Built a giant electromagnet for magnetized fusion and plasma research

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RobertMendelsohn
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Joined: Mon Jul 20, 2020 8:58 pm
Real name: Robert Mendelsohn

Built a giant electromagnet for magnetized fusion and plasma research

Post by RobertMendelsohn » Mon Mar 15, 2021 7:53 pm

Hello all,

I recently finished construction of a giant electromagnet, which I needed to continue my research into magnetized fusion (pre-magnetization for pinches) as well as exploring other topics in highly magnetized plasmas. I wrote a long post with technical information, equations, etc. in an interactive cloud notebook here (https://community.wolfram.com/groups/-/m/t/2219944) and wanted to share a photo as well as some important things I learned along the way. Feel free to leave a comment or PM me with questions if you are interested in building something similar! Most non-professional electromagnets online were little more than wire wrapped around a nail, so I did a lot of research to see how an "amateur" (bad term in my opinion) could produce a high-performance electromagnet. For those short on time, the top things to know:

1. Use aluminum wire instead of copper wire - it is 3x+ cheaper while 60% as conductive.
2. Different gauge wires are sometimes nonsensically cheaper or more expensive - my #2AWG wire was way cheaper than #4
3. Bonding to electrical insulation can be very difficult. In my case the wire was polyethylene, and I had to use a specialized hot melt adhesive (Infinity 500, great stuff)
4. Use very low forward voltage diodes for rectifying low-voltage output for best performance
5. Unless you need very high homogeneity, stick with a solenoid

This advice changes if you need a pulsed magnetic field, which may be the subject of a future post.

Edit: specifications. ~5 feet tall, ~1 foot diameter. 400 wraps of #2AWG wire, power supply designed for easily outputting 67A continuously, resulting in a central field of 160mT, in a 2 inch diameter, 2 foot tall section with +-2% homogeneity. Can be pulsed much much higher with my pulse forming network. It weighs about 125lbs (just the electromagnet).
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Rich Feldman
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Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: Built a giant electromagnet for magnetized fusion and plasma research

Post by Rich Feldman » Mon Mar 15, 2021 10:25 pm

Nice work there. What's your field modeling program?

The familiar conductor-procurement issues have also been explored in my old DIY electromagnet thread,
in which a steel yoke concentrates the field in an air gap much shorter than pole diameters:

* For a given DC magnetic field, electric power requirement goes down as coil winding volume and conductor mass go up.
* Efficiency is unaffected by the number of turns into which coil winding area is partitioned. Conductor thickness may be selected for economy of procurement, just like Robert reported, if designer is free to trade off V and I (for constant product) in the power supply.
* Aluminum may be more economical than copper. For a given winding volume & magnetic strength, Al needs more electric power than Cu. But if Al coil is made thicker, to match electric power requirement of Cu, it will still be lighter and may cost substantially less.

One caveat:
Copper in wire and strip form is usually almost pure, with conductivity just over 100% IACS.
Aluminum wire that's made for conducting electricity is close to 61% IACS.
But aluminum in wrought forms like bars, sheets, and strips is usually an alloy, with much less conductivity. For example, 6061-T4, 35.5 to 41.5% IACS.

Robert, can you tell us more about your power supply and magnetic field measurements?
Thanks!
All models are wrong; some models are useful. -- George Box

RobertMendelsohn
Posts: 31
Joined: Mon Jul 20, 2020 8:58 pm
Real name: Robert Mendelsohn

Re: Built a giant electromagnet for magnetized fusion and plasma research

Post by RobertMendelsohn » Tue Mar 16, 2021 3:47 am

Hi Rich,

Great points about the conductivity. I looked up the specific alloy beforehand and got the electrical data and it did indeed fall a bit short of the standard quotes value for aluminum, but not enough to be too appreciable. Smart to have those figures beforehand. I talk more about the power supply in the linked blog post/interactive notebook, but I am using two variacs feeding two 10x step down transformers (3300W per) which are then rectified by a full bridge rectifier with high-current very low forward drop diodes. The electromagnet itself acts to smooth the current ripple, but there is an additional inductor and capacitor for smoothing. The plasma processes in question are very short timescale so the ripple is relatively unimportant, but could be further reduced with capacitors and perhaps a pi filter. The low forward voltage drop diodes are really the key components, because for low voltage high current applications that voltage drop will eat away at the available voltage and consume a LOT of power. My four diodes are hooked up with silver epoxy to some really large heat sinks. Hope this helps! Ask me any questions, I'm happy to be a contributing member of this forum...

Sincerely,

Robert

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