Quasi-monoenergetic electron-beam generation using a laser accelerator for ultra-short X-ray sources

Abstract

Two types of electron acceleration methods have been conducted to generate quasi-monoenergetic electron beams. Multi-MeV quasi-monoenergetic high-charge electron beams were generated at Korea Electrotechnology Research Institute (KERI) from self-modulated laser wakefield acceleration by using a collimator with a 2 TW (1.4 J/700 fs) Nd:glass/Ti:sapphire hybrid laser system and a supersonic nitrogen gas jet. The peak electron energy was 3.6 MeV, and the energy spread was 4 MeV. These electron beams are useful for the generation of short-pulse X-rays in the water window region, which is 250 eV - 500 eV (2.5 - 5 nm), by using Thomson scattering. The calcualted photon spectrum indicates the scattered photon covers the water window region. This can be used for a high spatial and temperal resolution microscope for medical imaing. To generate higher-energy electron beams with small energy spread, a laser wakefield acceleration experiment with a sharp downward electron density gradient was conducted with a 100 TW laser system at Advanced Photon Research Insistitute (APRI). With the electron density gradient, some background plasma electrons could be locally injected in the laser wake wave and a small energy spread was expected. Using the pre-pulse, we could generate sharp downward electron density gradients. The gradient scale length was 20 μm for a 25 % density change. With this electron density gradient, we could get more reproducible electron beams than we could without the density gradient.