Figure 1. Conceptual illustration.

A plasma wake propagating along the z direction can be probed by an electron bunch propagating along the x direction. After traversing through the wakefield, the probe will form a density pattern on a screen placed in the yz plane at an appropriate distance, due to the momentum modulation caused by the wakefield. The wakefield structure can consequently be reconstructed by analyzing the density modulation. The unit c/ω₀ equals to 0.127 μm, where c is the light speed in vacuum and ω₀ is the frequency of the 800 nm laser.