Abstract
The dynamics of laser ionization-based electron injection in the recently introduced plasma photocathode concept is analyzed analytically and with particle-in-cell simulations. The influence of the initial few-cycle laser pulse that liberates electrons through background gas ionization in a plasma wakefield accelerator on the final electron phase space is described through the use of Ammosov-Deloine-Krainov theory as well as nonadiabatic Yudin-Ivanov (YI) ionization theory and subsequent downstream dynamics in the combined laser and plasma wave fields. The photoelectrons are tracked by solving their relativistic equations of motion. They experience the analytically described transient laser field and the simulation-derived plasma wakefields. It is shown that the minimum normalized emittance of fs-scale electron bunches released in mulit-$\mathrm{GV}/\mathrm{m}$-scale plasma wakefields is of the order of ${10}^{\ensuremath{-}2}\text{ }\text{ }\mathrm{mm}\text{ }\mathrm{mrad}$. Such unprecedented values, combined with the dramatically increased controllability of electron bunch production, pave the way for highly compact yet ultrahigh quality plasma-based electron accelerators and light source applications.
Highlights
Acceleration of charged particles in plasma is an extremely promising, emerging new method, as it can exploit accelerating and focusing electric fields that can straightforwardly reach tens of GV=m or more [1], permitting extremely compact accelerators along with high current, short pulse, compact phase space or high brightness beams
We have investigated the fundamentals of phase space density diluting effects due to the dynamics induced by the ionization laser in an underdense plasma photocathode
This analysis has overcome the need to include information on many different time scales through a hybrid numerical analysis, where the equations of motion of the injected electrons are obtained using the analytical fields of a bi-Gaussion laser pulse, and the plasma wakefields are taken from the results of VORPAL PIC simulations
Summary
Acceleration of charged particles in plasma is an extremely promising, emerging new method, as it can exploit accelerating and focusing electric fields that can straightforwardly reach tens of GV=m or more [1], permitting extremely compact accelerators along with high current, short pulse, compact phase space or high brightness beams. With sufficiently intense driving beams, the plasma electron displacement induced may result in completely electron-rarefied plasma blowout cavities that trail the driver pulse through the plasma, and support the desired large-amplitude plasma wakefields, having peak electric fields E near ‘‘wave breaking,’’ E $ EWB 1⁄4 kpmec2=e, where mec is the electron rest energy and e is the electron charge [7] This scenario stands in contrast to conventional metallic cavities, which are stationary in the laboratory frame, and a long array of such cavities are needed to accelerate to high energies. These electrons are caught and form a tiny, ultrahigh quality bunch that is copropagating with the plasma wave at the end of the blowout, profiting from maximized energy gain This scheme has been shown in simulations to yield beams with high current (hundreds of amperes) and unprecedentedly low emittance; these are high brightness beams with very strong promise as new-generation XFEL drivers.
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