
Step III  The Radiation FieldIn an FEL oscillator the radiation parameter are well defined by the seeding radiation field and the enclosing cavity. Otherwise (FEL amplifier or SASE FEL) they typically need to be optimized. Although SASE simulation significantly reduces the impact of the waist position ZWAIST and the Rayleigh length ZRAYL parameters it is recommended as a preoptimization step to run steadystate simulation (ITDP = 0). Thus the following is valid for both, FEL amplifier and SASE FEL. A good estimate of the radiation wavelength is given by the resonant formula with =XLAMDS, =GAMMA0, _{u}=XLAMD and a_{u} =AW0. Anyhow diffraction or emittance effects might slightly change the resonant wavelength. The Raylength length should be chosen so that the initial beam size is similar to the beam size of the gainguided radiation field in the exponential amplification regime. If the radiation waist is at the undulator entrance (ZWAIST=0) the initial beam size is simply given by with z_{r} =ZRAYL. While the input radiation power is a welldefined input parameter for an FEL amplifier its value has to be estimated for steadystate simulation modelling SASE FELs. With the definition of the FEL parameter an approximation of an equivalent input power (shot noise power) of the SASE FEL is with f_{c} =FBESS0 or the value GENESIS 1.3 would calculate in the case that FBESS0 is set to zero in the input deck, _{b}=(RXBEAM + RYBEAM)/2, I_{p} = CURPEAK, I_{A} = 17 kA, P_{b} =I_{p}mc ^{2}/e as the electron beam power and N=I_{p} /ce as the number of electrons per radiation wavelength. With the parameters for the magnetic field, electron beam and radiation field a run of GENESIS 1.3 should produce some gain. If the results are satisfying the input deck can be used for more advanced simulation (field errors, timedependent simulation). Otherwise further optimization of the input parameters is needed. 