Hybrid dc–ac electron gun for fs-electron pulse generation

We present a new concept of an electron gun for generating subrelativistic few-femtosecond (fs) electron pulses. The basic idea is to utilize a dc acceleration stage combined with an RF cavity, the ac field of which generates an electron energy chirp for bunching at the target. To reduce space charge (SC) broadening the number of electrons in the bunch is reduced and the gun is operated at a megahertz (MHz) repetition rate for providing a high average number of electrons at the target. Simulations of the electron gun were carried out under the condition of no SC and with SC assuming various numbers of electrons in the bunch. Transversal effects such as defocusing after the dc extraction hole were also taken into account. A detailed analysis of the sensitivity of the pulse duration to various parameters was performed to test the realizability of the concept. Such electron pulses will allow significant advances in the field of ultrafast electron diffraction.