Acceleration element for femtosecond electron pulse compression.

Abstract

An acceleration element is proposed for compressing the electron pulse duration in a femtosecond photoelectron gun. The element is a compact metal cavity with curved-shaped walls. An external voltage is applied to the cavity where a special electric field forms in such a way that the slow electrons in the electron pulse front are accelerated more than the fast electrons, and consequently the electron pulse duration will be compressed. The distribution of the electric field inside the acceleration cavity is analyzed for the geometry of the cavity. The electron dynamics in this acceleration cavity is also investigated numerically. Numerical results show that the electron pulse front and pulse duration can be improved by compensating for the effects of space charge and the initial energy spread of photoelectrons with a Lambertian angular distribution. Depending on the design parameters and the shape of the electron pulse, for a femtosecond electron gun with an electron energy of 30 keV, 10(3) electrons per pulse, and an electron drift length of 40 cm, the electron pulse duration can be reduced from 550 to 200 fs when using a compensating cavity with an average radius of 1.7 and 5.6 cm in length. Electron pulses shorter than 200 fs can be achieved if the length of the drift region is reduced.