An Optical Approach to Emittance Compensation in FELs

We present a new approach to compensating for the emittance in very short wavelength free electron lasers (FELs). The idea is based on the realization that the impact of finite emittance is to wash out the phase coherence of the electrons after passing some distance through the wiggler. This occurs because the electrons undergo betatron oscillations and those electrons with the largest transverse motion must travel a longer path. The new approach is to compensate for this by introducing an intense optical beam collinear with the electrons. If the beam has a transverse Gaussian profile in the field then the core electrons see on the average a higher field strength and undergo larger oscillations and can retard the core electrons sufficiently to allow them to stay in phase with the electrons with large betatron excursions. This paper presents the derivation of this effect, details of the physical interaction and simulation results for sample cases. Limitations as to the practicality of the approach are also discussed. >