High School Fusor: Fusor 0 Retirement, New Lab Space, and a Fusor 1 Pump-down Test
Over the almost month since I last updated here, I have made monumental progress on my fusion project. Not only have I actually began working with the metal Fusor 1 chamber, but I have achieved first light of the full Fusor 1.1 system and generated plasma within that chamber.
In the interest of concision and clarity (as well as so that I can cite a specific, relevant post in my fusor.net plasma club application), I am going to split this long progress update into two parts.
This post will detail:
- My final Fusor 0 tests and an analysis of the results
- The first pump-down test of Fusor 1.0
- My new lab space in the attic
The subsequent post (that I will publish concurrently) will include:
- An overview of the Fusor 1 project
- Information about Fusor 1.1 assembly, vacuum sealing, and pump-down tests
- Data and images from the first light of Fusor 1.1
- A very rough timeline for future runs and Fusor 1 progress
and will serve as additional information for my plasma club application (the significance of which is discussed in a previous post). I hope that this solution makes this update not only easier to read but more informative as well since a lot has happened and I would like to record it all in some detail.
Before I began assembly on Fusor 1, I did another three Fusor 0 tests, bringing the total to seven consecutive successful Fusor 0.4 runs. With those runs, I began investigating an interesting and unexpected anomaly that I had noticed during the second test where the voltage and amperage delivered by the NST would fluctuate predictably even when the voltage delivered by the variac remained constant.
To investigate this phenomenon, I did two long duration Fusor 0.4 tests that attempted to mimic the conditions under which I noticed the fluctuations. I ran the pump for ten minutes before activating and slowly turning the variac up at intervals of ten volts. I waited over a minute at each interval and monitored the changes in voltage and amperage. Due to issues with the steel grid overheating and beginning to soften and warp, the duration that I maintained a specific NST input voltage gradually lessened as voltage increased and I did not gather data above 60 volts from the variac.
While voltage and amperage fluctuated slightly at higher voltages from the variac, there were significant changes in voltage when 20V were sent to the NST. The maximum and minimum voltages read over a 200 second interval were 585V and 466V respectively, giving a very significant 119V range. Current fluctuated roughly inversely to voltage and ranged from 3.6mA to 4.6mA.
This very pronounced change in voltage and amperage was not reproduced at any other NST input voltages measured but was repeated at 20V two times.
Initially, I thought that this was because as plasma was generated within the chamber, the atmosphere within grew increasingly ionized and conductive, lowering the voltage needed to maintain plasma. This would cause the magnetically shunted NST to output more amperage as designed since the chamber would mimic the neon gas tubes that the NST is meant to ionize.¹
Another potential conclusion I made from the data was that the energetic bombardment (in many forms) from the plasma generated within the grid caused significant outgassing² which increased the pressure within the chamber. Due to its construction and design, Fusor 0 would be particularly susceptible to outgassing³ so it is very likely that this occurred to some degree. This increase in pressure, though, would lead to an increase in voltage, not a dip, so although outgassing almost certainly occurred in Fusor 0 and would likely influence the voltage and current, it could not — as far as I know — be the cause of this specific phenomenon.
Although I am still unsure as to why the voltage dipped and amperage increased in the first place, I am even more unsure as to why the dip and peak seem to level out and asymptotically approach a point. Unfortunately, I have no way to measure the ionization or pressure within the Fusor 0 chamber, so further investigation will be very difficult and is still ongoing (even with Fusor 1).
This phenomenon did not occur at or above 30V. At the input voltages above 20V that I tested, NST output voltages always stayed close to level at 560V and fluctuated less as input voltage increased.
As I said above, I ran the tests for shorter periods of time at higher voltages as to not overheat the grid. Additionally, I did each test sequentially without deactivating the NST between which might have lead to some inaccuracy.
I had plans to do more tests for longer durations but work on Fusor 1, which has pressure metering and a generally higher quality design, progressed at a much faster rate than I initially expected. Because some components (primarily the NST and vacuum pump) are required for both Fusor 0 and Fusor 1, I had to retire Fusor 0 early⁴.
Fusor 0 was not only my first step in the fusion project, it was also my first major project ever. I learned some incredibly valuable skills and made plenty of mistakes and was overall quite successful in building a functional (if limited) plasma device. The much more advanced Fusor 1 will allow me to experimentally explore and attempt to answer many of the questions raised with Fusor 0 and will take me one step closer to fusion.
