Ion channel focusing in FEL wigglers

Accumulation of ions inside FEL (free electron laser) wigglers can significantly alter electron beam transport and lead to a reduction of laser gain. This is of particular importance to devices driven by RF accelerators with continuous operation and with long wigglers. It is shown that while the mechanism for ion channel formation is the same as in electron beam storage rings, the problem appears less severe because the electron beam makes only a single pass through the system. In addition, the electric field of the beam together with the magnetic field of the wiggler causes an E*B drift, which can provide an effective mechanism for ion removal.<>