Simulations of CSR and LSC induced microbunching in the presence of a laser heater

We present a study of microbunching instability in the FACET-II linac, in which the amplification and damping mechanisms are analyzed separately. Our simulations investigate the gain induced by Longitudinal Space Charge (LSC) and, critically, the damping caused by nonlinear terms in the beam transport transfer map. We show theoretically and through simulation that these nonlinear effects can produce damping several orders of magnitude stronger than predicted by linear theory. Experimental evidence validates these findings. A quadrupole scan performed in the FACET-II dogleg reveals that an interaction between coherent betatron oscillations and transfer map nonlinearities shifts the point of minimal damping away from the expected linear condition of R_51 = 0 The strong agreement between our simulations and the experimental data demonstrate that a thorough understanding of nonlinear dynamics is essential for high-brightness beam transport.