Core material selection for high-current DC solenoid

[lichen]

Hello! This is my first post on Physics Forums. Seems like a good community. I will endeavour to contribute.

Over the next few weeks I will have to build a test rig for a fiber-optic sensor I'm developing. The sensor is for high magnetic field strengths, so the test rig must essentially produce a strong (up to 5 Tesla!) uniform static magnetic field.

My plan is to construct the high-current solenoid with suitable core material, then have a small hole bored along the central length of the core, in which the fiber sensor can sit for maximum magnetization.

I don't know a great amount about the commercially-available selection of core materials.

My question is, basically, what is the most suitable core material for my application? Consider that the saturation induction must be very high (5 Tesla), relative permeability should obviously be large, and the material does not have to be suitable for high-frequency applications. Price is not an issue, within reason.

Many thanks for any suggestions/advice.

Philip

 

[berkeman]

Can you just use a strong permanent magnet? If you need to be able to modulate it or turn it off, then I think you can just use a high-perm "soft" magnetic material -- some variation on iron, I would think. You want it to be "soft" in the magnetic sense, so that when you turn off the field, you do not get much of a residual field.

http://en.wikipedia.org/wiki/Magnetic_hysteresis

Also, you don't want to drill a hole in the material to put your sensor in -- the B-field will just bypass the hole and stay in the magnetic material. What you want in stead is a small gap in the magnetic material. You should use material that is in the shape of a "C", where the material is continuous except for a small cut or gap. That gives you the lowest magnetic "reluctance" for the magnetic circuit, and the highest field in the gap.

If you can use a permanent magnet, you should use a "C" shaped magnet with as small a gap as possible.

 

[lichen]

Thanks. I will look into using a strong permanent magnet. The problem is I must be able to control the field strength (range from 0 to 5 T).

I didn't realize that the field would simply go around the gap... That's unfortunate. Using a C shape is not really suitable for my application...

I hear that 5 Tesla will saturate "by a factor of 10" any commerically available core material. Is this true?

If so, how on Earth does one produce such a high strength magnetic field?

Thanks again.

 

[berkeman]

Hmmm. Not sure. Plasma confinement fusion uses pretty big and high-field magnets:

http://hypertextbook.com/facts/2000/AnnaWoo.shtml

http://coolmagnetman.com/magflux.htm

How wide of a gap do you need for your probe? Do you need to continuously (smoothly) vary the field strength from 0-5T, or could you do it in discrete steps of 1T (by varying the gap size, for example)?

 

[lichen]

Discrete steps would be acceptable. I suppose a permanent magnet could be used, but it is not really desirable since it would have to be under mechanical control and couldn't be 'turned off'.

 

[f95toli]

5T magnets are generally just superconducting solenoids with a relatively small bore (a few cm at most). However, winding a 5T magnet is as far as I understand pretty complicated and it not something I would recommend unless you really know what you are doing.
It is possible to make 5T magnets using normal wire (copper) but then you will need active cooling of some type (usually water cooling) since the resistance of the wire will be very high. Normal magnets are quite rare (I have never seen one IRL) and are probably quite a bit more expensive than superconducting magnets.
High-field magnets are -as far as I know- never made using any form of core material at all.

You will also need a pretty good current source, for 5T you will probably need about 50A. Moreover, it needs to have some form of protection circuit in case something goes wrong (there is a LOT of energy in an energized 5T magnet).

In case you haven't already guessed: 5T systems are pretty expensive.
We are currently planning to buy another small magnet where I work (something like 2T) and the total cost will probably be around £5K or so (and that does not include the cryogenics since the magnet will be fitted to an existing system).

 

[lichen]

So, perhaps I should rule out the possibility of achieving 5T.

If I were instead to attempt 2T (I think this is within the saturation limit of iron core?), why should I be expecting to pay up to £5K? If I was to set out and buy a core, enamelled high current wire, and accessories, and build it myself, would it still cost that much do you think?

Also, do you think I would need a form of active cooling even for a 2T solenoid? What kind of temperatures do these windings reach?

Thanks for the advice.

 

[lichen]

OK, so I doubt I can use an air core, because the current requirements would be gigantic.

If I assume I will buy a core, what core material will give me the highest saturation magnitization? Does anyone know? I know iron's is quite high (about 2T).

Thanks.