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 50 MeV beam was propagated through a 1–3 cm long plasma with density around 10¹⁴cm^-3. Beam size was measured as a function of propagation distance. For a ratio of collisionless plasma skin depth to beam spot size k_pσ_r = 0.33, no significant reduction in focusing was observed. Reduced focusing was measured for k_pσ_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.