From The Crop Circle Ship Wiki
A cavity magnetron is a high-powered vacuum tube that generates coherent microwaves. They are commonly found in microwave ovens, as well as being used in various radar applications.
 In the crop circle ship
A Magnetron or Klystron and the associated Hydrogen Container sit at the top of the Large Tetrahedron Structure. The device sends Microwaves down through the Inner Core of the Large Tetrahedron Structure. (Note: This is only an extremely rough representation.)
Below, the Hydrogen Container may also be marked as HC and the Magnetron may be marked as M.
Note that the Hydrogen Container is often shown as cone shaped.
 Also see