Harnessing the electron bunch in the wake of the laser
Fig.1 Snap shot of transversely chirped laser superimposed over the plasma electron density profile at various time to illustrate the off-axis injection of particles in the bubble. Plasma has a linear transverse density gradient.
The potential of the blowout regime for several applications has been confirmed in numerous experiments. The large accelerating fields associated with the blowout, which can trap and accelerate plasma electrons (self-injection), lead to the generation of quasi-mono-energetic multi-GeV electrons. In addition, the linear transverse focusing forces associated with the bubble are ideal for the generation of x-ray radiation, as the accelerated electron beams perform betatron oscillations in the ion channel. The beams obtained from LWFA also have potential to drive a free electron laser. For these applications it is crucial to control and manipulate the injection process that determines the charge, energy, energy spread and strength parameter for x-ray radiation. Although several techniques have been proposed for this purpose, including the use of short plasma down-ramps, the use of transverse external magnetic fields, and through the beating structures associated with counter or cross-propagating lasers, through ionization mechanisms, and longitudinal frequency chirped laser, here we present our results from the multi-dimensional PIC simulations, performed using OSIRIS, to show the possibility of using transverse chirped laser to control self-injection.
A transversely frequency chirped laser pulse, due to group velocity dispersion, develops a pulse front tilt (PFT), and the laser pulse is deflected away from the initial laser axis. This can be attributed to the transverse plasma-density gradient caused by the tilted pulse front, which corresponds to a varying refractive index perpendicular to the laser-propagation direction. This can result into optical steering of the trapped electrons in the bubble. Similar drifts in the laser centroids can be caused by pre-existing transverse plasma inhomogeneities. By properly choosing the chirp coefficient and density gradient, we can control/nullify the drift in the laser. However, due to the PFT, the nonlinear wake (bubble) remains asymmetric which leads to off-axis injection.