French Fusor: summary translation & ?'s

[guest]

All-

John Hendron posted link: http://www.ensicaen.ismra.fr/~bolmont/t ... fusor.html
This is a French fusor effort, utilizing copper electrodes and glass standoffs in the chamber. Chamber appears to have rubber gasket seal (like a pressure cooker), and black plastic electrical tape is seen holding outer grid. Current is introduced through coaxial cable to 20cm dia. outer grid and 3cm dia. inner cathode.

The image on this site of "...un plasma mauve, pour la plus grande partie localisé autour de la petite sphère..." (loosely translated: "... a mauve plasma, for the most part localized around the small sphere...")shows exactly what I have seen so far in my demo fusor. Indeed with no pressure gauge, I doubt I get below 10 microns yet with rough pump only.

They go on to describe initiating this effect at high voltage and note it is sustainable to ~400 V, also just as I observe. They go on to note the extinction of the plasma at lower pressure (as cited in this forum many times), and conclude that large wire thickness and insufficient voltage (they go up to 15 kV) inhibit formation of plasma within the cathode.

They cite their optimum pressure as 2 mbar, but give no indication of Star Mode. Summary Table from their work, translated:
Pressure (in mbar) ......Observations
7,1.10-5 ..........................Sparks, slight color violet.
1,2.10-4 ..........................Even phenomenon in more intense.
7,1.10-4 ..........................White sparks, green color. Mauve halation around the small sphere.
2.10-3..............................Beautiful plasma mauve, very intense. It is the best mode.
1-10 mbar......................Very clear reduction in plasma.
12 mbar .........................More plasma.
(see website for one small photo of plasma, and full report en francais)


As electrical tape seems to out-gas substantially (at least in my hands), it is surprising to see it used. My experience with coaxial cable has been that it leaks. Thus, I cannot reconcile the pressure measurements with the materials of construction, however the extinction of the plasma at lowest pressure does seem consistent. Perhaps someone could comment on these observations, and enlighten me, as my results are so far like this reference, and I wish to progress.

Best Regards
Robert

[guest]

Anyone can plainly see French metric sticky tape is far superior to any old garden variety imperial duct tape when it comes to high vacuum work!!!!!

regards
Mark Harriss

[guest]

You also might say that if you have a big enough pump and a small enough chamber, you can cover up a lot of sins of both omission and commision.... Still, that tape is probably a horrible congealed mess after exposure to vacuum and a touch of plasma.

[Richard Hull]

I don't wonder at the lack of star mode! One will never see a star until the system is very clean and most of the water vapor is pumped out. With tape and freshly soldered grids, outgass and virtual leak are the active terms.

The star forms without fail in long pumped, clean systems at or near 5-8 microns.

Sparking is normal in all freshly soldered fusors in power is applied in the 200-100 micron range. This is the solder nasties vaporizing and supporting near short circuit plasma currents due to glow cleaning.

I tend ed to run fusor I and II at high currents (100ma) at higher pressures of 100microns or more until all sparking ceased. One can rarely impress more than 1500 volts across such a load.

Richard Hull