Wakefields in a Cluster Plasma*

We report the first study of large amplitude Langmuir waves in a plasma of nanometer-scale clusters. The shape of these wakefields is captured by an oblique-angle frequency-domain holography diagnostic for the first time. The wavefronts are observed to curve backwards, in contrast to the forwards curvature of wakefields in uniform plasma. The first wakefield period is longer than those trailing it. Next, the experimental data is compared to fully relativistic 2D particle-in-cell simulations and a new injection mechanism is found. Our simulations indicate that this mechanism leads to a greater maximum momentum and energy spread compared to electrons trapped in the uniform case.