Figure 4

PIC simulation results.

(a) Electron density distribution together with the laser pulse, where the black dashed elliptical circle marks the portion of electrons performing resonant betatron oscillations. (b) Trajectory of a test electron within the black dashed circle in the laboratory reference. (c) The same test electron in the reference frame of laser (, where is the location of the peak laser field). A, B and C denote the electron birth position (released from a nitrogen ion), injection position and resonant oscillation transition point. The injection, weakly resonant and strong resonant oscillation processes are shown in blue, red and black lines, respectively. (d), (e), (f) The corresponding plots for helium. The bubble size in Fig. 4(d) is a little larger than in Fig. 4(a), which is due to the stronger laser self-focusing and self-steepening effects for helium.