Microbunch instability in wigglers

We studied the microbunch instability in wigglers induced by coherent synchrotron radiation (CSR) theoretically and numerically for the first time. This instability occurs only at very small energy spread, and reaches maximum when the energy of electrons and the peak value of the magnet field adopt specific values. Results show that the instability may slightly exist in the wiggler of Beijing Free Electron Laser (BFEL), but does not happen at all in the wigglers of the proposed China Test Facility (CTF) for the X-ray free electron laser.