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Do MRI magnets oscillate during charging or even during persistant mode due to parasitic capacitance? What problems can this cause?

I am planning a low temp superconducting toroid inductor with multiple layers operating in persistent mode under liquid Helium. I am considering if I have to plan for a serious a problem with LC oscillation due to parasitic capacitance between layers.

I can imagine that this may cause quench problems, problems with heat causing Helium boil off etc.

Do MRI magnets, superconducting magnets in general experience an electrically quiet ramp up, or does oscillation have to be mitigated. I've never read that there is a problem, but a physicist friend with some experience with DC circuits mentioned the possibility. I know that there will be parasitic capacitance and so it seems reasonable that all such equipment would have that problem.

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  • \$\begingroup\$ I think the self or parallel resonant frequency must be designed to be much greater than the RF coils for controlled impedance. \$\endgroup\$ Jul 18, 2018 at 1:13

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The only problem that an LC resonance would introduce is some ringing at discontinuities.

You may rest assured that we treat changing the field of a superconducting magnet in much the same way that you would do dental work on a nuclear polecat; very, very carefully and slowly. The economic cost, and physical danger, of a quench that could result from messing the process up is too high.

The process is (a) ramp up the current into the superconducting switch, until the external driven current matches the magnet current (b) open the switch (c) ramp the current to the new value (d) close the switch.

We take much precaution before step (b), as the voltage generated by the residual delta current could exceed the insulation rating of the magnet. Any ringing due to self capacitance is completely irrelevant, the damage will have been done in that first excursion. In fact, the self capacitance is a mitigating feature, that voltage spike has to charge the self capacitance, and so it limits the voltage.

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I don't think so. The Q will not be infinite, so any ringing would die off.

You obviously cannot increase the current quickly in the coil. The only moment of discontinuity is when the charging is complete but the delta in inductance (from the heat switch) is small so there might a small amount of ringing. While it is charging the damping comes from the series resistance in the (normal) wires running to the coil.

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