Bell Jar Grid


To use a glass bell jar for a fusor, the glass must be pyrex. Otherwise, a stray electron or ion beam  causing localized heating of the glass, can result in an implosion. Pyrex bell jars of a reasonable size are not infrequently seen on Ebay, but unfortunately we couldn’t find a seller willing to sell outside of the USA. Persistence paid off, though, and eventually we obtained a 12 inch diameter by 12 inch high pyrex bell jar from a Canadian seller.


A fourteen inch square piece of 1/4 inch thick 6061 T6 aluminum was obtained locally. The edges and corners were rounded and polished. 1 inch and 1 1/2 inch hole cutters were purchased, and the required holes cut through the aluminum using a drill press. The 1 1/2 inch hole was cut in the center for the high voltage feed-through, and two 1 inch holes cut for the vacuum pump connection and for the vacuum gauge sensor.

Bulkhead fittings were used to attach the vacuum lines (QF25) and the high voltage feed-through (QF40). Each of the three bulkhead fittings is fastened to the base by stainless steel 6-32 screws, using tapped blind holes. The holes had to be at least 3/16 inch deep, but not penetrate the upper surface of the aluminum. Using a 3/8 inch aluminim base instead of 1/4 inch would have made this operation a lot less harrowing! Finally, mounting holes were through-drilled in the 4 corners of the base plate.

The entire base plate was then polished, and finally cleaned with acetone.

The base was then mounted about 8 inches above a scrap plywood base, using 1/4 inch all-thread rod, with pieces of 1 inch PVC tubing used as spacers.

Bell Jar to Base Seal

We simply used a generous layer of vacuum grease on the ground glass sealing surface of the bell jar to create a seal with the base plate. This works well, except that whenever vacuum is removed from the system totally, even if the bell jar is not disturbed, the seal is lost. The bell jar then has to be removed, and the surface  re-coated.

Electrical Feedthrough and Support for Inner Grid

A 20,000 volt Electrical Feedthrough with a QF40 flange was purchased from Kurt J. Lesker. This was purchased with its final use in the actual fusor in mind. A higher voltage rating would have been desirable, but cost was an issue. It was felt that with care, and good high voltage practices, the feedthrough could be used at much more than its rated 20,000 volts.

Time will tell whether or not this was false economy!

The photo shows the inner grid supported on the end of the inner conductor of the feedthrough. An insulating tube of alumina ceramic (available from McMaster Carr) has been slipped over the conductor, and protects it from ion bombardment.

In the photo, the bell jar and the outer grid have been removed. The discolouration of the metal base is due to electron and ion bombardment. (For the same reason, the bell jar itself now gives the impression that it was made of smoked glass!)


With higher voltages and currents required for fusion, it was obvious that something like a stainless steel enclosure had to be built within the glass bell jar to protect it. However, a glass covered opening in the stainless enclosure would be required to allow the poissor to be viewed.

In the photo is the large stainless steel cooking pot that was modified to make a protective shield within the bell jar. The handles were removed, the lid discarded, and a 2 inch circular hole was cut into the base.

In the photo is the completed bell jar, ready for fusion experiments. Inside is the stainless pot, upside down, with the 2 inch hole now facing up and covered with a square of 1/4 inch glass. This allows the webcam to view the interior while protecting the bell jar from stray electron beams. The 1/4 inch glass gradually darkens and has to be replaced, which of course involves opening the bell jar.  Vacuum grease was used as a seal between the bell jar and the base plate. In the photo on the right, the excess vacuum grease can be seen around the base of the bell jar.



Inner and outer stainless steel grids under construction.

The inner grid shown is the one used in our initial demonstration fusor. Both inner and outer grids were made by first welding the individual rings, and then welding the rings into a grid.

The inner grid currently in use is slightly smaller than the one shown.

“Equipment to Build the Equipment” – A Homemade Spot Welder

The inner grid of a fusor is usually a spherical shape, and usually created from wire. Although silver-soldering can be used to form the wire into a sphere, the most popular way is to use resistance welding (spot welding). Unfortunately, most commercially made spot welders are designed for much larger material than the approximately .030 stainless steel wire that we wished to use. We therefore built a simple welder ourselves. The heart of our welder is an assembly of 14 – 4700 ufd, 50 volt electrolytic capacitors in parallel, for a total of 65800 ufd. A simple transformer-rectifier is used to charge the capacitors, through an inrush-current limiting resistor.

Discharge is controlled by a small contactor salvaged from a very early Amana Radar Range microwave oven. To reduce the instantaneous current when the contactor closes, the output goes through an inductance made by winding #10 wire onto the core of a salvaged microwave oven transformer. This is necessary because the electrolytic capacitors are not designed to withstand extremely high instantaneous discharge currents. The discharge energy can be controlled by varying the voltage that the capacitors are allowed to charge up to.

Discharge Electrodes were formed in several different shapes, from several sizes of copper pipe and tubing. An anvil made from a copper pipe tee and end cap can serve as one of the electrodes. A pair of clothes pegs with copper jaws was mounted to hold pieces of wire that need to be joined end to end.

The apparatus is assembled on a pair of wooden bases, and although not pretty, it does a credible job of spot welding the stainless steel wire.

The cobbled-together resistance (spot) welder. On the left base is the (small) 25 volt transformer, bridge rectifier, and the storage capacitors. The large “transformer” is the current-limiting inductance, and to its left is a small contactor used to control the output.
On the right is the base for the anvil and electrodes. The large black microswitch is used to control the contactor.