Max Epstein Fusor 1

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Maxwell_Epstein
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Max Epstein Fusor 1

Post by Maxwell_Epstein »

Hello all!

I have been working on and off (mostly off because of school) on my fusor project for approaching two years now. I made a post on the Images du Jour page about a year ago, but have decided at Mr. Hull's advice to move a more permanent record of my progress to this forum instead.


Fusor 1.3 Design
My current fusor design is based around a stainless steel 6" CF conflat cross vacuum chamber. The rest of the vacuum system has been substantially upgraded since my last post. It now includes a TPU-170 turbopump and TCP-300 controller connected to the chamber through a right angle valve. I have a Precision D-25 roughing pump with a valved connected to the turbopump via a manifold consisting of an up to air valve and Varian 531 thermocouple.

The HV supply is a pretty basic 12kV NST supply. I am using a tantalum grid and feedthrough that I bought from Steven Haid on the items for sale section of this forum. I am measuring grid voltage with a Fluke 80K-40 HV probe and current over a 10Ω resistor between the NST case and ground. I have not yet tested this system with the turbopump in operation.
The system in its current configuration - without the HV system attached
The system in its current configuration - without the HV system attached

Turbopump Pumpdown Tests
Once I connected the turbopump and sealed the system, I conducted a series of tests. More runs are required to fully characterize the system and to remove residual moisture and other contaminants. However, I believe that the data I collected is promising.

Although I have two thermocouple connection points, I have blanked off the KF-25 port on the foreline as I only have one functional thermocouple tube. I attached that tube directly to the chamber itself.

My procedure for these tests was to close both valves and activate the pump until the oil stops gurgling. I then opened the foreline valve which allowed the roughing pump to pull air out of the turbopump. Once the oil stopped gurgling again, I opened the turbopump --> chamber valve and began monitoring the pressure in the chamber.
All data from Fusor 1.3 Pumpdown test 1
All data from Fusor 1.3 Pumpdown test 1
After two successful roughing pump tests, I activated the turbopump for the third test once chamber pressure reached 200 microns (I waited for such low vacuum mostly because I was still unsure if everything would work and wanted to "soft" start as much as I could). I first brought it to its standby speed before increasing to maximum speed.


Next Steps
I plan to first condition the chamber and attempt to flush as much moisture from the system and roughing pump oil as possible. I hope that process will bring the leak rate down significantly.

After that, I will attach my NST HV system and hopefully observe some low pressure plasma!
See my needlessly verbose fusor blog here: https://highschool-fusioneer.medium.com/!
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Maxwell_Epstein
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Re: Max Epstein Fusor 1

Post by Maxwell_Epstein »

Considering the leak rates I have observed, is this a level to be expected with a new, unconditioned system?

As it stands, I first measured a leak rate of about 0.8 microns per second in just the main chamber starting at 200 microns. When I included the turbopump in the sealed system in a subsequent test, my leak rate jumped to ~2.2 microns per second. I have had some leak problems with that turbopump before. Will this leak impact anything in a meaningful way? It's worth noting that in the third test, when starting at 30 microns, my leak rate in the chamber (not chamber + turbopump) was 0.2 microns per second. That's clearly better, but is that still too high? Is this the sort of problem that simple conditioning of the chamber and burn in will solve? What other steps can I take to improve my leak rate?

I know that's a ton of questions, I am mostly just wondering if I have a serious leak problem that I'll have to address. These are some of the lowest vacuums that I've worked with so far so I am really looking for some context to my numbers. I am also scouring other fusor.net posts.

Thank you!
Max E.
See my needlessly verbose fusor blog here: https://highschool-fusioneer.medium.com/!
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Liam David
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Re: Max Epstein Fusor 1

Post by Liam David »

Sounds like outgassing + a bad leak to me. The outgassing will subside, at least for micron-level vacuums, when you pump for at least a few hours continuously.

My reasoning is as follows:

Your first pump-down, (from atmosphere?), bottomed out at ~200 microns. The chamber leak rate was linear at 0.8 microns/s.

Your second pump-down, (from atmosphere?), bottomed out at ~200 microns. The chamber + turbo leak rate was linear at 2.1 microns/s.

Thus: The turbo has an independent leak and/or is outgassing more than the chamber. If outgassing, this is not surprising due to the high blade surface area. Contaminants can also accumulate in turbos during normal operation.

Notice: The leak rates are linear with time, which typically suggests a real leak. However, the duration of your measurement is short and so the rate could decrease asymptotically over longer times. A real leak is linear essentially to atmosphere, while outgassing will level the pressure off well below atmosphere.

Your third pump-down included the turbo, which brought the pressure to ~30 microns. The chamber leak rate was linear at 0.27 microns/s.

Notice: The pressure was lower due to the higher effective pumping speed of the turbo vs. the mechanical pump.

Notice: The leak rate decreased after the turbo pumped for a few minutes. This suggests at least 0.27/0.82 ~ 1/3 of the gas load in your initial pump down and leak was from outgassing. The upper bound real leak rate for the chamber is 0.27 microns/s, and likely lower since things aren't fully outgassed.

It all seems like outgassing so far, so why do I posit a real leak or two? The final pressure achieved with the turbo is very high, even accounting for the high roughing pressure of ~200 microns. While turbos don't achieve anywhere near their ultimate compression ratios for all but the cleanest systems (mine hits ~5e5 vs. its rated 1e8 at 4e-9 torr), yours is ~10 and the pump is rated for 8e8 for N2.

P.S. Good luck with college apps!

Notice: We don't know where the leak is (before or after the high-vacuum angle valve).

What I suggest doing is the following: Let the system pump for several hours with just the mechanical pump. Log the pressure over time. Shut the valve and let just the chamber leak for ~ 1hr while logging the pressure. Pump it back down and then let both the chamber and turbo leak for another ~1hr. Next, pump with the turbo for a few hours and repeat the two leak tests I just described. Also, the 1hr isn't critical and varies by system. Log data till something interesting happens, like the curve flattening significantly, or till you're pretty sure it'll be linear for a long time. This should give you a pretty good idea of what's going on.

P.S. Good luck with your college apps!
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Maxwell_Epstein
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Re: Max Epstein Fusor 1

Post by Maxwell_Epstein »

Thank you Mr. David!

I really appreciate your response and your thought out reasoning. I see and understand what my problems may be and what I should do as my next steps. I will set up a longer duration mechanical pump test in the next few days to determine what sort of outgassing/leak I am dealing with.

Max E.
See my needlessly verbose fusor blog here: https://highschool-fusioneer.medium.com/!
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