Enhanced pointing and charge properties of electron beams from few-TW laser wakefield acceleration with an asymmetric density profile in a sub-millimeter nitrogen gas jet

This work demonstrates the feasibility of creating a sub-millimeter, subsonic nitrogen gas jet using a 178-μm diameter orifice to conduct laser wakefield acceleration (LWFA) with 1-TW, 40-fs laser pulses. More importantly, our findings reveal that using a blade to impede part of the gas flow and create an asymmetric density profile with a shortened down-ramp leads to a notable reduction in pointing fluctuations and an increase in the total charge of the output electron beams. As evidenced by the corresponding particle-in-cell simulation, the laser intensity is more effectively sustained toward the downstream end of the shaped gas jet, allowing for effective excitation of low-amplitude plasma waves that help preserve the accelerated electrons over the target rear side. In contrast, the pulse intensity drops significantly within the rear side of the unshaped gas jet, resulting in continuously diminishing plasma waves and decreased beam charge. The steeper gradient of the density down-ramp in the shaped gas jet also leads to a more rapid increase in the plasma wavelength over a reduced propagation distance, which helps mitigate the dephasing of accelerated electrons and increase the charge at the high-energy side of the spectrum. Our study paves the way for the future development of few-TW LWFA using a subsonic gas jet with sharp edges to further enhance the properties of output electron beams.