Diagnostics for plasma-based electron accelerators

Plasma-based electron accelerators rely on excitation of light-speed plasma density waves and associated ultrahigh electric fields to accelerate electrons to ultrarelativistic energy. Intrinsic time and length scales of these plasma waves are typically a few tens of femtoseconds and micrometers and result in ultrashort electron bunches with often even smaller temporal and spatial dimensions. In this article the progress and challenges of diagnosing the plasma waves, the excited fields, and ensuing particle bunches are reviewed.