Comparison of ion-channel guide and magnetic field focusing in a FEL

Nonlinear simulations show that compared to an axial guide magnetic field, the ion-channel guide field has the advantageous properties in confining the electron motion and stability, resulting in smaller velocity spread and configuration spread of the electrons. The intrinsic factor of this physical phenomenon is that the ion-channel guide field provides the electron motion a smaller Larmor radius and slighter guiding-center excursion from the wiggler axis than an axial magnetic field does. It is also found that over-focusing may occur if the ion-channel guide field is too strong.