Amateur polywell?

[Chris Bradley]

Did you isolate them, or did they form part of the electrical circuit that is the gas-discharge bulb of the fusor (i.e. were they electrically connected to inner or outer grids)?

[ScottC]

They "sandwiched" the inner SS grid in pairs (one outside, one inside) because they couldn't stick to the inner grid itself. I would assume some portion of the current was flowing through them. I think I tried just laying one on top of the grid also.

Scott

[Doug Coulter]

For what it's worth here are some of my observations.

I have used 6v 2A quartz halogen bulbs (from lawn fixtures) with very good success for electron emitters. BillF found me some Y2O3 to coat them with, and it helps greatly, while being relatively resistant to occasional air exposure. A low voltage/high current filament is best, and the one from a magnetron ought to be very good too, but is sort of 90 deg turned the wrong way..Low volts is good as there won't be as much difference in potential across the filament to foul up efforts at focus. I built an E-gun based on this to fool around and be a heat source that used .7" ID, 1.1" OD pieces of bronze pipe between pieces of 1" pyrex for the lensing and accelerators, and it works fairly well. With the small 6v bulbs and Y2O3 I run about 2.1-2.5 volts and get about 30 ma emission (depends on the voltage gradient to suck the space charge off the virtual cathode). Without the coating, it takes about a volt more, and they don't last nearly as long. I use a diamond wheel in a dremel tool mounted on the toolpost of my lathe to cut the tops off the bulbs and leave a smooth surface to seal onto the vacuum system with apezion wax. Hysol or JB weld would probably work too, but you wind up replacing these once in awhile...I mount mine to a fat piece of copper rod I machined out on my lathe to be a Pierce diode, and I put a heatsink on the outside of this to keep it cool so the apezion doesn't melt in use.

Another promising candidate is tungsten-rhenium alloy wire from type C thermocouples, which is supposed to be a lot more resistant to oxygen. It is certainly more ductile than pure tungsten.
Haven't tried this myself so far, but I get the stuff from Nanmac, and it's available in small gauges (but the price will make you wince). It has a lot more resistance than pure tungsten, so you can use a fatter size for the same length.

Triple carbonate filaments as used in normal vacuum tubes will not take exposure to air; once activated they have pure Ba, Sr, and Ca which is what does the emission -- it's the same stufff used as a getter! I have successfully rejuvenated them once or twice, but that's it, and each time they emit less than the time before. The older ones (thicker coatings) are the ones that can go 3 times. You could reuse a CRT gun but you'd have to replace the emitter with something more robust -- I plan to try this myself, as they are better precision than things I can easily make here (but small aperture and low current).

For low volts and high current, the best source is probably going to be a switching supply from an electronics surplus house. You can also make one by chiseling off and rewinding the secondary from a microwave oven transformer -- we have done that here a few times. Take out the extra iron between the windings, and if you want it to run cool, add about 20 turns in series with the primary, as these normally run deep in core saturation and get hot. We use them to heat evaporation tubs in the few volt hundred amp region with great success. But the size of the filter caps you would need for DC with low ripple at high current is prohibitive. So the switcher is the way to go, and not all that expensive. IIRC I got a 3.3v 150A supply from Marlin Jones for under $100.

Seltzman found out you can't push enough Fluoinert through a tiny tube to get much cooling, even with a positive displacement pump. If you need fat tubing you could probably use mineral oil as a coolant just fine. I get mine at McMaster-Carr, it's very pure and a great insulator. I am using it here to replace the PCB oil in some old X ray supplies I am rebuilding with success.

Try to keep away from plastics, They may not all (or always) be as bad as stated above, but they are easy to heat to the point of trouble in a vacuum, even the rather expensive polyimide and teflon. Even pyrex glass can be a problem as it will shatter if suddenly heated by a stray beam -- a real mess to clean up (and if you're running a turbo, can wreck it). I use alumina. For protecting my main windows, I use another pyrex disc, again from McMaster. This way when it gets coated, I can just dunk it in the pickling tank from my electroplating setup. I hold them up to the original port with some tiny CuBe clips around the edge so they are easy to R&R. This avoids having to put acid on the nice glass to metal seal of a main window or door. Mica also works pretty well for this.

Pix -- here is the emission source and the whole thing mounted on my big tank. I floated the focus/emitter power supplies on the little table so I can apply serious HV to the whole mess to get a powerful beam that enters the tank at "ground" potential. It works surprisingly well for an empirical design (eg the "looks right method"). The lenses are machined to fit inside the pyrex segments except for a ring in the middle which is where the apezion goes to seal them. I plan to rebuild this to use closer lens element spacing which should work better. This nets about 20ma to a target in the tank at whatever HV I'm willing to put on it. The ring mount into the tank itself is the final accelerating electrode, at ground potential. I adjust the focus and emission pots with a long fiberglass stick with a screwdriver bit mounted on the end. To work, this needs to run at the E-5 mbar level or below, but if you let in some noble gas you can run it for awhile at a slightly higher pressure and see the beam. I use a couple of gel cells to run the filament and focus power supplies, the latter of which is a CCFL inverter and some diodes that uses a 3 terminal voltage regulator on the input to adjust the voltage.