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Measuring a *really* fast magnetic field change

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Hi all,

I have a rather basic question about Hall sensors. I need to measure a pulsed magnetic field (say at 1 Hz) that is approximately 1 femtosecond (!) in length. Is this even remotely possible? Obviously even triggering the Hall sensor is difficult, but assume that that Hall sensor is triggered "correctly", so that it could feasibly measure the signal if able. The Hall sensor has some finite time that it measures the voltage across the Hall element, and so I wasn't sure if with such a short pulse the Hall sensor would a) completely miss the signal, or b) measure some integrated or averaged version of the magnetic field.

I don't think Hall sensors generally have such a high bandwidth, but I thought I'd ask people more knowledgeable than me :)

If you are curious, the application is in particle physics, where we want to measure the charge in an electron beam moving very close to the speed of light. The length of the bunch is only about 500 nm,  which is around a femtosecond. Yes, there are other ways to measure the charge, but I think there are some good reasons to do it this way (if it is possible, that is).

Thanks in advance!

While the pulse is 1 fs long, the magnetic field will spread this out to a few fs, at least.  But, I doubt a Hall sensor can respond to this, as the field has to penetrate the somewhat conductive bulk of the sensor.  I think everybody uses magnetic coils to sense things like this.  A quite small coil will do, but you often have to shield the coil to prevent pickup of ambient electrostatic fields.  The shield needs to be broken at one point to avoid creating a shorted turn.  Then add a fast amplifier to the coil, and you should be set.  Watch out for capacitance of coax cables.


No.  First of all, a femtosecond pulse is an optical frequency.  So you should be thinking more like "measure light" than "hall sensor"

Second, unless your sensor is within a few hundred nanometers of the charge, and the sensor itself is a few hundred nanometers in size the magnetic field pulse will be much longer, defined by the size/separation of the sensor rather than the charge packet.

Just curious, isn't the amplitude of the signal imposed on the beam position monitor you probably have any way directly proportional to the amount of electrons in the beam?

@ejeffrey, these are fair points. I haven't looked into "how long" the magnetic pulse would be, I just assumed it would be pretty short. The sensors wouldn't be hundreds of nanometers away, the idea was to put them perhaps a centimeter away. Regardless, it is certainly worth looking into further (and probably finding out the measurement is still not feasible, but oh well). Thanks.


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