Predictions and expected performance for the VUV OK-5/Duke storage ring FEL with variable polarization

The OK-5 FEL is the first distributed optical klystron (DOK) comprised of four electromagnetic wigglers and three bunchers. In this paper we focus on the expected performance of the OK-5/Duke storage ring FEL which will operate in the UV and the VUV ranges of spectrum and will generate intense beam of Compton backscattered γ-rays. We present the calculation of the main parameters of the OK-5 VUV FEL such as its tuning range, gain and lasing power. Based on these calculations, we present the predictions for the OK-5 Compton γ-ray source. We extend our prediction to the OK-5/Duke storage ring FEL with the low-emittance lattice and low impedance vacuum chambers. We also discuss briefly the mode in which one of the wigglers is tuned on a harmonic of the OK-5 FEL to provide optimal conditions for harmonic generation in VUV and soft-X-ray ranges.