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A comprehensive experimental study of focusing of relativistic electron beams with overdense and underdense plasma lenses is being conducted at the Beam Test Facility at LBNL. Short (15ps rms) electron bunches, from the 50 MeV LBNL Advanced Light Source injector are transported through laser produced plasmas. The electron beam spot size and divergence at the plasma lens is adjusted using quadrupoles. The plasmas are 1-5 cm long with densities of 10/sup 13/-10/sup 14/ cm/sup -3/. By changing the laser intensity and shape, the plasma density and profile can be controlled. This allows for exploration of both the charge and current compensation regimes, by changing the ratio of the plasma wavenumber, k/sub p/, to the electron beam size, /spl sigma//sub r/. Experimental results on the production of plasma through two-photon UV ionization and electron beam diagnostics have been presented earlier. Here we present results from experimentally observed plasma focusing for overdense lenses in charge and current compensation regimes. Detailed interferometric results from the production of highly overdense plasmas are also discussed.

First time, self-consistent particle-in-cell simulations of electron injection into a plasma wake field by colliding laser pulses are presented. This method for injecting electrons in a laser wake field has been proposed recently by Esarey et al. (1997). An intense pump pulse generates a fast wake field. Forward going and backward going injection pulses collide at some distance behind the pump pulse generating a slow ponderomotive beat wave which can be used to inject plasma electrons into the fast wake field for acceleration to high energies. We have modified XOOPIC to run numerical simulations of the colliding pulses scheme. We have analyzed the influence of plasma and pulse parameters, so that we are able to study electron injection as it varies with amplitude, wavelength, and phases of the pulses. We will present preliminary results.