RF beam spread in PPM confined-flow TWTs

PPM focusing of TWTs (both helix and coupled-cavity types) is widely and advantageously practiced, but heretofore little use was made of the fact that the beam becomes stiffer when magnetic flux is permitted to thread the cathode. This increased stiffness may alleviate beam control problems such as excessive beam spread under rf interaction or beam instabilities due to ion effects. While most very high-power linear-beam devices, which are solenoid focused, rely on confined-flow focusing (space-charge balanced flow with flux threading the cathode), few PPM-focused devices have attempted to exploit the confined-flow principle. This paper analyzes the benefits and liabilities of confined-flow PPM focusing. Computer simulation data are presented to show how rf interaction causes the beam to defocus and how the system parameters Brms/Bb, λp/L, and perveance affect the control of beam focusing under rf drive.