Abstract
The shot-noise driven microbunching instability can significantly degrade electron beam quality in x-ray free electron laser light sources. Experiments were carried out at the Linac Coherent Light Source (LCLS) to study this instability. In this paper, we present start-to-end simulations of the shot-noise driven microbunching instability experiment at the LCLS using the real number of electrons. The simulation results reproduce the measurements quite well. A microbunching self-heating mechanism is also illustrated in the simulation, which helps explain the experimental observation.
Highlights
The microbunching instability seeded by shot noise and driven by collective effects can significantly degrade the quality of the electron beam before it enters the free electron laser (FEL) undulators
It consists of an S-band photoelectron gun, a short section of linac (L0) before the laser heater, two bunch compressors and three linac sections
The start-to-end macroparticle simulations using the real number of electrons reproduce the microbunching instability experimental observations at the Linac Coherent Light Source (LCLS) quite well
Summary
The microbunching instability seeded by shot noise and driven by collective effects (primarily space charge) can significantly degrade the quality of the electron beam before it enters the free electron laser (FEL) undulators. An initial small density modulation inside the electron beam (even from shot noise) can cause sufficient energy modulation from the longitudinal space-charge force in a section of the linear accelerator. Such energy modulation causes larger density modulation through a dispersive magnetic optics element such as a chicane. With the help of an X-band transverse deflecting cavity (XTCAV) [19], the longitudinal phase space can be imaged at the end of the accelerator revealing the detailed structure arising from the microbunching instability To better understand these experimental results, we have done start-to-end macroparticle simulations using a real number of electrons.
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