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Mr. Fusion
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Head Scratching
October 20, 2002 — 16:50

After an exhausting week at work, I managed to get a few hours in on this
weekend into the fusion project.  I’ve been squirreling out some few hours
between builds and short regressions, as well as reading up on various things
that I need to learn about.

Of the material that I had already milled out for the cage, I’ve had a 50%
success ratio.  I tried to use the band saw to cut the pieces to the
correct angle but quickly discovered that this was pretty much an impossibility. 
So I used a rotating vice to give me the correct angle, and milled out the ends
after cutting them to the correct length on the band saw.

I started off with 24, 4.5" pieces with the correct half-dihedral angle
milled into the bar.  I messed up three by bending them when I tried to
mill too fast.  D’oh!  But that still left me with 21 that were pretty
nice.  However, when I started putting them together into triangles, I
found I really only had 12 pieces that were accurate enough to work (4
triangles).  Hmmm.  I don’t know what went wrong on the remaining
pieces.  The triangles just didn’t come together.  I’ll have to see
how my jig is set up and make darn sure that I get these out accurately.

However, a 50% rejection ratio for my first run isn’t that bad.  I have
a process now and I can whip these puppies out pretty fast.  Hopefully I’ll
have some more time this week to get all the pieces out.

I’m still faced with the problem of how to construct the cage.  I tried
my hand at welding them with my MIG welder.  Jeesh!  Considering the
small pieces, the MIG seems like a clumsy jackhammer.  Hmmm…  So
I’ve been thinking about using epoxy to glue them together.  Two concerns. 
First is the out gassing in the vacuum.  When you’re trying to get down
below 10-7, you don’t want a bunch of epoxy preventing this. 
Second concern is electrical conductivity.  The entire cage is supposed to
be held at the negative potential, and epoxy is an excellent insulator.  Or
is it?  Turns out there’s a number of silver impregnated epoxies on the
market specifically designed to deal with this.  Problem is one of heat and
again, out gassing.  Lesker vacuum
sells some conductive epoxy, but they specifically limit its use to 10-3
– about 4 orders of magnitude greater than my target.

And then there’s the heat issue.  The last thing I want to have happen
is for the epoxy to give way just when the system is starting to work because of
overheating.

Is nothing easy?

Obviously not.  So I’ve ordered some conductive epoxy to see how well it
will hold up.  Don’t need that much to keep it together, and perhaps I can
combine the non-conductive epoxy with some strategic dots of conductive epoxy
such that the cage is held together entirely with high vacuum, high strength
epoxy, and just electrically connected via dots of conductive epoxy.

I don’t want to braze the cage as that will result in even more sources of
out gassing and other nastiness…  We’ll see.


I took the advice of many and have committed to a custom chamber for the
system, shown on the right.  Pretty simple, really.  The sphere
provides positive mounting and vastly smaller chamber volume.  Also, the
guns (electron and ion) can be mounted outside and the nightmare of electrical
feed throughs is eliminated. 

I also went out and obtained a little portable B&W TV to take apart. 
$25, but what the heck.  Little sub-miniature gun in the back – all very
cute.  Should be relatively easy to dismantle.

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