IEC FUSOR

The following entries outline the design and construction of a basic Farnsworth – Hirsch fusion reactor, with the intent of using it as an emission source. This project was a joint undertaking by myself and a friend, and was completed using a high vacuum chamber on loan from our University’s physics department. Note that the following information primarily serves to document my progress, and not to instruct the reader in any way. Readers are advised to exercise extreme caution before attempting to construct their own.

HIGH VACUUM CHAMBER

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Construction began with a stainless steel vacuum chamber approximately 18 inches in diameter and 32 inches tall. Fortunately most of the hardware was included, but one window was cracked and the the chamber’s interior was coated in a thin white film that resembled spray paint. A silicon wafer that had been suspended in the chamber by the previous owner had shattered, and small shards were visible almost everywhere. It took almost a week of cleaning between classes and overnight to fully remove them. The white film was insoluble in acetone and ethanol, and as of writing, this residue has not been fully removed.

GAUGES AND CONTROLLERS

When the chamber was given to us, the lowest resolution gauge attached to it was a Pirani gauge which bottoms out at around 10^-4 Torr. It’s possible that the white powder inside wasn’t a problem for the previous owners when measuring pressure in this region, but to sustain deuterium fusion, our gauge pressure would need to drop another order of magnitude. It became clear that there was no way of knowing the extent of the problem without using more accurate gauges. Fortunately, I was able to find an ion gauge online that was moderately priced and prepared a mounting location while it shipped. The downside of buying second-hand lab equipment is that many instruments are manufactured by small, highly specialized companies that change names, go out of business, or otherwise do little to archive their documentation. Unsurprisingly, I wasn’t able to locate a datasheet for this old gauge, but found the pin-out myself.

Ion gauges are effectively triode vacuum tubes that use thermionic emission, a high electrical potential of about 1.5 kV, and a grid to motivate current flow. Just like a conventional triode, current flow is inversely proportional to the atmosphere present in the tube, so these gauges can theoretically measure down to nothing. A few of the grad students I’ve spoken with when borrowing vacuum parts shared that ion gauges are notoriously inaccurate at lower pressures.

POWER SUPPLY ELEMENTS

Our fusor will be supplied with approximately 20 kV at 20 mA. These figures were decided upon after investigating similar setups, determining the limitations of our resources, and recognizing the inherent safety concerns of operating the fusor with an arbitrarily high power input. X-rays and the threat of lethal arcing are already present and should be limited if possible. A ceramic, high vacuum feed-through was found with a nominal rating of 30kV and maximum rating of 35kV.

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/!\ Postponed /!\

COVID-19 put an end to this in mid-March. I intend to continue as soon as I’m permitted back on campus.