FEL gain parametrization

See,e.g.,W. B. Colson:Classical free electron laser theory, in:Laser Handbook, edited byW. B. Colson, C. Pellegrini andA. Renieri, Vol.6 (North Holland P.C., 1990).

I. B. Bernestein andJ. L. Hirshfield:Phys. Rev. A,20, 1661 (1979);P. Sprangle, C. M. Tang andW. M. Manheimer:Phys. Rev. A,21, 302 (1980);G. Dattoli, A. Marino, A. Renieri andF. Romanelli:IEEE J. Quantum Electron., QE-17, 1371 (1981).

G. Dattoli, A. Torre, C. Centioli andM. Richetta.IEEE J. Quantum Electr., QE-25, 2327 (1989).