V.N. Litvinenko (a), S.H. Park (b), I.V. Pinayev (a), Y. Wu (c)
FEL Laboratory, Department of Physics, Duke University, Durham, NC 27708, USA
For number of years the wavelength of 200 nm was a psychological barrier for FEL oscillators. The progress towards shorter wavelength was marginal since the OK-4/VEPP-3 storage ring FEL lased at 240 nm in October 1988. The lack of robust highly reflective mirrors and low FEL gain were premier showstoppers. It took about nine years and new helical optical klystron at UVSOR to move the 240 nm limit down. The recent advances in increasing FEL gain at the NIJI-IV and the OK-4/Duke storage ring FELs allowed to push this limit slowly to 226 nm, 217 nm and to 212 nm in 1998. In August and October of 1999 the OK-4 FEL lased in 193.7 - 209.8 nm range using electron beam with energies of 500-800 MeV. It was the first FEL lased below 200 nm. Previous attempts to lase with the OK-4 FEL at 193 nm failed because the fast mirrors degradation.
Improvements of the OK-4 FEL gain above 10-15% per pass and the use of custom made composite dielectric mirrors (by Limonics Optics Group, Ontario, Canada) made this lasing possible. The measured average out-coupled power was 5 mW with 16 mA of electron beam current. In July 1999, we also operated the OK-4 FEL in giant pulse in near UV range and observed first indication of third coherent harmonic generation around 120 nm.
In this paper we report results of the last year's lasing below 200 nm. We discuss the performance of the OK-4 FEL and short wavelength mirrors. We also present up-grades of the systems and results of recent experiments at short wavelengths and high harmonics generation. Finally, we discuss the possibilities to extend lasing range of storage ring FELs to 100 nm and below by both enhancing their gain and reducing mirror's degradation.
