Electron beam probing: essentials for the direct measurements of plasma wakefields *

Direct experimental characterization of plasma wakefields is highly valuable for understanding the wake evolution in various regimes as well as the injection mechanisms. While optical methods of wakefield probing have been extensively studied and have successfully demonstrated the ability to capture shadowgraphs of the plasma wakes, such methods directly rely on the change of the index of refraction and are not applicable for diagnostic of low-density plasmas. Instead, an ultrashort relativistic electron beam can be used to visualize the plasma wakefields: when passing through a wake, an electron beam density is modulated by the electric fields of the wake. Once imaged, this density modulation provides an insightful snapshot of the wake structure allowing for the direct characterization of the wakefield. We present the essential considerations necessary to implement the direct wakefield visualization technique using an electron beam as a probe. The discussion is focused on the electron probing experiment implemented at the Accelerator Test Facility (ATF) of Brookhaven National Laboratory (BNL).