Observations of relativistic beam focusing by a passive plasma lens have demonstrated a reduction in focusing strength due to plasma return current. A 50 MeV beam was propagated through a 1-3 cm long plasma with density around 10 14 cm 23. Beam size was measured as a function of propagation distance. For a ratio of collisionless plasma skin depth to beam spot size k p s r 0.33, no significant reduction in focusing was observed. Reduced focusing was measured for k p s r 1.1, where a significant fraction of the inductively driven return current in the plasma flows within the beam. The observations are in good agreement with an envelope equation model and with particle-in-cell simulations.

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Theoretical studies are made of radio frequency structures which can be used to condition electron beams so as to greatly reduce the stringent emittance requirements for successful lasing in a free-electron laser. The basic strategy of conditioning calls for modulating an electron beam in the transverse dimension, by a periodic focusing channel, while it traverses a series of rf cavities, each operating in a TM210 mode. In this paper, we analyze the cavities both analytically and numerically (using MAFIA simulations). We find that when cylindrical symmetry is broken the coupling impedance can be greatly enhanced. We present results showing various performance characteristics as a function of cavity parameters, as well as possible designs for conditioning cavities.