Design and use of Small Rogowski Coils for use with Large, Fast Current Pulses

Rogowski coils are commonly used in pulsed power applications to make an indirect measurement of very large currents. Problems arise with this technique when coil dimensions are forced to be small. Ideally, a Rogowski coil has a very large number of turns, making it less sensitive to current location, noise, and winding variations. For a small coil this can cause two problems: 1.) a large number of turns yields a large output voltage, and 2.) the spacing between turns becomes very small, making shorting between turns a problem. One way to combat both of these problems is to minimize the voltage induced in each turn of the coil. This can easily be done by increasing the diameter of the coil, placing the coil's turns in regions of lower dB/dt. However, when a coil's outer dimensions are fixed, the only way to decrease output voltage is by decreasing the area of each turn. Since Rogowski coils are typically constructed using standard sizes of 50 Ohm cable it would seem that there is a lower limit to how small the area of each turn can be. However, by including a "counter-turn" with each turn on a coil the effective area of each turn can be made as small as desired. Tests were performed using coils having diameters from 2.0-2.5 cm and 5-10 turns. Current pulses ranged from 20 kA to 250 kA with rise times of 250 ns and 100 ns respectively.