Diagnostics for plasma-based electron accelerators

Plasma-based accelerators that impart energy gain as high as several GeV to electrons or positrons within a few centimeters have engendered a new class of diagnostic techniques very different from those used in connection with conventional radio-frequency (rf) accelerators. The need for new diagnostics stems from the micrometer scale and transient, dynamic structure of plasma accelerators, which contrasts with the meter scale and static structure of conventional accelerators. Because of this micrometer source size, plasma-accelerated electron hunches can emerge with smaller normalized transverse emittance (epsilon(n) < 0.1 mm mrad) and shorter duration (tau(b) similar to 1 fs) than bunches from rf linacs. Single-shot diagnostics are reviewed that determine such small epsilon(n) and tau(b) noninvasively and with high resolution from wide-bandwidth spectral measurement of electromagnetic radiation the electrons emit: epsilon(n) from x rays emitted as electrons interact with transverse internal fields of the plasma accelerator or with external optical fields or undulators; tau(b) from THz to optical coherent transition radiation emitted upon traversing interfaces. The duration of similar to 1 fs bunches can also be measured by sampling individual cycles of a copropagating optical pulse or by measuring the associated magnetic field using a transverse probe pulse. Because of their luminal velocity and micrometer size, the evolving structure of plasma accelerators, the key determinant of accelerator performance, is exceptionally challenging to visualize in the laboratory. Here a new generation of laboratory diagnostics is reviewed that yield snapshots, or even movies, of laser- and particle-beam-generated plasma accelerator structures based on their phase modulation or deflection of femtosecond electromagnetic or electron probe pulses. Spatiotemporal resolution limits of these imaging techniques are discussed, along with insight into plasma-based acceleration physics that has emerged from analyzing the images and comparing them to simulated plasma structures.