The goal of the coherent harmonic generation free-electron laser (CHG-FEL) experiment in NSRL is to gain a 266nm coherent radiation and a large spectral ratio which is defined as the ratio of coherent intensity to incoherent intensity in infinitesimal bandwidth and solid angle aperture. The intensity measurements are made through a spectrometer whose spectral and angular aperture is much larger than the actual apertures of coherent radiation and smaller than those of incoherent radiation. So the measured ratio is integral ratio integrated over the actual apertures of the measurement system. This paper is mainly on giving a formula and designing a computer program to calculate the spectral ratio according to the bandwidth and solid angle aperture of the measurement system, taking into account the measured magnetic field of optical klystron and the energy spread of the electron beam. The code will soon be employed in our next turn experiment.

Undulator field errors influence the electron beam trajectories and lower the radiation quality. Angular deflection of electron beam is determined by first field integral, orbital displacement of electron beam is determined by second field integral and radiation quality can be evaluated by rms field error or phase error. Appropriate ordering of magnets can greatly reduce the errors. We apply a modified simulated annealing algorithm to this multi-objective optimization problem, taking first field integral, second field integral and rms field error as objective functions. Undulator with small field errors can be designed by this method within a reasonable calculation time even for the case of hundreds of magnets (first field integral reduced to 10^-6T·m, second integral to 10^-6T·m² and rms field error to 0.01%). Thus, the field correction after assembling of undulator will be greatly simplified. This paper gives the optimizing process in detail and puts forward a new method to quickly calculate the rms field error and field integrals.

The design study of the undu1ator for Shanghai deep ultra violate free electron laser source (SDUV-FEL) is presented. The optimum undulator parameters for the FEL performance have been studied. The scheme of focusing and segmentation is discussed. The requirements of undulator magnet field and main technical demand are given.