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Farnsworth Fusor   -   2016/01/15Viewed 731 times this month, last update: 2016/02/07



"I think I'll build a fusion reactor" - something I probably said a few months ago.

A Farnsworth Fusor is a nuclear physics demonstration device; (probably) not a practical means of generating power, but a way to achieve real nuclear fusion of hydrogen atoms into helium. In the process x-rays are produced, a very small amount of neutron radiation is created, and a fantastic light show is made, so fantastic to see it has been called a "star in a jar".

My intent is to demonstrate some fusion, take some great pictures, and try not to electrocute myself, injure myself with an implosion, or burn the house down; for fun!

Theory of operation:
A Hirsh-Farnsworth Fusor is an intertial electrostatic confinement device intended to fuse hydrogen into helium. It functions by ionizing gas of the hydrogen isotope deuterium, then attracting the deuterium ions toward the center of a chamber by means of electric fields. The outer casing of the chamber is positively charged, which repels deuterium ions. A "grid" of wires arranged in a sphere in the center of the chamber is negatively charged, attracting the ions, but allowing them to zip right by toward the center, out the other side, and accelerate back again. The difference in potential between the grid and case causes Paschen Discharge, ionizing some of the deuterium gas. When the deuterium ions are accelerated fast enough through the inner grid, some smash together in the center of the reactor with enough energy to fuse. In the process, the deuterium ions emit light, causing an eerie glow from the center of the chamber, as well as x-ray radiation due to deceleration and acceleration forces. If fusion is achieved, some neutron radiation is also released, as well as high-energy helium ions.

My fusor equipment is arranged this way:


Build Log:
I started with a very old, very rusty steel propane tank found on craigslist for $100. It took me a few days to cut off unnecessary structure, and remove all the rusted-on plugs. The tank is 0.25" thick steel, and weighs several hundred pounds.
The next step was to cut down the tank to a more managable size. This required two long circular cuts with the plasma cutter to remove about two feet of total length.
The tank could then be stood up on a rolling stand, and welded back together.
A frame was then built all the way around the tank out of two-inch square tube with angular mount structs, intended to give anti-crushing strength to withstand vacuum. I then cut out a large door hole. I was concerned that the door hole would be a weakness in the structure, causing it to fold like a beer can under vacuum, so I welded additional square bars along side the door openings. These also provide the bracket function for the door bolts.
The plate cut out of the door hole then gets a flange, and a window. This was very tricky and had to be cut off and re-done in order to get the flange to line up with the tank door. The tolerance is about half the width of the o-ring cord used. 3/8" extra-flexible cord helps.
All welded up, cleaned and ready for painting. For window glass, I'm using 0.5 inch thick polycarbonate from McMaster Carr. The pressure of the door under vacuum compresses a 3/8-inch o-ring seal made from o-ring cord stock.
The tank holds vacuum very well (no measurable loss in over a week at full vacuum) but to do real fusion, a simple piston vacuum pump will not suffice. To that end I purchased this large diffusion pump on ebay.It is an Edwards Diffstack-160 with 6-inch orifice, valve header, and 1300 watt heater.
A hole is cut into the tank for the diffusion pump, and a matching flange is welded on. All 0.25-inch thick stainless steel from Industrial Metal Supply.
Starting with an elbow section of 6-inch stainless steel pipe I got from Machinery and Equipment, I created a flanged elbow pipe to mount the diffusion pump to the tank.
Here's the complete vacuum system, holding vacuum very well! In order to present a uniform ground potential, the inside of the tank was ground down to clean metal, then painted with a conductive graphite paint. This required me to spend a few hours curled up fully inside the tank, holding a 2-horsepower 9-inch angle grinder. That is officially the worst, loudest sound in the universe.
Fully plumbed the vacuum and water cooling ports of the diffusion pump, and tested the resulting maximum vacuum. It's difficult to precicely calibrate my vacuum guage, but I estimate I'm getting down to about 4 millitorr.


The high-voltage power supply. The circuit design is based on fusor.eu. The parts are mounted to a frame of acrylic plastic, is placed in a grounded steel container, and covered in transformer oil. This transformer outputs 40kv at full input voltage, which is rectified to DC by the big diodes, and smoothed out by a capacitor. A variac is used to vary the input voltage, and thereby control the output voltage. I added a few important safety features, as 40kv is enough to kill a person instantly: the case is conductive and grounded, the HV cable is covered in grounded braided wire and though most of the HV components are rated for use in air, everything is submerged in oil which protects against arcing and over-heating.
The grid sphere I made of 0.029" stainless spring steel wire. Spot welded together with a home-made spot welder as in this Instructable.
Of the elements, deuterium is the easiest to fuse, but it turns out that acquiring deuterium is almost impossible for a private person living in the USA. However, heavy water, which is D2O can be purchased from United Nuclear, and the deuterium can be separated from the oxygen via electrolysis. My equipment for doing this is a model airplane fuel tank, a tiny fuel cell/electrolyser from Fuel Cell Store, some tubing, and a balloon to collect the deuterium gas. The fuel cell is powered from the 3.3vdc rail of a AT power supply.
First plasma! This is the first time I got real Paschen Discharge, rather than just destructive arcing within the vacuum chamber. Most of the discharge is between the left side of the grid, and the wire mesh I placed over the intake pipe to the diffusion pump. I put the screen there to keep the pump clean, but it's obviously unbalances the ground plane within the chamber, so my next test will be without the screen, with a smaller grid, and more effort made to center the grid within chamber. I will also insulate the rest of the hanging wire, as there is significant discharge there. This is a about 4mtorr and 30kv with just air remnants in the chamber.
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