Quench spreading on multiple coils superconducting magnet

[hsinhui]

The multiple coils superconducting maget is uaually quench protected by bypass diodes.

The heat spreading model is mentioned on textbook for coil to coil quench spreading.
But the heat spreading is not fast enough for magnet quench protection.
Because the theoretical quench protection should be "fast" spreading for all coils.
(With initial quench in one coil only.)
So I need the bypass quench heater to accelerate the quench process.
Is it true?

I think the current decay causes the quench spreading from coil to coil through
mutual inductance or self-induced voltage.
This type of quench spreading is faster than heat spreading model.
If the theory above is correct?

If it's true, how can I evaluate it through simulation?
I cannot find any information about field changing quench on superconducting coil.
Thank you very much!

 

[Bob S]

Quench protection in superconducting magnets is a difficult but solvable problem. If a small section of a superconducting coil goes normal, there is a very small V=x·I·R_x voltage drop (where R_x is the resistance per unit length of normal (non-superconducting) cable and x is the length of the normal cable) in addition to the expected V= L·dI/dt signal across the entire coil (where L is the coil inductance). Once the I·R drop is detected, the magnet current has to be shut off, and often the current has to be switched to an external resistive load R_ext to speed up the L/R_ext current decay time constant. If I(t) is the magnet current as a function of time, the total heating per unit length of cable is ∫I(t)²·R_x dt (joules per meter), which has to be minimized to prevent the cable from melting. But if the dI/dt is too fast, then the inductive voltage V = L·dI/dt will arc over inside the magnet. Coil heaters are used to heat the coil and make the normal zone spread faster, make the IR voltage drop larger, and distribute the coil heating. Quench bypass circuits (either active SCRs or passive diodes) are used to shunt current around quenching magnets. In the case of shunt diodes, the V=I·R voltage drop of the normal zone is opposite polarity to the V = L·dI/dt voltage of the decaying current, so heating the coil is necessary to make the IR drop larger than the L·dI/dt voltage and make the diodes conduct. The dI/dt by itself does not induce quenching in other coils, excepting in coil designs where eddy current losses inside the superconducting cable (caused by dB/dt) produces heating..

Some superconducting magnets are designed to fully absorb the quench heating of the superconducting wire and not damage it by overheating.

Bob S

 

[hsinhui]

Dear Bob S, thanks for your great answer!
So the quench spreads by the means of heating only.

In the persistent mode operation,
there is no way to transfer the coil energy to dump resistor.
If the bypass quench heater is necessary for the multi-coils SC magnet?

Thanks a lot!

 

[Bob S]

A real problem in a superconducting magnet is if a small normal zone (quench) appears, and propagates very slowly. In this case, a short section of wire will (in many cases) overheat and melt the wire, unless it is detected and the heaters are fired. Detecting the normal zone depends on having a detectable IR drop, which requires a large normal zone. Some magnets do not need a bypass dump resistor. The bypass dump resistor is just to get the energy out fast and minimize the amp-squared seconds in the normal zone. It is not needed in some magnets (e.g., persistent magnets), if they are capable of absorbing their own stored energy.

Bob S