Answer
We would need to change the electric potential of the two semi-infinite end cylinders.
Work Step by Step
If the electric potential of the two semi-infinite end cylinders is $-\infty$, then the positron would be attracted to the end cylinders, since the positron has a positive charge.
To trap the positron in the central cylinder, the electric potential of the two semi-infinite end cylinders should be $~~+\infty$
Therefore, we would need to change the electric potential of the two semi-infinite end cylinders.
