Phase-Selective Excitation of Betatron Oscillations by Nonadiabatic Magnetic-Field Switching
Nonadiabatic removal of an external transverse magnetic field provides a phase-selective mechanism for controlling betatron oscillations in laser wakefield accelerators. When the field is switched off on a timescale shorter than the betatron period, the equilibrium orbit shifts abruptly and acts as an impulsive transverse drive. The induced motion interferes coherently with the preexisting betatron oscillation, leading to phase-dependent enhancement or suppression of the oscillation amplitude. A theoretical model shows that the excitation is governed by the dimensionless switching parameter $\chi=\omega_\beta L_s/c$, which distinguishes nonadiabatic and adiabatic regimes. Particle-in-cell simulations confirm the predicted scaling and demonstrate controllable modulation of the betatron radiation spectrum while leaving longitudinal acceleration largely unaffected. These results establish magnetic-field switching as a direct mechanism for phase control of relativistic betatron oscillations in plasma-based accelerators.