FEL Amplifier Start-up from Shot Noise

Abstract

Till now we have considered the electron beam as a continuous medium when describing the theory of the FEL amplifier. To some extent this is an ideal­ization, since in reality the beam current is produced by a large number of moving electrons. If we consider the microstructure of the electron current, we find that electrons enter the undulator randomly in time and space. When the electron passes the undulator, it interacts with the electromagnetic field produced by other electrons and also emits radiation. So, we can expect that the FEL amplifier should possess intrinsic noise properties. Taking into ac­count this complicated picture, we can formulate several problems. The first one is the region of applicability of the steady-state theory of the FEL ampli­fier: namely, to what extent can we neglect the noise properties of the FEL amplifier? The second one is what kind of physical effects are hidden in the steady-state theory. Finally, we study how these physical effects can be used in practical devices.