---------- Forwarded message ---------- Date: Wed, 8 Oct 1997 18:06:12 -0400 (EDT) From: richard hull To: hvlist@Anchorage.ab.umd.edu Subject: Farnsworth/Hirsch Fusor (III) Here is the third segment in this series. It is now assumed that you have a viewing vacuum chamber which can be pumped down to about 100 microns (.1mm) or less (preferably less - 10u -.01mm). All safety rules apply to vacuum materials. Have a protective shield over all vacuum chambers, especially regular glass chambers of unknown origin. Implosions are violent and dangerous to you and any visitors or bystanders. We are now ready to construct the actual internals of the fusor. This consists of two spherical wire grids. One large and one small. These are probably the hardest part of the system to construct. I have a 10" diameter chamber and I settled on an 8" outer sphere and a 1.5" diameter inner grid sphere. You should work around this inner/outer ratio if possible. If you have an 8" chamber, then you should allow a minimum of 1/2" clearance for the outer grid, thus ,a 7" out grid is needed. I would not recommend an inner grid smaller than 1" (tough to fabricate) The actual ratio is not critical and this is a rich area for experimentation. I used 308 stainless wire .030 in diameter (mig welding wire) I have a mig welder and used some from the internal spool. Tom Ligon and R.W. Bussard also use stainless wire grids. Tantalum wire would be ideal for a bigger neutron generating system where actual temperatures rise with increasing power. The wire diameter and the wire material is not critical for a demo fusor, but whatever your choice, choose something which will not outgas under heat. Brass is a big no-no as the zinc in it is horrible. The outer grid is the easiest to make. I started by making 6 circles or loops of wire exactly 8" in diameter. I closed the loops by hard silver soldering them. I used a jewelers oxy/acetylene micro torch. I used 90% pure silver solder (jewelers brand) and not the lower purity form of industrial silver solder. Tom Ligon and others tend to spot weld their grids together. Spot welding allows for higher temp operation if done well. Three of the circles are placed together to form "lines of longitude" or "great circles" as on a globe. They are evenly divided to 60 degrees and welded or silver soldered at the "poles". Some sort of jig must be devised here. (you are on your own) Next, the remaining three circles are added similarly, but at the equator with the same 60 degree separation and welded or silver soldered into place. The process above is repeated with constructing the smaller spherical grid, but the task is much more difficult and demanding. The wire diameter gets in the way here and you had best compute and number the grids steping larger to allow for the double wire thickness. Six equal rings will crush to an oblate and be hard to work with. A good bit of practice and plenty of screw ups are common here, so make the outer grid system first which is easier and will build confidence. You should now have two grid systems made up. A big one and a small one. The small one is sort of ready to go. The big one needs more work. The big wire sphere must have a central refractory, insulating sleeve or tubing running from some point outside the vessel toward its center to hold and mount the smaller inner grid system and carry power to it. This might demand a custom opening in the larger grid system. I chose to make a 3" diameter circle of wire and place it about the "polar region" of my big sphere as a "Latitude band". I silver soldered it to all intersecting wires. next I clipped out the Northern polar region of the wire frame. This left a nice 3" hole in the top of the large "globe". Next, the glode must be suspended or mounted in the chamber in some fashion. My chamber had a lip or neck down near the base. I created a circle of wire which would just fit on the ledge of this lip and then did some fancy measurements and calulations which resulted in my soldering four struts to the outer part of the larger grid and connecting them to the "lip ring" such that the sphereical outer grid was suspended in mid-air within the chamber and electrically connected to the "lip ring" Finally, I bored and tapped a 10-32 hole just at the lip ring and added a stainless bolt and washer to the assembly with the head internal to the chamber, and as the screw was tightened, a piece of indium foil under the washer gripped the wire, made contact, and held the entire outer grid fast to the chamber at this one point. It is very inmportant to retain a perfect vacuum seal to the chamber, so I placed a miniature rubber "O" ring over the screw outsde the chamber and slathered it with Inland's Execlin 3 vacuum grease. I then placed a nut on the protruding threaded screw and tightened to seal off the threads and chamber. This was now a stud to connect the positive lead of the power supply to the outer grid. I also planned ahead and aligned the axis of the outer grid to allow for a nice open viewing area in an unobstructed part of the chamber. This allows a clear view" of the inner grid where all the action is going on. My Nalgene chamber had a nice hollow handle on top and I also bored and tapped it likewise for 10-32. I next took a 3" piece of 10-32 "all-thread" and Ag soldered a straight stainless wire to it on one end. I purchased an alumina hollow tube 3/16 O.D. and 1/8" I.D. from Mc Master p/n 8746K14 - $5.90. This I used as a hanging conduit of the stainless wire. I drew the wire tight through the tube and now I had a tube with a 10-32 all thread on one end and a connecting piece of stainless wire out of the other end. To this I Ag soldered the small inner grid sphere and cut off the excess stainless wire after soldering. It is important that there is little slack in the small sphere/tube/all-thread assembly. I now threaded the all thread through the top of my chamber half and sealed as in the stud for the bigger grid with vacuum grease and "o" ring. I added a pretty white HV insulator atop the chamber and lowered the smaller spherical grid down into the larger suspended outer grid through the "polar" hole as I placed the lid on my chamber. a lot of pre-planning and measurement assured that the little spherical grid was close to center in the larger grid structure. The top insulator connection is for the high negative potential going to the smaller inner grid. You must improvise and adapt for you chamber. This involves: 1. Supporting the outer grid 2. Allowing for some sort of entry of the central grid and its insulated suspension system. 3. Connections for the two grids to HV supply which are 100% leak proof. positive to outer grid system and negative to the inner, small grid. The above is a tall order and lots of careful measuring and planning are needed. Once all the above is done, vacuum out the chamber and test for leaks. Richard Hull, TCBOR