I modify the code so that the velocity can be adjust to 20km/s, however, you better to change the time step to smaller value.
The unit for change has been change to e (1.6*10^{-19})$
The program will change the scale if the particle come out right/left.
The bounce off the wall (above/below) kept the same.
Click the eye (image) to display the simulation
[eye]