Observation of Return Current Effects in a Passive Plasma Lens

Observations of relativistic beam focusing by a passive plasma lens have demonstrated a reduction in focusing strength due to plasma return current. A 50MeV beam was propagated through a 1 - 3cm long plasma with density around 1014 cm-3 . Beam size was measured as a function of propagation distance. For a ratio of collisionless plasma skin depth to beam spot size kpσr=0.33 , no significant reduction in focusing was observed. Reduced focusing was measured for kpσr=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. copyright 1999 The American Physical Society