Fusor 1.0 Pumpdown:
After assembling the Fusor 1.0 chamber on my clean, indoor desk, I carried it out to the workshop and set it on the table that previously held Fusor 0. I attached it to the vacuum pump PVC vacuum line that I used for Fusor 0 with a new 1/4" NPT barbed hose adapter, using Teflon tape in all NPT attachments to make the connection as vacuum-sealed as possible.
I attached the thermocouple tube to the KF-25 connection on the chamber and set up the controller. The former fusioneer on fusor.net that I bought the tube and controller from said that he had calibrated the gauge system against a higher quality, digital gauge so although this one gauge is my only way to measure pressure, I believe that it is accurate as I trust the seller and have no reason to think otherwise.
After attaching the thermocouple and double-checking that all flanges were firmly attached, I activated the vacuum pump for the first pumpdown attempt of Fusor 1. I immediately heard a loud hiss coming from the chamber and the thermocouple gauge did not dip below atmospheric pressure so I knew that I had a big leak. Once I had located the leak, I removed the flange, a 6" CF reducer, and attempted to re-seat the Viton gasket before re-sealing the chamber. I had lots of difficulty getting the gaskets to work properly and not slip during initial assembly as well as during this pumpdown attempt, so I had to admit that I was doing something wrong although I had no idea what.
Luckily, re-seating the gasket stopped the hissing and in my second pumpdown attempt, I successfully pulled a vacuum that the thermocouple read as approximately 380 torr.⁵
I had previously inferred via comparison with qualitative data on fusor.net that I had not yet reached 1 Torr of pressure in Fusor 0, but that I had come relatively close. Additionally, although the physics and math surrounding phase transition to plasma are too complex and multifaceted for me to claim any level of assuredness in saying this, a Paschen’s Law⁶ calculation shows that the voltage required to generate plasma at 380 Torr is over an order of magnitude higher than what was used to successfully create plasma in Fusor 0.
Although I realized this at the time, I did not make any further pumpdown attempts at that time. Instead, I contacted the engineering director at LDS Vacuum who had previously helped in validating my account with them. He gave me a lot of great advice regarding many areas of vacuum systems and suggested that I use vacuum grease with the Viton gaskets to improve the seal. When I eventually re-sealed the chamber prior to the first light test, this advice was invaluable and helped me to achieve an end vacuum orders of magnitude deeper than 380 Torr.
New Lab Space:
Although this is a less interesting update than my actual plasma experimentation (which is already of dubious interest to anyone other than myself), having gotten a dedicated space to work on the fusor project will prove to be a huge step as far as increasing what I can do.
Upgrading to a much less cluttered and significantly cleaner space will allow me to assemble high vacuum systems without worrying too much about contamination.
Because I will be off to college (hopefully taking a working fusion reactor with me) in less than two years, my parents allowed me to set up a lab in the unfinished attic of a farm building. This is probably the cleanest this room will ever be, so I made sure to take pictures now instead of later :)
While the largest development since my last post was undoubtedly achieving first light of Fusor 1, the more incremental tests and developments recorded here are equally important to the fusion project. The new lab space has opened up so many opportunities for projects and research, progress is only going to speed up from here!⁷
Notes & Citations:
³ There were very many potential sources of outgassing in the Fusor 0 chamber including vacuum grease, non-vacuum epoxy, trace amounts of water from the milling of the aluminum flanges, trace amounts of isopropyl alcohol from the cleaning process, trace amounts of water on the borosilicate tube from the cleaning process, tape glue on the grid, flux on the grid, trace amounts of water on the grid, and many others.
⁴ I am beginning to entertain plans to convert the leftover Fusor 0 chamber into a sputtering system sometime in the future so Fusor 0 may not be retired permanently. We’ll see.
⁵ The gauge I am using is analog and calibrated to read pressures from 0 microns to 2000 microns so pressures above 2000 microns (2 torr) cannot be measured accurately. The gauge needle rested about half way between atmosphere (approx. 760,000 microns) and 2000 microns.
⁷ Of course, now that I say that something bad will happen and progress will be significantly delayed… I guess I’ll knock on wood.