Fusor Forums

The idea is more refined than that but it has the essentials.
HV feed throughs and view ports and end flanges are things we need on a Cylindrical fusor and this is an idea to provide all three at one go using a combination of ideas from this forum.
The body of the fusor is a SS tube over a base plate. The top is an transperant acrylic end plate with the HT cathode mounted directly in in the centre.
The acrylic can't stand heat and can't stand plasma.
The HT cathode is liquid cooled to stop heat from the cathode
The acrylic plate has a anullar glass plate suspended below it on thin SS tube standoffs. The glass plate stops short of the cathode by 10mm and the ss case by 10mm. The cathode has an alumina tube surround (T/C case) with a glass or ceramic disc part way down the alumina tube arranged so the plasma cant see the gap between the anular glass plate and the alumina tube through to the acrylic end plate.

This idea embodies the sacrificial glass plate for view ports
the alumina tube for cathodes
and liquid cooling.

Should the glass becoume conductive ( a problem mentioned elsewhere) this shouldnt be an issue since there is aleast 10mm of vacuum between the glass and HT and the glass and ground.

What have i forgotten. -- Oh yes you need excess pumping capacity for the acrylic out gassing.

comments?

Derek -

If I understand the physical relationships, correctly, it seems worth a "go"...

The acrylic will outgass at all temps, but if your pump is big enough, you can probably maintain low pressures sufficient to at least get a demo going.

If you could lay hands on a piece of tempered plate glass, and get it bored to size for your HV feedthrough, you could at least side step the outgassing problem.

The real hazard with glass-like materials, is their low thermal conductivity. Many times lower than metals. This makes them susceptible to very high localized temps, if electron or ion bombardment happens. Bell jars are very convenient for some types of experiments, but have this issue, as well.

Sounds like you're making a good start. Wish you well with the next steps..

Dave Cooper

Build it and see.

The glass annular shield in this is sacrificial so hot spots arent too bad . at higher powers most of the glass annulus can be replaced with a sheet stainless annulus with a smaller hole covered by a glass disk.

the key advantages of Acrylic are its cheap, transperant and can be drilled and tapped for the various bits and pieces. especially if its 20mm thick

I think I have excess pumping power in an edwards Diffstak 63, (115 l/s) with a pfieffer Uno 5 backing pump(5 m3/hr). Roughing is an Edwards E2M18 19m3/hr.

And of course I can always replace the acrylic end plate with a SS plate when I have worked out whats what.

I have an acrylic viewport 20mm thick covering an area of 100mm X 120mm and love it,
It sits on an o ring in a groove and I easy pull 10-6 and have not noticed outgassing but I don't have a valve to seal off. I have a sacrificial poly carb shield. I can't even see any distortion in the surface reflection under vacuum over this large area.
Good Luck.
Steve.

This is over a 260mm aperture (I 've bought a new ISO 250 by 580mm high full nipple to provide the SS cylinder) . I thought a 25mm thick piece would work.

25mm is generously thick for that diameter.

This DIY solution looks a lot like your design.
http://4hv.org/e107_plugins/forum/forum ... post_79819
The builder figured that a 1/2 inch acrylic sheet was strong enough, but he chose 3/4 inch for less deflection, and a strength safety factor of over 20 before drilling.

That thread also has a picture of a semiconductor-equipment loadlock cover spanning over 300mm, made of polycarbonate only 22mm (7/8") thick. And a link to online strength-of-materials calculator that can do bending of round flat plates.

Derek
Acrylic is fine ie perspex
Polycarbonate tends to sublime ie evapourate. I found this out when I made some sacrificial viewport protectors out of polycarbonate 2 of at 2" diameter 0.125" thick. Vacuum would not go below 1 x ten to the minus 6 millibar until I took them out where upon the vacuum went back down to 4 by ten to the minus 8 millibar. This is a twenty litre chamber pumped by a 1000 L/sec turbo.

Any implications in having a 31 litre fusor chamber as opposed to more common 1 ~ 2 litres?

Hi Derek,

Outgassing, permeation, and adsorption all increase with chamber surface area, degrading the quality of the vacuum unless pumping speed is also commensurately increased. However, I don't anticipate you would have difficulty evacuating your planned 31-liter volume to fusion-suitable pressures using a Diffstak-63. The role of the diffusion or turbo pump for typical fusor operation is pretty undemanding, serving mostly as a speed booster in a pressure region where the forepump's speed is near zero.

One thing to consider, if the fusor chamber is indeed literally going to be a "dustbin," is the quality of the metal. I see a lot of "stainless" articles in kitchen and home-improvement stores that look nice, but are a poor grade of stainless that is magnetic and nickel-plated. (Sometimes I wonder if they're even technically stainless steel underneath). Also, check for longitudinal seams that could be mechanical weak points or leak sites. Make sure no other coatings (e.g. clear lacquer) are on the metal.

-Carl

Derek Mitchell wrote:
> Any implications in having a 31 litre fusor chamber as opposed to more common 1 ~ 2 litres?
1. Lots more surface area to hold gases and moisture, not to mention more volume. So it will need a bigger pump, or more time to pump down, and (indirectly) consume more D2.

2. More damage if it implodes from inadequate strength.
Not a problem with [edit] your "ISO 250 by 580mm high full nipple" SS cylinder.
But things like rail car tanks can be downright hazardous when evacuated...
http://www.youtube.com/watch?v=2WJVHtF8GwI

in this case the "Dustbin" is direct from Edwards in Crawley.
My other smaller "dustbin" is a casing from a large 1979 turbopump.
Although in my case quality of materials and construction are not a issue (see note on British rail engineering) your points are generically apposite.

I think I can do without the water cooling for an initial stage with a little bit of clever but simple construction.
The feedthroughs for this approach are of course all metal . So the challenge is how to make a good enough thermal insulator in metal to protect the acrylic

The idea is to have a SS flange with a stainless thin walled tube extending from about 25 mm vacuum side to 75 mm non-vacuum side where it capped with a solid piece of stainless. The electrode held only at this cap runs concentrically in side the tube in to the vacuuum side. Thus the hot electrode conducts thermally only to the cap and not into to the acrylic and the tube acts a heat shield for radiated heat.

Note the tube would extend into the vacuum side at least the thickness of the acrylic..

The scale of this contruction would work well with silver soldering.