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MPI Magnet Spacing
This question fundamentally impacts the rest of the design and is on the docket for the first questions to be asked. Within this page, a select group of intertwined design parameters will be discussed. Given the likelihood that future groups will modify our initial design to fit other specific needs, and the wide-reaching impact of changing the FFL spacing, we have included this discussion as a qualitative starting point for other group's designs.
If you move the magnets closer together you get higher resolution images because the gradient goes up but as the gradient increases, if the field of view is maintained, this gradient increase puts a similar on the shift coils. So if the gradient is doubled, the FWHM of the FFL is halved, which doubles the resolution, and doubles the amount of field the shift coils need to supply.
If you move the magnets closer together, the copper bore needs to shrink a proportional amount. For small changes, this is probably fine and besides the heating effects on the shift coils likely won't have a major impact. But as the bore gets substantially smaller, unless you reduce the drive coils too (which reduces the Rx coils) you will start to have a greater proximity effect of the tube on your drive coils, lowering the efficiency of those coils requiring more power increasing heating problems etc.
The length-to-spacing ratio of the permanent magnets should be kept approximately the same due to edge effects of the FFL, so if you reduce the diameter of the bore and bring the PMs closer together, you can also get away with shorter PMs to a degree, and therefore shorten the shift coils.