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Archived - Cathode? I don't need no stinking cathode!

Posted: Thu Apr 30, 2009 8:11 am
by Dan Tibbets
Well, at least for demo purposes , I don't need a negative power supply.
I added a second feedthrough to my Pressure Cooker demo fuser. When a positive biased power supply was hooked up to the outer grid and the central grid was grounded there was essentially no visible difference in the glow discharge compared with a setup with a central negatively biased grid with a periferal anode or ground.
If the periferal grid was used as an anode or cathode with the central grid floating the appearence was ... interesting. With the periferal cathode I presume the appearence would be similar to a single large grid (not tested). There was no central collection of glow- most was diffuse or towards the walls of the vessel. Perhaps with larger voltages and better vacuums the appearance would be more typical. See the picture below.

The inner grid was ~ 0.75 inches in diameter and the outer grid was ~ 3 inches in diameter. The irregular shape of the outer grid is due to a quick effort to create some symetry out of a sloppy bailing wire assembly, not due to any cleaver geometry considerations.

Dan Tibbets

Re: Cathode? I don't need no stinking cathode!

Posted: Thu Apr 30, 2009 2:37 pm
by Richard Hull

You can certainly do this in glass, but not in metal. The metal shell would be hot in relation to ground killing anyone touching it. If you floated a postivie supply metal chamber then either electrode could be dangerous.

Nothing new here.

Richard Hull

Re: Cathode? I don't need no stinking cathode!

Posted: Thu Apr 30, 2009 2:55 pm
by GEOelectronics
I've long thought that two power supplies, one positive, one negative with respect to gorund, two insulators, each only for 1/2 the needed voltage, and an ungrounded cathode structure ( internal) with the chamber grounded would work.

It would certainly work in glass. The question is, how large would the metal chamber need to be so that it's grounded structure wouldn't interfere?

Such a configuration would allow spark plugs for insulators.

George Dowell

Re: Cathode? I don't need no stinking cathode!

Posted: Thu Apr 30, 2009 3:14 pm
by Dan Tibbets
Both the positive and negative power supplies were grounded to the metal shell. I was figuring the electrons and ions would be attracted to the opposing grids more than the grounded shell. When the central grid was floated, I just disconected it from any power cable, and so long as I did not touch the recessesed contact it was isolated. The case remained grounded in all configurations.

I'm not sure if the negative and positive feeds could be configured without having a common ground- the shell..

Dan Tibbets

Re: Cathode? I don't need no stinking cathode!

Posted: Thu Apr 30, 2009 7:46 pm
by MarkS
Excellent work - especially on the cheapo pressure cooker fusor.

Why bother with two power supplies? I think you could take a transformer, bridge the output, and connect the positive side of the bridge to the anode, and the negative to the cathode. Since the e-fields are weaken by the inverse square law, the case wouldn't have to be much much larger then the anode.

Re: Cathode? I don't need no stinking cathode!

Posted: Fri May 01, 2009 3:21 am
by Dan Tibbets
Using the seperate leads from a center taped transformer to feed two seperate grids (one positive and one negative) makes sense, but the voltages would be out of phase (each 1/2 wave rectified). I'm not sure how the ions would interact with these intermittant fields, especially at low frequencies such as 60 Htz. Capaciters would help. I wonder if having different strength capaciters on each arm would have any interesting effects on the ions- sort of a low frequency POPS effect. I suspect much higher frequencies would be needed.
Could RC resonant circuits be applied to this system? Could it be done on the cheap at the voltages and currents expected?

Dan Tibbets

Re: Cathode? I don't need no stinking cathode!

Posted: Fri May 01, 2009 10:56 pm
by DaveC
Dan -

Potential difference is potential difference. It does not matter what we select to call "ground". That's only a reference point. What matters for the fusor, is that central grid is negative relative to the outer shell or electrode. What matters for safety, though is something else.

If the shell is electrically above (or below) ground potential, it becomes a major safety issue, and probably will adversely affect your gages, too. Be really careful with this mode of operation. Other than just for personal edification, I would strongly recommend against any continued operation like this. Too dangerous, unless you've got a good insulating shield around it. You would also need insulating hoses and etc to keep the peripheral hardware safe.

Some years ago I did demo fusor in a glass bell jar, with only the inner wire grid, at negative potential. The "shell" was simply the baseplate of the bell jar. Then I added a plate off on one side of the inner wire grid. Whether the plate was there, grounded or not didn't seem to affect the appearance of the central glow and rays , etc.

At the pressures the fusor operates, ions can be generated almost anywhere and then are attracted toward the more or less spherical inner grid. Arriving at the center and being largely neutralized, they create the central glow, which tends to be generally spherical with protrusions (rays) depending on the pressures, cleanliness and etc.
Bugles, bright spots and transitory asymmetrical glows are signs of local hot spots that usually go away with continued operation, as the surfaces clean themselves.

Dave Cooper