  In order to meet requirements of the synchrotron radiation users, a fully
coherent VUV free electron laser (FEL) has been preliminarily designed. One
important goal of this design is that the radiation wavelength can be easily
tuned in a broad range (70-170 nm). In the light of the users' demand and our
actual conditions, the self-seeding scheme is adopted for this proposal.
Firstly, we attempt fixing the electron energy and only changing the undulator
gap to varying the radiation wavelength, but the analysis implies that it is
difficult because of the great difference of the power gain length and FEL
efficiency at different wavelength. Therefore, dividing the wavelength range
into three subareas is considered. In each subarea, a constant electron energy
is used and the wavelength tuning is realized only by adjusting the undulator
gap. The simulation results shows that this scheme has an acceptable
performance.
