I’m considering simultaneously building 3 different devices:
With this approach we lower the barrier to success. Building a fusor is comparatively easy next to a superconducting polywell. I expect a fight getting down to 10e-08 torr, but I don’t expect trouble getting to 10e-03 torr. While I’m futzing with the vacuum, I can get the hang of operating a fusor. Valuable experience sooner.
While disassembling the vacuum chamber last night, I removed the probe for the vacuum gauge that came with the chamber. Now, I assumed the chamber came with a pirani gauge because the readout for the guage only went down to 10e-3 torr. But looking at the probe, I’m pretty sure this is a nude ion gauge:
I think this is it’s pinout.
I’m preparing to have the core fabricated. I have a number of considerations to consider.
Welding. We have to weld the lids to the chassis.
Maybe TIG welding will work. My concern is that heat affected zone will damage the SC coils inside. We have ~2.5 mm from the surface to the coils. Laser welding has a much smaller heat affected zone. TODO: get a quote for laser welding from EB Industries. Can anyone comment of the viability of TIG welding for this sitation?
Surfacing. The product that comes back from prometal has a rough surface which we need to machine so that the lid mate well.
Previously I tried wet sanding. This worked decently. However, I wasn’t able to get the deeper surface imperfections, it took a lot of sanding. The outer ring of the torus half saw more material removed than the inner ring, which means that the inner rings mate very tightly, but the outer ring has about a 0.25 mm gap.
There is a surfacing machine here in the shop. It’s large enough to accommodate the lids, but not large enough to accommodate the chassis. The surfacing machine uses a magnetic vise, so the work piece must be magnetic. The sample parts we ordered from prometal are magnetic, however the next parts will be made with a less magnetic stainless steel alloy (the chassis should not be magnetic).
We may need to take surfacing into consideration for the design of the part. ie, we may need to include some extra material on the prometal part, so that after we surface it, we have a perfect half torus.
UPDATE: Stuart told me about Lapping which seems to be an advanced for of wet-sanding.
CAD problems
I’m using BRL-CAD to generate my parts. Lately I’ve been getting this error when I try to export to STL: class_lu_vs_s: loop transits plane of shell/face? I can’t proceed until I overcome this bug.
Even when the STL export works, it takes forever to render an STL with the resolution I need for production (I’m talking days here). This is really cramping my flow.
Permeability of the Core
We are building a superconducting core. There will be liquid nitrogen at atmospheric pressures inside the core (and connected to outside of chamber via a fluid feedthrough). The core can’t be so permeable as to leek nitrogen into the vacuum which would poison the reaction. Speaking of pressure differentials, the core must withstand the pressure from the inside. To calculate this pressure, I think we need to know the internal surface area of the core.
I’m toying with idea of buying this decommissioned MRI to extract it’s superconducting cable. I’ve spoke with a few people at Philips to gauge the feasibility of the idea. So far nobody knows. Extreme recycling.
What is this yellow tube for, and do I need to worry about it?
The docs say:
The pumping station is fitted with air cooling which can be used for ambient temperatures up to 35 °C as standard. Conversion to water cooling is possible if required (water cooling on request).
Does this mean air cooling is included in the unit? Or do I need to attach an air pump (presumably to that yellow tube). What kind of pump/fan would I use?
There is a large fan pointed right at the base of the turbopump. I think that’s the built in air cooling. Also I wonder what the electronic component is just before the yellow tube.
Based on these docs, I think it’s a “Venting Connection”. But what is that? It’s seperate from the “Fore Vacuum Flange”
UPDATE
Talked to the maker of the pump (very helpful). Basically that tube is used when you want to re-pressurize the chamber. Sometime people use nitrogen for re-pressurization so water vapor doesn’t enter the chamber.
He said not to use the heater w/air cooling (the heater is for bake-out of the pump). And he said to check the oil packet prior to use, which I’m doing now.
UPDATE2
The oil packet gets a clean bill of health from the manufacture, this pump is ready to rock:
Last night I took a double espresso, and got down to business setting up the vacuum chamber. I’m removing the top portion of then chamber, elevating the chamber, and connecting the pump. All the connection are dry (without gasket) until I’m comfortable the setup is correct.
Here is a time lapse video of the transformation (made with TimeLapse for iPhone):
The new turbo pump just arrived. The Turbopump itself has has a 4.5″ conflat flange for an inlet. Attached to the turbopump inlet is a 4.5″ to 2.75″ reducer.
I think as a first step, I’m going to skip the heroics of putting the chamber on it’s side, and just buy a 2.75″ 90˚ conflat elbow. Cheap and easy to install. I realize this is not an optimal connection for the pump (skinny with a bend). But ultimately I have time, and if it takes days to get down to vacuum… so be it.
I’ve moved to a cheaper short term (one month) apartment to reduce my burn rate. Also did away with most of my personal possessions to keep light and mobil. So now the lab is my home base. I’ve got it setup:
I must return my work laptop soon, so I got a new macbook with 4G RAM, you can see on the desk.
Very exciting. The YBCO arrived today. This is what $1,105.00 worth (13 Meters) of insulated YBCO superconducting cable looks like:
The ribbon itself is very very thin and flexible. Much more flexible than I was expecting. Here is a close up of the ribbon:
That’s just some scotch tape at the end.
The lead time for this YBCO is about 3 months. So you really have to order in advance.
Today I plan to order the first prototype of the chassis.
Also going to get the dewar flask filled with some liquid nitrogen and attempt to build a superconducting coil!
While we’re on the topic of superconducting cable. Would it be possible to buy a used MRI machine and extract the superconducting cable? Even if this is possible, it would be using low temp superconducting cable, which of course requires both liquid helium and liquid nitrogen.
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