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Direct observation of plasma-lens effect.

A plasma lens for 18-MeV round-shaped electron beams was experimentally studied; the lens is based on self-focusing due to shielding of the space charge of a particle beam by a quiescent plasma. A differential pumping technique was applied to separate the plasma chamber from the linac acceleration duct. It is shown that overdense plasmas successfully focus the electron beams. Another finding is that the plasmas appear to reduce the transverse emittance.