Emittance oscillations in a mismatched electron beam

An electron beam is mismatched when the radius at injection differs from the equilibrium radius during propagation. This results in oscillations in the beam envelope such that beam radius (r) depends on propagation distance (z). The authors produced the mismatched beam condition in the laboratory by injection of a relativistic electron beam (0.7 MeV, 4 kA, 100 ns) into a grounded conducting tube (1 m long x 10 cm diameter) which contains air at 320 milliTorr. Beam electrons ionize a channel in the background gas and expel plasma electrons from the channel. Plasma ions that remain in the channel neutralize the beam space charge permitting beam focusing. This is known as propagation in the ion-focused regime