Suppressing shot noise and spontaneous radiation in electron beams

Shot noise in the electron beam distribution is the main source of noise in high-gain FEL amplifiers, which may affect applications ranging from single- and multi-stage HGHG FELs [1] to an FEL amplifier for coherent electron cooling [2]. This noise also imposes a fundamental limit of about 10⁶ on FEL gain, after which SASE FELs saturate [3]. There are several advantages in strongly suppressing this shot noise in the electron beam, and the corresponding spontaneous radiation. For more than a half-century, a traditional passive method has been used successfully in practical low-energy microwave electronic devices^* [4, 5] to suppress shot noise. Recently, it was proposed for this purpose in FELs [6]. However, being passive, the method has some significant limitations and is hardly suitable for the highly inhomogeneous beams of modern high-gain FELs. I present a novel active method of suppressing, by many orders-of-magnitude, the shot noise in relativistic electron beams. I give a theoretical description of the process, and detail its fundamental limitation.