Efficiency and energy spread in laser-wakefield acceleration.

The theoretical limits on efficiency and energy spread of the laser-wakefield accelerator are investigated using a one-dimensional model. Modifications, both of the wakefield due to the electron bunch, and of the laser pulse shape due to the varying permittivity of the plasma, are described self-consistently. It is found that a short laser pulse gives a higher efficiency than a long laser pulse with the same initial energy. Energy spread can be minimized by optimizing bunch length and bunch charge such that the variation of the accelerating force along the length of the bunch is minimized. An inherent trade-off between energy spread and efficiency exists.

Bunch self-focusing regime of laser wakefield acceleration with reduced emittance growth.

A new regime of laser wakefield acceleration of an injected electron bunch is described. In this regime, the bunch charge is so high that the bunch wakefields play an important role in the bunch dynamics. In particular, the transverse bunch wakefield induces a strong self-focusing that suppresses the transverse emittance growth arising from misalignment errors. The decelerating longitudinal bunch wakefield, however, is not so strong that it completely cancels the accelerating laser wakefield. In fact, the induced energy spread can be compensated by exploiting phase slippage effects. These features make the new regime interesting for high beam quality laser wakefield acceleration.

Simulation of laser wakefield acceleration of an ultrashort electron bunch.

The dynamics of the acceleration of a short electron bunch in a strong plasma wave excited by a laser pulse in a plasma channel is studied both analytically and numerically in slab geometry. In our simulations, a fully nonlinear, relativistic hydrodynamic description for the plasma wave is combined with particle-in-cell methods for the description of the bunch. Collective self-interactions within the bunch are fully taken into account. The existence of adiabatic invariants of motion is shown to have important implications for the final beam quality. Similar to the one-dimensional case, the natural evolution of the bunch is shown to lead, under proper initial conditions, to a minimum in the relative energy spread.