Enhanced Harmonic Spontaneous Radiation using a Novel Undulator

摘要

In a typical free electron laser (FEL), the electron beam interacts with a dipole211u001eundulator that has a sinusoidal magnetic field variation; the electron motion is 211u001esinusoidal in the plane transverse to this field and emits odd-numbered harmonic 211u001eradiations along the axis. However, one need not limit the choice of undulator 211u001efield profile to the sinusoid, providing other profiles result in significant 211u001eadvantages. In connection with the IFEL accelerator, in the past we have pointed 211u001eout that the use of an undulator profile that approximates a square wave will 211u001eresult in an enhanced acceleration gradient, by as much as a factor of two 211u001e(equivalent in effect to an increase of laser drive intensity by a factor of 211u001efour). This improvement (essentially at the fundamental FEL resonance) results 211u001elargely from the fact that, for a given peak undulator field amplitude, the rms 211u001eelectron acceleration obtained from the square wave undulator is larger than that 211u001efrom the sinusoid; furthermore, the electron orbit is stable as well. In this 211u001epaper, we find additional advantages that should result particularly at harmonic 211u001enumbers f 1 if the undulator field profile is nearly 'square wave' namely, a 211u001elarge enhancement of the harmonic spontaneous power radiated, together with 211u001eenhanced FEL gain. The modification of undulatory to enhance FEL gain has been 211u001eexamined in the past, usually with a particular design in mind, but with similar 211u001econclusions.