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Comment by Waterluvian | Hacker News Reader

Why do MRIs need zero resistance vs incredibly low resistance?

Because the resistance is the limiting factor in how strong a magnetic field can be made in a given volume using the conducting material.

Copper across an area of 1 cm ^2 does about 300A, compare with https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8047038/ for what some superconductors can do.

Link says 90 MA per cm^2, or 30,000X more than copper.

Gulp!

I think you are missing a zero.

You are right!

300,000X more than copper.

Typo converting from brain to screen. I should have used exponential notation and the mistake would have been more obvious.

That's not an answer to that question.

I think it's a fair question. I guess the answer is "because we have superconductors, and no material with 'very small resistance' instead".

Also note that while there is zero resistance you still have parasitic currents and general interaction with the rest of the environment. So no perpetual motion for us today!

Even a 'very small resistance' would likely be prohibitive. Pure silver for instance is an excellent conductor, but when you're talking about the last little bits it's orders of magnitude difference between that and a superconductor, and those orders of magnitude difference in resistance translate into orders of magnitude more current. So even a small resistance would cause your MRI machine to have a resolution so low as to be unusable.

Actually, there are infinite orders of magnitude between the resistance of silver and that of a superconductor.

MR imaging is possible with permanent magnets and resistive (copper wire wound) magnets at low field strengths, below 0.35T or so. Above that the heating of the magnet windings becomes excessive and it would be very difficult to maintain a stable enough field strength.

Superconducting magnets are very nice as long as there is no quenching. The material used for conductors must be mechanically stable and perform consistently from one production batch to another. One reason why current high tc superconductors are not popular...

The material used for conductors must be mechanically stable and perform consistently from one production batch to another. One reason why current high tc superconductors are not popular

I was going to ask: Why don't we use the current high temperature superconductors, and start building grid interconnects? I guess part of the answer lies in the cost that would be incurred because of the mechanical properties of the existing superconductors.

The current best "High temperature", standard pressure superconductor only works at -150ish degrees Celsius. That's not as bad as cryogenic superconductors, but it is still a blocker for large scale use.

In essence, my comment is asking, "Why is it a blocker?" Very efficient thermal insulation is something we already have.

Because it is easier to keep a localized thing cool than something that is 100's of km long. That's why iceboxes are boxes and not ice ribbons and why most applications of superconductors right now are using them as coils for magnetic field and volume economy. For instance there is one grid component that uses superconducting coils as inertia free stabilizers, they can be used to source and sink current very rapidly to absorb transients in the load. This allows older and less stable grids to be used to transfer wind power because the power over time is a bit lower. Allowing those peaks unfiltered onto the grid would cause parts of it to go down.

Because it is easier to keep a localized thing cool than something that is 100's of km long.

If we simply decided to make this kind of thing a priority, we could probably manufacture suspension components at scale. (Or create small tunnel boring machines and bury them?) We wouldn't need to replace all of the lines. We'd just need enough interconnects to make transferring more power economical.

I think that the initial question was "why a 0 resistance compared to a ridicously low resistance". And my point is that it's easier to get a superconductor than some material with "ridicously low" resistance. As you said, silver is unusable for potent magnets, and such is any other non-superconducting magnets.

Probably if we had materials with a billionth of the resistance of silver they would work, but we haven't. And we have superconductors, luckly. :)

They don't!

MRI scanners work just fine with regular electromagnets, or even plain old normal magnets. However, work better with stronger magnetic fields. In practice normal magnets end up being extremely large and heavy, and electromagnets end up using massive amounts of power. Both options are also limited in their field strength because getting enough stuff close together is tricky.

A superconducting magnet is an electromagnet which is way smaller and uses orders of magnitude less power. For extremely high field strength MRIs they are the only viable option, and for regular MRIs they are often the best option.

The more you crank up the field and gradients the better resolution you can get. Being able to take MRI of a beating heart depends on that sort of thing.

The current is ridiculously large and would lead to an unacceptably high power consumption and thermal expansion with the resistivity of even a good metal (copper).

To create very, very strong magnetic fields.

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