The possibility of obtaining high-energy electron beams of high quality by using a low-density homogeneous plasma and a low-intensity laser (just above the self-injection threshold in the bubble regime) has been explored. Three-dimensional simulations are used to demonstrate, for the first time, an energy-spread of less than 1%, from self-trapping. More specifically, for a plasma density of 2×1018 cm−3 and a laser intensity of a0=2, a high-energy (0.55 GeV), ultrashort (1.4 fs) electron beam with very low energy-spread (0.55%) and high current (3 kA) is obtained. These parameters satisfy the requirements for drivers of short-wavelength free-electron lasers. It is also found that the quality of the electron beam depends strongly on the plasma length, which therefore needs to be optimized carefully to get the best performance in the experiments.

The propagation of an intense (a0=3), short-pulse (L∼λp) laser through a homogeneous plasma has been investigated. Using two-dimensional simulations for a0=3, the pulse-length and spot-size at three different plasma densities were optimized in order to get a better quality beam in laser wakefield accelerator. The study reveals that with increasing pulse-length the acceleration increases, but after a certain pulse-length (L>0.23λp) the emittance blows-up unacceptably. For spot-sizes less than that given by kp0rs=2a0, trapping is poor or nonexistent, and the optimal spot-size is larger. The deviation of the optimal spot-size from this formula increases as the density decreases. The efficacy of these two-dimensional simulations has been validated by running three-dimensional simulations at the highest density. It has been shown that good quality GeV-class beams can be obtained at plasma densities of ∼1018 cm−3. The quality of the beam can be substantially improved by selecting only the high-energy peak; in this fashion an energy-spread of better than 1% and a current in tens of kA can be achieved, which are important for applications such as free-electron lasers.

Nonlinear, large amplitude, plasma waves are excited in the wake of an intense laser pulse propagating in a cold plasma, providing acceleration gradients ~ GeV/m. Linear analytic theory has shown that the wakefield amplitude is optimal for a certain ratio of the pulse length and plasma wavelength [1,2]. Here we present simulation studies to optimize the wake amplitude. The wake amplitude of a Gaussian pulse profile is maximized with respect to the laser pulse length. Using two dimensional simulations, it is seen that for a Gaussian pulse profile, the optimal pulse length derived from linear theory is also close to optimal (higher charge, smaller energy spread) for the generation of a quasi-monoenergetic electron beam in the bubble regime of laser plasma interaction. We also show, by varying the spot-size but keeping the intensity constant, that the magnitude of the trapped charge is a function primarily of the ratio P/Pc.