Generation of Ultra-Short Bunches

Electron sources providing bunch length down to the fs regime and below are of increasing interest for applications as time resolved electron diffraction or ultra-fast magnetics, but also as injectors for advanced accelerator experiments. Analytic and numerical studies demonstrate that sub- fs bunch length can be generated at low charge with simple ballistic bunching schemes if nonlinearities in the longitudinal phase space are corrected. The correction can be achieved by adding a higher harmonic rf system or by operating the rf gun in an unusual phase range in the so-called stretcher mode. The presentation will give an overview of ballistic bunching concepts and correction methods. Limits and options for ultra-short bunch generation at low energies will be discussed