Figure 1: The positron trapping mechanism (from [30]). The horizontal axis is the position along the trap axis. The left vertical axis is the voltage seen by particles. The magnetic field strength is shown by the green curve, with the scale on the right vertical axis. In blue, the electron potential well filled with electrons is drawn, reducing the apparent voltage shown by black curve with the value on the left vertical axis. The positron potential is shown well in red. When a positron bunch arrives, the entrance electrode voltage is low (dashed dotted line). It is then increased and positrons go back and forth (it is depicted by red arrows) between this gate and the downstream part of the trap. They pass many times through the electron plasma and are eventually slowed down and fall into their well. The presence of residual helps the final catching of positrons. In order to reach the desired number of trapped positrons, the positron well has to be enlarged and deepened during the accumulation process from around 200 V to around 1 kV. Positrons may also be trapped in the potential well formed between the entrance electrode and the electron well, but its depth remains constant and the induced loss is small.