Magnetically shielded electron guns with a center magnetic post

The principles of operation and design are described for magnetically shielded guns with a center magnetic post through the cathode. The CMP gun is a relatively simple and highly versatile new type of gun for gyrotrons and other transverse wave tubes. Essentially an annular beam is generated in a magnetically shielded region and passes through a short region of radial magnetic field into the uniform field interaction region. Magnetic flux is transmitted through the cathode by means of a soft iron post on the axis. A small current coil may be mounted over the center magnetic post to adjust the enclosed flux. The beam transverse energy may thus be conveniently adjusted from zero to the case where the entire beam rotates about the axis at the cyclotron frequency. In the latter case it is shown that the constant initial flux provided by the CMP geometry meets a necessary condition to provide constant axial velocity for balanced flow at high beam perveance.