Time constant and pulse magnets

[trilex987]

Greetings

I've been reading about these magnets such as these:
http://www.lanl.gov/orgs/mpa/nhmfl/magnets.shtml

Take a look at the pulse durations (it says in notes at the bottom "Total pulse length including decay"

It's all nice but I don't understand one thing.
How can they make the current rise so fast? If the time constant of such a magnet is L/R, you have to have huge resistors, and by that the power rating in kA range would be more than any power generator on Earth can make
(consider having L of say 0.2 mH, and current of say 20kA, to get a time constant of 1 ms, you need to have resistance of 200 ohms, and that gives you a power of 80GW which is insane)

Am I missing something here?

 

[Enthalpy]

Well, such magnets are indeed insane.

The time constant isn't necessarily L/R but can be sqrt(LC). R isn't a desired feature. Generally unavoidable, but you may perfectly feed the magnetic energy back in a capacitor. Where did you get 0.2mH? I didn't see it in the linked page.

80GW is an available power, at least for a limited duration. In fact, many experiments with high currents have higher powers than that. They generally discharge quickly a big capacitor. Flywheel also existed, see "homopolar generator".

Many lasers have instant power MUCH bigger than 80GW... But the pumping energy is spread over a longer time.

I built a much more reasonable setup to magnetize SmCo magnets. 1m3 chemical capacitors, a 3kA thyristor used at 30kA monopulse, 500V, in a massive sturdy homebuilt coil (copper sheet and wood). It achieved 7T for 10ms. Cables already jumped when the current flew, and got instantly lukewarm. Fun.

But with 70T or even 250T, their toy is another breed of cat. Power, energy, force scale up as the square of the induction.

If you want to see high electric powers and interesting energies, have a look at Sandia's "Z-machine".

 

[astrokreng]

Greetings,

Thank you for your question. Time constant and pulse magnets are indeed fascinating subjects in the field of electromagnetism. Let me try to provide some insight into your question.

First, let's clarify the concept of time constant. The time constant of a circuit is the time it takes for the current or voltage to reach approximately 63% of its final value after a sudden change. It is determined by the product of the inductance (L) and resistance (R) of the circuit, as you mentioned. In the case of pulse magnets, the time constant is important because it determines how quickly the magnetic field can be generated and sustained.

Now, let's address your concern about the power rating needed for such fast current rise. You are correct in saying that a large power source is needed to generate such high currents. However, pulse magnets are designed to operate at high voltage, which reduces the power required. This is achieved by using capacitors and inductors in the circuit to store energy and release it quickly when needed. In addition, the pulse duration is usually very short, on the order of milliseconds, which also reduces the overall power needed.

Another important factor to consider is the design of the magnet itself. Pulse magnets are typically made of superconducting materials, which have zero resistance when cooled to very low temperatures. This allows for extremely high currents to be generated without the need for large resistors. Furthermore, the magnetic field produced by the magnet can be focused and directed in a specific direction, reducing the overall power needed to create a strong field.

In summary, pulse magnets are able to generate high currents and strong magnetic fields by utilizing high voltage, energy storage components, and superconducting materials. I hope this helps to clarify your understanding of these fascinating devices.