Experimental demonstration of dynamic focusing of a relativistic electron bunch by an overdense plasma lens

Dynamic focusing of a 3.8 MeV electron bunch, a few collisionless skin depths long ∼3c/${\mathrm{\omega }}_{\mathit{p}}$, by an overdense, thick plasma lens has been demonstrated. Because of electron inertial effects, the head of the bunch is virtually unaffected by the lens while the rest is focused to varying degree. Time-resolved measurements performed 31 cm downstream of the plasma lens show that, in time, the bunch pinches from an initial size of 2.7 mm (FWHM) to about 0.57 mm and then expands, in reasonable agreement with theory.