Optimization of the beam quality in ionization injection by a tailoring gas profile* Project supported by the National Natural Science Foundation of China (Grant Nos. 12005297, 11975308, and 11775305), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDA25050200), the Fund of Science Challenge Project (Grant No. TZ2018001), Natural Science Foundation of Hunan Province, China (Grant No. 2020JJ5651), and the Fund of the State Key Laboratory of Laser Interaction with Matter (Grant No. SKLLIM1908)

A new scheme is proposed to improve the electron beam quality of ionization-induced injection by tailoring gas profile in laser wakefield acceleration. Two-dimensional particle-in-cell simulations show that the ionization-induced injection mainly occurs in high-density stage and automatically truncates in low-density stage due to the decrease of the wakefield potential difference. The beam loading can be compensated by the elongated beam resulting from the density transition stage. The beam quality can be improved by shorter injection distance and beam loading effect. A quasi-monoenergetic electron beam with a central energy of 258 MeV and an energy spread of 5.1% is obtained under certain laserâ€"plasma conditions. [ABSTRACT FROM AUTHOR]