Simulation of the influence of secondary electrons on the performance of potential depressed collectors for TWTs

Significant differences have been found between measured and simulated data for collectors designed with codes which disregard the influence of secondary electrons. To solve this problem, various codes have been developed. These codes assume that secondary electrons are only emitted in a single direction and ignore their spatial distribution. We have developed a code in which secondary electron yield is calculated from an empirical formula and the emitted secondary electrons are assumed to have cosine spatial distribution. To represent this distribution, for each primary beam point of impact, the emitted secondary beam is split into 3 beams; one beam normal to the impact surface and the other two at an angle to the normal. The normal beam carries half of the current and the rest of the current is divided equally between the side beams. The code uses a two dimensional, axis symmetric Finite Element Method (FEM) for modeling. FEM has been chosen because of its ability to model accurately arbitrary shaped structures. We designed a ku-band (14.25 GHz), 2-stage TWT by using the developed code.