Transverse-Field Traveling-Wave Tubes with Periodic Electrostatic Focusing

Traveling-wave tubes are described in which the wave-propagating structure constitutes a smooth, balanced transmission line for radio frequency signals while at the same time providing a space-periodic focusing field at zero frequency. Experimental structures for the 500-900 mc band have wave velocities of 1 to 2 per cent of light and characteristic impedances of about 500 ohms. A simplified theoretical analysis is given, based on the readily visualized concept of transverse electron waves in a focusing field. In order to get gain, the electron stream must travel faster than the circuit wave by a substantial margin which is determined by the strength of the focusing field. The gain is proportional to the square root of the beam current. Experimental data are presented concerning the relation between focusing field and effective beam velocity, and between beam current and gain. They are in good agreement with the theory. Noise figures as low as 6 db have been measured.