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Time constant and pulse magnets

  1. Nov 7, 2008 #1

    I've been reading about these magnets such as these:

    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?
  2. jcsd
  3. Nov 11, 2008 #2
    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".
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