Effects of nonideal conditions on resonantly enhanced betatron radiation in a plasma undulator.

Laser wakefield acceleration (LWFA) provides a compact platform for generating bright, ultrashort, and tunable x-ray betatron radiation. When the betatron oscillation wavelength matches the laser centroid oscillation period in a preformed plasma channel, resonance occurs, leading to a rapid amplification of the electron betatron oscillation amplitude and significantly increasing the photon energy and yield of the sources. However, nonideal conditions from laser-plasma interactions may drive electrons out of resonance rapidly. In this work we use particle-in-cell simulations and theoretical analysis to identify four distinct mechanisms that disrupt the resonant growth: the nonlinear focusing force at large radii, the dephasing between the wakefield and the electron beam, the acceleration or deceleration of the electrons, and the decay of the laser centroid oscillation amplitude caused by phase mixing among different laser slices. Detailed particle tracking reveals that a delicate balance between dephasing and acceleration can sustain electrons in resonance for an extended period. Building upon these physical insights, we optimize laser, electron beam, and plasma channels to significantly enhance the emitted radiation. These results provide valuable guidance for developing high-yield and high-energy betatron radiation sources based on LWFA.