Figure 1: Potential distribution (left) and electric field (right) in the collector. Both are simulated with CST PS. The value of the electric field corresponds to the size and color of the arrows. The direction of the arrows indicates the field direction.
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.
Figure 2: Beam Trajectory simulated with CST PS.
The secondary electron emission implemented in CST PS is a full probabilistic model according to Furman [1]. 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 material properties.
Figure 3: Beam trajectories for 4 different initial energies simulated with CST PS.
The depressed collector has been studied for four different initial beam energies. Figure 3 shows the results of the CST PS simulation which have been reproduced with permission of M. J. de Loos, S. B. van der Geer, Pulsar Physics (see also [2]). 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 and the next stage is used.
Figure 4: GPT results of the beam trajectories [2].
References:
[1]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.
[2]M. J. de Loos, S. B. van der Geer Pulsar Physics, Nuclear Instr. and Meth. in Phys. Res., Vol. 139
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