# How are Superconducting Coils charged?

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1. Mar 21, 2017

### John Morrell

Superconductors can be used to store energy in the form of magnetic fields, because the current in a superconductor can persist indefinitely. In fact, some large power grids are already using this as a way to regulate power flow in the grid.

My question is how are the coils "charged"? How do we get current into them, and how do we get useful power out of them?

2. Mar 22, 2017

### Simon Bridge

You charge them by attaching them to a power source.
You get useful power out by the same method only in reverse.

The puzzle is that you didn't already figure this out, so I'm guessing you have some objection?

3. Mar 22, 2017

### f95toli

I am guessing the question is really how you get current into a superconducting solenoid if it is a closed loop?
The answer is that you use a heater to heat a piece of the superconducting wire above the critical temperature; this makes it resistive and you can then use a normal current source to ramp up the current in remaining solenoid (no current will flow into the resistive bit since the resistance of the rest of the wire is essentially zero). Once you've reached the desired current you can turn off the heater, when the temperature of the piece of wire goes below Tc it becomes superconducting again and all the current will flow around the (now closed) solenoid, in a well designed magnet the current will decay very, very slowly. This is what is known as persistent mode.

Power supplies meant for superconducting magnets do all of this automatically. However, you can also do it using "normal" electronics. I've built a couple of small solenoids that could be put into persistent mode and they actually worked quite well.

4. Mar 22, 2017

### rbelli1

What exactly is abnormal about regular superconducting magnet power supplies?

BoB

5. Mar 23, 2017

### John Morrell

That makes sense. I didn't initially think of this simply because I thought that it would be really difficult to generate or maintain really high currents in a non-superconducting wire like that for any length of time without melting wires and stuff.

I'm still curious though, if you have a loop already charged up and you want to charge it up further, would you just have to dump all the existing current into your charging device until you could further increase the current?

It's all a bit weird to think of, I guess, because I'm used to power sources applying voltages instead of currents...

6. Mar 27, 2017

### f95toli

There is nothing "abnormal" about them. However, they are explicitly designed to work with superconducting solenoids; meaning they are low-voltage (1-2V ) high current (60-120A) supplies with features such outputs for the heater and -more importantly- quench protection. This means that these supplies can not be used to e.g. drive a normal solenoid.

7. Mar 27, 2017

### f95toli

No, the supplies are designed so that you can just turn on the heater; this will then make the current flow via the supply and then you can ramp the current up/down.