Starting fusor build

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Luke Weston
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Joined: Thu Mar 07, 2019 7:32 am
Real name: Luke Weston

Starting fusor build

Post by Luke Weston » Tue Apr 23, 2019 2:00 am

This chamber is a standard CF 2.75" 4-way cross with a viewport on one side ("left") and the HV feedthrough on the other ("top")

Although not strictly necessary at this level of vacuum, standard copper CF gaskets are used on the chamber flanges because that's what I had.
Maybe if I need to buy some more I'll get Viton instead for reusability, but I do like all-metal systems where practical.

At the moment a large-diameter unusual adapter nipple is used as a "stand" to hold up the device, with an internal wad of Al foil jammed in there as a plug against dust ingress. Just temporary. Later this port ("bottom") will be connected to the vacuum throttle valve and vacuum system.

The remaining port is used by the 2.75" CF to NW16 adapter ("right") which holds the vacuum transducer. There is also a 1/8" NPT NW16 tee inserted in line here at the moment (a tiny bit of Apiezon M on the thread) with a 1/8" Swagelok adapter, and this will ultimately connect to the gas delivery system (1/8" SS316 tubing and 1/8" Swagelok standard components throughout).

At the moment I've capped it closed for vacuum testing with a little stub of SS tubing soldered closed at one end and swaged at the other end.

These grids, in the three-ring "atom" style, are assembled from 1mm W.

I also have 0.8mm Ti, 0.8mm 316 SS, and 0.8mm Ta. The Ta is much more malleable than W, with a melting point almost as high, but can't be soldered, and I don't have the means to weld it with TIG under argon or similar. I can wet it with solder, with the right flux and enough temperature, and cleaning the parts before brazing of course, and therefore the W seems like the most successful choice so far.

Ti would have been an interesting material due to hydrogen (and deuterium) sorption into the Ti matrix. (Pd could also be used as a good H sponge material, but clearly the cost would be very high and the melting point is even lower than Ti which is already getting low relative to W. Or uranium wire?) I have access to more Ti wire (0.8mm) but surprisingly W has proven the easiest to fabricate nicely.

Given its extremely high melting point the W is ideal - I thought it would be nearly impossible to fabricate but it has been OK, and if the temperatures on the grid get that high the Ag braze will let go before the W wire does.

By solder I mean brazing with hard Ag solder - with bifluoride flux and gas torch.

I'm getting grid diameters that are slightly variable due to fabrication tolerance, but they are about 15-16 mm. If we take the 2.75" CF inner diameter to be 1.5" (ignoring high-field effects at the cross edge) then this gives us a chamber diameter to grid ratio of 2.4-2.5 or so. (Wire diameter 1mm.)

As with all 2.75" CF cross systems the HV applied to the grid before breakdown occurs is clearly an important limiting factor - we know neutrons are certainly possible, it's a matter of operation technique, the right pressures, the right HV voltage and the grid geometry.

A long stalk of sintered alumina tube is used - to evaluate it as a DIY feedthrough for high-ish voltages.
This doesn't have the convolutions characteristic of HV insulators - but this is just a matter of creepage path length and it could always be made longer.

After the W grid is soldered together, a length of stainless steel M3 allthread (a bit longer than the alumina) is soldered onto the grid, then the grid/stalk assembly is cleaned to remove oxide and flux residue etc.

Internally, the alumina tube is bonded onto the stainless steel allthread with JB-Weld for its strength, and a little bit of Hysol 1C capping the exposed edge at either end in case there are outgassing issues - not sure how vacuum-compatible the JB-Weld is.

A blank CF 2.75" flange is drilled to fit the alumina tube OD with tight tolerance and the feedthrough stalk is fitted into its hole, checked for the appropriate depth to reach the centre of the CF 2.75" cross, cleaned and set into place with Hysol 1C on the inside and outside.

These little MKS901P hybrid Pirani/piezo transducers are nice and small and simple.

From 1 kTorr (a bit over atmospheric) down to 10 uTorr - low enough vacuum for most of our purposes.

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Richard Hull
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Real name: Richard Hull

Re: Starting fusor build

Post by Richard Hull » Tue Apr 23, 2019 4:10 pm

Nice stuff. good luck.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Bruce Meagher
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Joined: Fri May 13, 2011 3:25 am
Real name: Bruce Meagher
Location: San Diego

Re: Starting fusor build

Post by Bruce Meagher » Tue Apr 23, 2019 5:59 pm


Here are a few comments on your nice setup...

Have you considered the expected temperature your grid will reach during operation? At fusion producing power levels will your silver brazed connections hold?

Jon R. had a nice post about his grid design and how he mechanically attached the grid to the stalk.


Having an exposed threaded rod (like shown in your picture) might not be ideal due to sharp points creating high field lines. Consider filing or turning these smooth.

The Hysol-1C you have at the bottom of the stalk will see high temps making it unsuitable for a neutron producing fusor.

Finally, I would add a 90 degree bend to your vacuum gauge. Current it’s looking directly at the grid.

For a demo fusor these should be fine, but things to consider for the future.


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