Emittance growth of a short electron bunch in circular motion

A short electron bunch undergoing circular motion produces space-charge forces that do not decrease with increasing bunch energy, unlike those induced by straight-line motion. These energy-independent forces can be separated into a noninertial space-charge force and a coherent synchrotron radiation force. These forces result in an energy spread in the bunch, and can lead to a potentially large emittance growth. These effects can take place in both (1) bunch compression systems used to increase the peak current and (2) the wiggler itself. Numerical estimates of the emittance growth in a wiggler for a 1 ps long, 1 mm radius, 1 nC electron bunch can be as large as 0.1π mm mrad per wiggle period; the energy spread can grow as much as 30 keV per wiggle period. These types of beam quality degradation may become significant for future, short-wavelength free-electron lasers requiring high-brightness electron beams, especially for self-amplified spontaneous emission operation.