stuff up.
The other construction point I see which could fail with multiple shots is the reliance on crimp to lug connections. Professional crimping creates a solder typ connection, these crimps all look hand tool made that in turn does not garuantee the fusing. For this type of application in my past I used non-insulated lugs, hand crimped, then hand soldered, or just straight-up soldered on lugs.
All in all, good fun. I do miss un-intended blowing of stuff up.
Today in the lab: constructing a Langmuir probe from ceramic tubes and a lightbulb filament. (At least, I'm guessing that's what's going on.)
https://picasaweb.google.com/FAMULUS.fusion/20110720
https://picasaweb.google.com/FAMULUS.fusion/20110720
Temperature, density, confinement time: pick any two.
Today he connected the magrid coil inside the vaccuum chamber to the coil power supply and fired the coil inside the chamber. It looks like everything's in place for him to start testing. Of course, EMC2 has a bigger magrid, but I imagine we'll see some real numbers from Famulus first.
Temperature, density, confinement time: pick any two.
I am still worried about his wire laying techniques. In the photos today he showed the coil power feedthrough with the coated wire running up into and through the metal pipe. He runs a high risk of abrasion over repeated firings with a corrosponding grounding failure and arcing. This could risk his vacumm pump as well as the coil power supply.