Can you collapse an inductors magnetic field instantaneously?
NO ….you can neither create nor collapse a magnetic field instantaneously since there is always some L and R time constant involved.
You CANNOT reverse the voltage on an inductor either (at least not in any practical circuit) and get lower discharge times.
Perhaps a simple example may help:
simulate this circuit – Schematic created using CircuitLab
In the circuit above the inductor is a very low value and may be typical of what you might have in a large air cored loop, though the value is not really important.
Here I've shown the inductor as having about 0.5 Ohm resistance, and fed from 12V by a 0.5 Ohm resistor. There is an L and R time constant to build up the magnetic field, and when you turn OFF M1 drive I've shown a Zener Gate feedback drive to dissipate the energy as quickly as possible.
In the schematic and values above it takes about 500us to increase L1 current to 10A, and about 20us to decrease it to zero.
The practical limit here is the voltage you allow to develop across M1. Increasing the voltage decreases time taken to discharge L1.
Update: If you want to delve into the math involved in characterizing and selecting components, supply voltage etc, this ST application note may be well worth reading.
It is worth noting:
Using a spark gap of any form is an unlikely method of discharging an inductor since it has a trigger voltage and a maintain voltage (which would actually extend the collapse time). Even with a professional vacuum/gas spark gap they take time to operate, usually in the 1us range for small units. However, once triggered they maintain at a much lower voltage (typically 70-100V) so don't provide the capabilities needed (maintain a high voltage).
Propagation of an EM field is always less than the speed of light and medium dependent (go study Maxwells equations). A constant field is not subject to propagation delay, only a changing field, so you actually need the current to be changing continuously to create EM wave fronts.
Update: Since the comment about being able to reverse the voltage across an inductor caused some commotion, let me add this:
You can of course use for example an H-bridge to feed an inductor and this will reverse the voltage, but it DOES NOT provide a shorter discharge time in any practical circuit, and the point of the OPs question was in seeking to collapse the field quickly.
Here is a schematic for an H-bridge using 100V, but only 10A (because of the FETs available to be simulated):
simulate this circuit
The waveforms look like this:
Notice that the rise and fall times of the inductor current are the same, and it's not possible to change them without changing the voltage applied.
In my original circuit the falltime was 0.1 of the risetime and could be dropped even further by altering the clamp voltage.
The only way to shorten the time taken to reach max current is by increasing the clamp voltage or decreasing the resistance (of the inductor). If you switch the voltage polarity on L1 then the rise and fall times are always equal.
It is only by clamping the back EMF of the collapsing field to a much larger voltage than the supply that you can reduce the falltime.