The acceleration of electrons injected in a plasma wave generated by the laser wake eld mechanism has been observed. A maximum energy gain of 1.6 MeV has been measured and the maximum longitudinal electric eld is estimated to 1.5 GV/m. The experimental data agree with theoretical predictions when 3D e ects are taken into account. The duration of the plasma wave inferred from the number of accelerated electrons is of the order of 1 ps. 41.75. Lx,52.40.Nk Typeset using REVT E X 1

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