The depressed collector is part of the beam energy recovery system of the Fusion Free Electron Maser (FEM). It works by collecting the beam and ensuring the suppression of back-streaming secondary electrons. This means that the collector has to be designed in such a way that secondary electrons are accelerated towards the back of the electrodes. To investigate this effect, a particle tracking simulation was perfomed in CST PARTICLE STUDIO® (CST PS). Figure 1 shows the simulated potential and field distribution inside the collector.
As can be seen from the trajectory plot simulated with CST PS (see Figure 2), the incoming electron beam is decelerated by the electric field. Due to the design of the collector, the trajectory of the beam ends on the rear side of the electrodes. The primary particles hitting the electrodes generate secondary electrons. However, these secondary electrons fall back to their origin and do not stream back....
The secondary electron emission as implemented in CST PS is a fully probabilistic effect using the Furman model . The self-consistent model incorporates the emission of secondary electrons depending on the kinetic energy and incident angle of the primary electrons. It can be easily included in the simulation by editing the properties of the emissive material.
The depressed collector has been studied for four different initial beam energies. Figure 3 shows the results of the CST PS simulations, reproduced with permission of M. J. de Loos, S. B. van der Geer, Pulsar Physics (see also ). The results agree very well with the GPT results shown in Figure 4. The graphics illustrate the functionality of the collector very clearly. The higher the initial energy, the deeper penetrates the beam into the collector, with later stages used to capture higher energy electrons.
 M. A. Furman and M. T. F. Pivi, "Probabilistic model for the simulation of secondary electron emission", Physical Review Special Topics, Accelerators and Beams, Volume 5, 2002.
 M. J. de Loos, S. B. van der Geer Pulsar Physics, Nuclear Instr. and Meth. in Phys. Res., Vol. 139, 1997.