Fourier limited micro-pulses in the OK-4/Duke storage ring FEL

The Super-modes are Fourier limited FEL micro-pulses predicted by Dattoli and Renieri in 1980. The OK-4 FEL at Duke, operating in the wavelength range from 193.7 nm to 730 nm, was the first to observe the Super-modes in a storage ring FEL in 1996. Since 1996, the up-graded diagnostics and improved control of the RF frequency allowed generation of Super-modes on a regular basis and systematic study of them. The Gaussian FEL micro-pulses with 1.3 ps RMS duration and Fourier limited RMS linewidth were generated in 1998–1999. In this paper we present the results of our studies and the comparison with theoretical predictions. We also present practical criteria for operating SR FELs with Super-modes.