The schematic drawing shows a shunt reactor with magnetic core whose central leg is built from several radially laminated cores. Such discs are assembled from many laminations of varying lengths, positioned radially (see also the image below).
The air gaps in the central leg are distributed uniformly to reduce the flux fringing.
The drawing was adapted from:
See also: 
shunt_reactor_with_and_without_winding_-_magnetica.png
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S. Zurek, Encyclopedia Magnetica, CC-BY-4.0
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