Construction of Nonconductive Internal Components of Vacuum Systems
Posted: Thu Jul 25, 2019 1:19 am
Simple fusors don't usually demand too much complexity in their internal construction. The grid goes on the end of the feedthrough stalk.
Things get more complicated if you're making ion guns, plasma diagnostics, accelerator systems such as BOTtles, or complicated ion/electron optics that don't really have much to do with fusors. If you do that, you need to have a bunch of metal parts that are mechanically connected but electrically insulated, all in the vacuum. And it appears to be a bit tricky to do this, and do it compactly.
Ways to do this as far as I know, include:
- Macor or Mycroy / Crystex machinable ceramic -- expensive, requires machine shop -- these are ceramic materials that machine somewhat like an even dustier and more brittle version of cast iron. Macor has a thermal expansion coefficient close to that of various 400-series stainless steels, which might be useful. All of them are quite expensive in any reasonable sized pieces -- there seems to be a bunch of off-brand Macor on Ebay though. Obviously these have all the advantages of ceramics in a vacuum. They also tolerate quite a bit of heat.
- PEEK, Ultem -- Insanely expensive -- PEEK is a very hard, strong, low-outgassing, chemical-resistant, high-temperature plastic. Ultem is much like PEEK but brown, translucent, and just less so, and also often much cheaper (but still expensive enough to discourage casual experimentation). And so they're usable in vacuums. Ultem sometimes comes up cheapish on Ebay. I wonder, though, how practical they are under plasma.
- 3D Printed PEKK or Ultem -- moderately costly, hard to do at home -- Stratasys Direct offers 3D-printed parts made out of PEKK, a very similar material to PEEK, or Ultem. Minimum cost for a part is about $40, and many parts of significant size and complexity can be manufactured for under $200. Make sure you don't have high definition mode turned on, it will triple the price. I have never tried this under vacuum, I worry that the somewhat porous FDM plastic construction might make for massive virtual leaks and/or enhanced outgassing. Also, NASA has released a paper on how to modify a hobbyist-grade 3D printer such as a Prusa to print Ultem.
- Ceramic or PEEK standoffs: --availability, cost, machining needs-- Threaded standoffs made of ceramic material or of vacuum-compatible plastics are available. Unfortunately, they are often very expensive, and making compact assemblies may require machining and/or cutting plate. Re-usability is a plus. You may also need to worry about virtual leaks from the trapped gas in the threaded holes, unless you drill or side-grind your screws.
- Glass Blobs -- skill, material constraints, strength, equipment -- If you're making a vacuum tube and already making lots of glass-metal seals, it does make some sense to just make more glass-metal seals to blobs of glass to hold your metal in place. Many small CRTs and vidicon tubes tend to have this.
- Epoxy blobs -- is this even a good idea -- We all know of Hysol 1C. If you add a bit of MgO powder to it, you can make it into a stiff paste rather than a soft semi-liquid glue. It's then possible to mold it around things and into shapes. I am extremely skeptical as to whether this is compatible with good vacuum quality.
- Terminal blocks -- You can get PEEK or ceramic bodied terminal blocks.
I am curious whether anybody has more experience with this, or better methods/ideas/materials.
Things get more complicated if you're making ion guns, plasma diagnostics, accelerator systems such as BOTtles, or complicated ion/electron optics that don't really have much to do with fusors. If you do that, you need to have a bunch of metal parts that are mechanically connected but electrically insulated, all in the vacuum. And it appears to be a bit tricky to do this, and do it compactly.
Ways to do this as far as I know, include:
- Macor or Mycroy / Crystex machinable ceramic -- expensive, requires machine shop -- these are ceramic materials that machine somewhat like an even dustier and more brittle version of cast iron. Macor has a thermal expansion coefficient close to that of various 400-series stainless steels, which might be useful. All of them are quite expensive in any reasonable sized pieces -- there seems to be a bunch of off-brand Macor on Ebay though. Obviously these have all the advantages of ceramics in a vacuum. They also tolerate quite a bit of heat.
- PEEK, Ultem -- Insanely expensive -- PEEK is a very hard, strong, low-outgassing, chemical-resistant, high-temperature plastic. Ultem is much like PEEK but brown, translucent, and just less so, and also often much cheaper (but still expensive enough to discourage casual experimentation). And so they're usable in vacuums. Ultem sometimes comes up cheapish on Ebay. I wonder, though, how practical they are under plasma.
- 3D Printed PEKK or Ultem -- moderately costly, hard to do at home -- Stratasys Direct offers 3D-printed parts made out of PEKK, a very similar material to PEEK, or Ultem. Minimum cost for a part is about $40, and many parts of significant size and complexity can be manufactured for under $200. Make sure you don't have high definition mode turned on, it will triple the price. I have never tried this under vacuum, I worry that the somewhat porous FDM plastic construction might make for massive virtual leaks and/or enhanced outgassing. Also, NASA has released a paper on how to modify a hobbyist-grade 3D printer such as a Prusa to print Ultem.
- Ceramic or PEEK standoffs: --availability, cost, machining needs-- Threaded standoffs made of ceramic material or of vacuum-compatible plastics are available. Unfortunately, they are often very expensive, and making compact assemblies may require machining and/or cutting plate. Re-usability is a plus. You may also need to worry about virtual leaks from the trapped gas in the threaded holes, unless you drill or side-grind your screws.
- Glass Blobs -- skill, material constraints, strength, equipment -- If you're making a vacuum tube and already making lots of glass-metal seals, it does make some sense to just make more glass-metal seals to blobs of glass to hold your metal in place. Many small CRTs and vidicon tubes tend to have this.
- Epoxy blobs -- is this even a good idea -- We all know of Hysol 1C. If you add a bit of MgO powder to it, you can make it into a stiff paste rather than a soft semi-liquid glue. It's then possible to mold it around things and into shapes. I am extremely skeptical as to whether this is compatible with good vacuum quality.
- Terminal blocks -- You can get PEEK or ceramic bodied terminal blocks.
I am curious whether anybody has more experience with this, or better methods/ideas/materials.