What happens if pushing the MRI QUENCH button? How the superconductive magnet loses the superconductivity?


In order to achieve the superconductivity in a magnet, the coil winding must be at extreme low temperature. Liquid Helium (4K) is for this purpose. For an MRI machine, what happens if you push the QUENCH button? This webpage ((http://mriquestions.com/what-is-a-quench.html)) states ” If the coil temperature rises above the superconductivity threshold (*Tc*), the winding suddenly develop a finite resistance.  The several-dozen amperes of circulating current passing through this elevated coil resistance create heat.  This heat causes a sudden, explosive boil-off of liquid helium. ”

Does this mean after pushing the QUENCH button, the coil temperature will rise? But how exactly pushing a button can make the coil temperature rise at the first place? It seems that the logic in this explanation in this webpage is: Push the QUENCH button -> coil temperature rises -> heat in the coil generates and increases -> liquid helium boils off.

But my understanding is that, after pushing the QUENCH button, the liquid helium will connect to room temperature in some way, such that liquid helium will boil off (becoming gas) and leave the coil winding container. At the same time, the temperature of coil winding itself rises, leading to the occurrence of a finite resistance. In this way, the magnet loses its superconductivity.

But I am not sure if my understanding is correct. I don’t know if it is true that pushing Quench button will first lead to liquid helium temperature rise. Even if it is true, I don’t know how. Could someone who knows this explain this like I am 5? Thank you very much!

In: Physics

> But how exactly pushing a button can make the coil temperature rise at the first place?

It activates an electrical heating element on or very close to the magnet itself. There are dedicated circuits for this emergency stop process, and the goal is to heat the magnet quickly to the point where it gains electrical resistance and the circulating charge can dissipate into heat, stopping the strong magnetic field.

Basics: If a coil has resistance and if current is made to pass through it, there will always be heat generated in the coil. Also more the current passed through a coil, higher the electromagnetic field generated.

Now coming back to your question. In order for MRIs to work, we need magnetic field of the order of 3 to 5 teslas. This is achieved by a superconducting magnet. In order to make a superconductor, we need a coil whose resistance is ideally 0 ohms. This is usually achieved by lowering the temperature of the coil, since metals tend to lower their resistance wrt lower temperature. The quench button basically undoes the coolant action. Thereby increasing the temperature of the coil. If coil temperature rises, resistance increases thereby adding more heat. At the same time, magnetic power of the coil is reduced because of the increased resistance.

Hence, no coolant will result in increased temperature which will result in higher internal resistance of coil and lower magnetic field generated.