Observations and simulations of transverse density waves in a collimated space-charge dominated electron beam

Experiments and particle-in-cell simulations demonstrate the appearance of wavelike transverse density variations in a space-charge dominated electron beam. Simulations show how an aperture located near the source gives rise to a nonequilibrium phase-space distribution with strong force imbalance confined to a sheath near the beam edge. Tracking of particles in this sheath, starting near the aperture's edge, reproduces well the onset of the perturbation. The subsequent evolution of the perturbation over about one meter suggests the appearance of a transverse wave. For the parameters investigated, simulations further indicate that the perturbation damps out over a few plasma periods without causing any rms emittance growth.