New results and prospects for harmonic generation in storage ring FELs

In this paper we review new results and prospects for harmonic generation in storage ring (SR) FELs. Traditionally, high harmonics were generated in SR FELs using conventional (IR or visible) pulsed lasers as a driver. Discovery of super-pulses with GW levels of intra-cavity peak power provided for new ways of generating coherent harmonics in the VUV and X-ray ranges. Super-pulses are generated in the OK-4/Duke SR FEL using gain modulation technique. As we reported at the previous FEL conference, the intra-cavity power on the OK-4 FEL was sufficient for effective generation of third and higher harmonics in the fundamental FEL wavelength. Recently, the OK-4/Duke SR FEL generated second, third, fourth, fifth and seventh coherent harmonics in the range from 37 to 135 nm. The tunability of the FEL wavelength provided for natural wavelength tunability of the harmonic radiation. Recent experimental results of the UV and VUV harmonic generation in the OK-4/Duke SR FEL demonstrated peak spectral brightness comparable with VUV SASE FELs. We discuss the prospects of the intra-cavity short-wavelength harmonic generation in SR FEL.