Grid heating by Current vs Ion impacts

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
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Rich Feldman
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Re: Grid heating by Current vs Ion impacts

Post by Rich Feldman » Sat Dec 24, 2011 6:29 pm

Frank beat me by a minute.
I was about to say, the current in those wire loops is zero at some point opposite the power connection - and that's where they seem to be the hottest!
If the current were nonzero at the point of symmetry, how would its direction (sign) be chosen?
The absolute current can only increase, monotonically, as you move from the stagnation point toward the power connection.

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Chris Bradley
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Re: Grid heating by Current vs Ion impacts

Post by Chris Bradley » Sat Dec 24, 2011 7:09 pm

OK, I'll go with that. So why the incandescence? Negative ion impact?

Do you think secondary electrons, generated at some point within the chamber at significantly higher potential, might do it? e.g. ion bombardment of the cermamic, charging it up +ve, then any secondary electrons would accelerate vertically down onto the top of the loops (as the field is vertical, they should be able to move in that direction as it is not crossing any fields)?

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Frank Sanns
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Re: Grid heating by Current vs Ion impacts

Post by Frank Sanns » Sat Dec 24, 2011 7:34 pm

The simplest solutions are usually the most likely. For heating of the grid, that would be positive ions for a negative grid bias or electrons for a positive grid bias.

Charged particles can move parallel to magnetic field lines and will not be affected. Only charged particles moving with some velocity perpendicular to the B field will feel a force and will follow curved paths.

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Re: Grid heating by Current vs Ion impacts

Post by Dan Tibbets » Sat Dec 24, 2011 8:55 pm

Several comments. The wires in the pictures 'look like cathodes (negative). Admittedly my appreciation of anode glows and cathode glows may be falsely applied here, but are you sure that the diodes are not negatively orientated now?. As for anode heating, I would assume it would be negative ions or electrons. The electrons have less momentum to transfer, but because of increased speed they would impart the same KE(?) to the anode. But, I generally do not see incandescent anodes.

A simple experiment- Take a low wattage light bulb (some filament thickness similar to the fusor wires and nonfrosted). Limit the current flow through the wire/ bulb to a few milliamps. Turn off all of the room lights, dark adapt your eyes, and then see if you can detect any glow- even if it is only a very dim red glow. Take a 15 Watt bulb for instance. In Europe it would have 220 V through ~ 3,000 Ohms (when the tungsten filament is hot) with a current flow of ~ 70 mA. Add a ballast resister of ~ 100,000 Ohms to get the current down to ~ 2 mA. The bulb is now a ~ 0.5 Watt incandescent bulb. I'm unsure how the black body emmited luminosity at the range of frequencies would scale, but I'm sure the light, if detectable by vision alone would be dim and very red.

PS: I'm not sure 2 mA through this light bulb would heat the tungsten wire to the degree where the resistance of the wire would be comparable to the bulb under rated loads. And, I suspect the wire thickness in your fusor may be more comparable to a thicker light bulb filiment that would be found in a 100 Watt or greater rating, so my above comparison is probably biased in favor of at least seeing some glow due to resistive heating.

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Re: Grid heating by Current vs Ion impacts

Post by Richard Hull » Thu Dec 29, 2011 5:01 pm

The heating here and the current is a function of the .12 Tesla field! Remove the field and no glow or at least no heating.

I again refer folks to my old Fusor videos and in both of them I show the effect of the magnetic field on a dull fusor II in simple glow mode. At a fixed voltage and current that causes a simple glow, the introduction of a magnetic field will result in intense heating of the grid.

I would think this would be due solely to electron impacts regardless of polarity due to the circling of same in a tight magnetic streamline. Without the field, the electrons would be more uniformly distributed in the discharge.

I move a pair of potent NdFeB magnets in towards the bell jar to touch the glass briefly and the inner grid incandeses as I pull them away the system returns to normal unheated (non-super hot grid) condition. I spend a good deal of time, (minutes), illustrating this. This was all part of my 2 year demo fusor experimentation, (97-98) to actually become intimate with plasma glows ionization, electron streams in fields, etc. This is why I beg would-be fusoioneers to create a demo fusor first and experiment, play, and stop to smell the roses. (read "Ionized Gases", while doing this).

What heats the grid is electron current leaving it, (high field emission), coupled with a mix of ionic impacts, both! Which predominates is a moot point ot the fusioneer, but of some concern to the hard physics types.

Richard Hull
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