Coherence Limits of Free Electron Lasers

The e-beam and radiation wave dynamics in the radiating and nonradiating beam transport sections of free electron lasers (FELs) are analyzed in the collective regime by use of a single transverse mode linear response formulation. This is employed to derive conditions for coherent operation of seeded high-gain FELs. It is shown that the level of incoherent self-amplified spontaneous emission radiation power can be controlled by adjusting the plasma oscillation phase in the nonradiating beam transport sections preceding the FEL, and that at short wavelengths the FEL coherence is limited by energy noise (rather than current shot-noise), and ultimately by quantum noise.