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What sort of decay curve can you plot from the magnetic field dissipating over time? Is it linear?

No. It basically doesn't dissipate at all until the helium is gone and it warms up.

Not a physist, but I remember from school when studying electrical resonators, the stronger the inductor the lower the resonant frequency. i.e. the stronger the magnetic field the longer it takes to turn back into current.

I would guess a strong magnet takes a while, probably minutes to shed its field.

This is a superconductor. It is basically on full strength until it suddenly isn't.

In the idealized resonator, the impedance is assumed zero what matters is the inductor strength and the capacity in the capacitor.

In a real MRI, I presume there is no capacitor to take the charge, I am guessing the limiting factor is the impedance of wherever the charge is going to go.

Saturation/critical current: zero resistance doesn't mean infinite current. all superconductors gain resistance if the current exceeds a value. This value is temperature dependent.(http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/scbc.html).

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