Design and Experimental Study of Double Periodic Permanent Magnetic Focusing System

In order to improve the periodic permanent magnetic (PPM) focusing the performance of traveling wave tubes (TWTs) for the higher current, the analysis and experimental study of the double-PPM focusing system were carried out in this article. The influence of the ratio of third harmonic to the first harmonic on the electron beam pulsation of the double-PPM focusing system was analyzed theoretically. A split-type double-PPM focusing structure is proposed to realize multiparameter harmonic regulation. A beamstick with the microbeam tunnel’s radius of 0.24 mm was designed and fabricated. The beam transmission of 98% was measured with the emission current of 289 mA corresponding the maximum beam current density of 626 A/cm2 at the voltage of 21 kV. The transmission distance is 104 mm from the anode entrance to the collector. Such a distance can cover the interaction circuit length of the almost millimeter and terahertz TWTs. The design and experimental study verify the split-type double-PPM focusing system, laying the research foundation for power enhancement of TWTs.