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The problem of radiation start-up in FELs is important in the simulation of virtually all FEL configurations including oscillators and amplifiers in both seeded-MOPA (Master Oscillator Power Amplifier) and SASE (Self-Amplified Spontaneous Emission) modes. Both oscillators and SASE FELs start up from spontaneous emission due to shot-noise that arises from the random fluctuations in the electron phase distribution. The injected power in a MOPA is usually large enough to overwhelm the shot-noise; however, this noise must be treated correctly to model the initial start-up of the harmonics. We discuss and compare two different shot noise models that are implemented in both 1D wiggler-averaged (PERSEO) and non-wiggler-averaged (MEDUSA1D) simulations, and a 3D, non-wiggler-averaged (MEDUSA) formulation. These models are compared for examples describing both SASE and MOPA configurations in 1D in steady-state and time-dependent simulations, and remarkable agreement is found between PERSEO and MEDUSA1D. In addition, 3D correction factors have been included in the MEDUSA1D and PERSEO, and show reasonable agreement with MEDUSA for a sample MOPA in steady-state and time-dependent simulations.
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