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Increasing use of cylindrical fusors

Posted: Sun Jan 12, 2020 10:11 pm
by Dan Knapp
The 21st US-Japan Workshop on Fusion Neutron Sources and Applications (formerly called Workshop on IEC Fusion) was held last month in Kyoto. Unfortunately, I did not attend; but I did see the list of presentations. Most notable to me was an increasing shift from spherical devices to linear devices both in the U.S. (Wisconsin “LUNA” device; cpb-us-e1.wpmucdn.com/blog.umd.edu/dist/d/568/files/2018/10/Bonomo_LUNA_paper_2018-uqhq93.pdf) and in Japan. An argument for this is that since gridded spherical devices seem to operate best in a multibeam mode, why not just optimize one pair of beams. The linear device offers definite advantages for the amateur in terms of simpler construction. It consists of a tube or ring(s) as cathode with anode plates at each end. The anodes are optimally made concave, but flat discs or just the blank flange faces will work. Given the demonstration that titanium electrodes give higher neutron yields, one would ideally make the electrodes from titanium. Titanium alloys are notoriously hard to machine, but pure titanium (designated grade 2) is no more trouble than stainless steel. I've also found it easy to spot weld. If one wants a more complex cathode, grid structures can be made from titanium welding rod. If I were to set out to make a linear device, I would use a conflat cross with the HV feedthrough on the port opposite the pumping port, and attach titanium discs to blank flanges attached to the other ports. I would use a length of titanium tubing with end edges rounded as the cathode. If one wants a viewport and/or other access, a five- or six-way cross would be in order. There have been a few linear fusor efforts reported here, but it appears that this approach would warrant more consideration in amateur fusion work.

To anticipate a question, the presentations from the Kyoto meeting will not be posted on the web for public access.

Re: Increasing use of cylindrical fusors

Posted: Mon Jan 13, 2020 1:15 am
by Jim Kovalchick
I'm not bought into titanium grids. I think they liberate gas too much for my liking.

Re: Increasing use of cylindrical fusors

Posted: Tue Jan 14, 2020 7:02 pm
by Roberto Ferrari
Hi Jim

Please, comment on your experiences with titanium and degassing.

Re: Increasing use of cylindrical fusors

Posted: Tue Jan 14, 2020 9:13 pm
by Jim Kovalchick
When I was breaking in my fusor, I gradually over successive runs would raise my voltage in steps with current relatively steady. I rode the Paschen curve by a combination of throttling closed the d injection and/or cracking my vacuum valve open a little more. I experienced good control on multiple runs up into the high 20's kV range. My only concern was that pressure runs a little higher than my big fusor. I wasn't too worried though because running at my target voltage would mean lower pressure. I was able to work my way into the 30's and started to get some neutron numbers.

On a run into the 30 kV range I was startled by rising pressure and current. I was able to throttle open the vacuum valve to my turbo to get current back, but as indicated by a high foreline pressure, I could see that the roughing pump was seeing a lot of extra flow. I was worried about my turbo.

I concluded that I either had sprung a leak or something was outgassing. When I shut off the high voltage and d injection, the chamber was tight as a drum.

I tried another run within a few hours. I didn't see outgassing again but my neutron numbers were way down.

Were my neutron numbers benefitting from a d loaded titanium grid? Did I unload it by getting the grid too hot? Im just a dumb engineer. Maybe others have good theories.

Re: Increasing use of cylindrical fusors

Posted: Tue Jan 14, 2020 10:05 pm
by Dan Knapp
The reports from the Kyoto group of greatly enhanced neutron production were for a titanium anode. In more recent work with a cylindrical fusor, they also used a titanium cathode, but it was a tubular cathode and not a wire grid.. A titanium wire grid cathode would definitely be expected to outgas with overheating.

Re: Increasing use of cylindrical fusors

Posted: Tue Jan 14, 2020 11:32 pm
by Roberto Ferrari
Tim and Dan

But degassing until emptied of gasses, right?
Or do you think that there is a "push-pull" action, loading and outgassing, rytmically?