Imperfections in magnet pole pieces

[Christoffer B]

TL;DR Summary

I'm designing a gap magnet for NMR experiments, and the steel pole pieces I have obtained has a 5 mm deep 1mm wide saw cut into the peripherie. Will that be catastrophic for field homogeneity?

Hi all, I'm fairly new here.

I'm currently designing an as-homogene-as-possible permanent magnet for low-field NMR and similar experiments. I'm on a fairly tight budget, and having finally found pole pieces for my magnets, I got them even though they have a small saw cut in the edge. approx. 5 mm from the peripherie towards the center, all the way through. Both magnets and pole pieces are cylindrical disks, 10 cm in diameter, magnets are 15mm thick and pole pieces are 5.

Asuming that i can get the pole pieces parallel and polished to a mirror finnish, how bad would you estimate these cuts would mess up my homogeneity? I plan on having a way of shimming the magnets regardless.

I've tried modelling the effect in FEMM (free magnet finite element software), and it looks negligible towards the center of the magnet, but obviously looking at field lines on an image isn't really quantitative. Please see attached.

Thanks for the interest!

--Chris

 

[marcusl]

In general you want homogeneity of 1 ppm or better through your sample volume to resolve individual lines. It can be worse if you just want to say "I got a signal out of the sample."

I can't picture your description. Can you attach a photo?

 

[Christoffer B]

Thanks for your reply! I'm hoping to get crude structural analysis resolution, so ppm level is likely needed.

Sorry for the poor description, here's a link to the discs I purchased:https://www.ebay.com/itm/102mm-x-5mm-Round-Disc-Circle-Mild-Steel-Sheet-Plate-Size-/264306833910?

I guess you wouldn't expect homogeneity right on the edge of pole pieces anyway, but I'm having trouble visualising the magnitude of the influence of the defect.