These plots show results from simulations of the APS thermionic rf electron gun, performed using the fully-electromagnetic particle-in-cell code 'spiffe'. The colored dots represent simulation particles, each of which simulates the contribution of a large number of electrons; the different colors show bands of particles emitted over a 20 picosecond interval from the cathode on the left-hand wall.
The arrows represent the electric field induce by the beam, while the crosses represent the magnetic field. One sees how the field travels with the beam, and how it is stronger at the head of the beam were the particles are bunched together. One can also see how the fields are slightly changed by beam-induced surface currents as the beam approaches the exit tube at the right-hand side.