Much Ado about Microbunching: Coherent Bunching in High Brightness Electron Beams

Abstract

The push to provide ever brighter coherent radiation sources has led to the creation of correspondingly bright electron beams. With billions of electrons packed into normalized emittances (phase space) below one micron, collective effects may dominate both the preservation and use of such ultra-bright beams.

Subradiant spontaneous undulator emission through collective suppression of shot noise

“…In the simple 1D model, the noise factor nearly reaches 0 for ϒ ¼ 1, with the maximum level of suppression limited by fundamental beam properties of emittance and energy spread [10,32,35]. In practice, a number of other longitudinal and transverse effects also inhibit full suppression.…”

“…First, because different parts of the beam have different values of n 0 , there is no single value of R 56 that suppresses the entire beam. (See e.g., [12,35] for a discussion of a Gaussian distribution in a short interaction.) The beam is also expected to have strong peak current dependence along the longitudinal axis.…”

Theory of Plasma-Cascade Instability

“…It can be also used to describe one dimensional or 2D instabilities. For example, it is easy to show that conventional longitudinal microwave instability [53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69][70][71] can be also described by this equation under simplified assumptions. Specifically, conventional theory of longitudinal microwave instability assumes that in straight sections the longitudinal motion is frozen and energy modulation resulted from accumulated space-charge forces is transferred into density by R56 of a magnetic system.…”

“…The PCI is the micro-bunching instability occurs in a beam propagating along a straight line. It differs from the well-known and well-studied conventional micro-bunching instabilities [53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72] requiring bending of the beam trajectory 1 .…”