Zero-Compression, Laminar Electron Beams From Convex Cathodes in Uniform Magnetic Fields

The need for enhanced performance of high-power RF vacuum electron devices has led to the investigation of multiple-beam, sheet beam, and annular beam configurations. A key issue with such devices is the magnetic field shaping required producing high-power, laminar beams. Field shaping is difficult when Pierce-type gun geometries are employed. The development of high current density cathodes makes the necessary beam power achievable without compression. Such cathodes can operate within a uniform magnetic field yielding advantages for both single and distributed-beam RF devices. However, the quality of the resulting beams presents problems. A project to optimize beam quality in zero-convergence electron guns was undertaken by Calabazas Creek Research, Inc. (CCR) and North Carolina State University (NCSU). The surprising result was that high-quality electron beams can be generated in uniform magnetic fields using convex (dome)-shaped cathodes. The underlying physics involves perturbation of the beam cyclotron motion by a nonadiabatic radial electric field impulse. This article examines this physical mechanism and extends the initial result to additional diode and beam geometries.