Effect of Frequency Chirp and Pulse Length on Laser Wakefield Excitation in Under-Dense Plasma

Laser wakefield acceleration (LWFA) is one of the prominent energy efficient techniques to generate high potential gradient ( similar to GeV/m) to accelerate electrons. The phenomenon of LWFA depends on several parameters of plasma as well as laser beam like frequency of the laser pulse, the amplitude of the laser electric field, the shape of the laser pulse, density of plasma etc. These parameters can be adjusted to optimize the energy gain of electrons. In the present paper, the wakefield excitation by both positive and negative chirped laser pulse in an under-dense plasma is investigated. Effect of laser pulse length on LWFA is also investigated. For this purpose, the 1D model of laser wakefield acceleration is revisited to obtain analytical expressions for the longitudinal wakefield generated behind the laser pulse. Curves are plotted to compare and discuss the effect of pulse length and chirp (both positive and negative). The results show that the positive chirp enhances while the negative chirp diminishes the LWFA. Also, the wakefield generated by a laser pulse is maximal when the pulse length is close to the plasma wavelength. These results could be very useful as a starting point for optimization studies for a real LWFA experiment which is energy efficient.