Title : Coherent radiation from inverse compton scattering sources by means of particle confinement in an optical lattice
Inverse Compton scattering (ICS) sources also named as optical undulator radiation sources are one of the promising compact tools to generate short wavelength radiation from electron beams based on the relativistic Doppler effect. Nonetheless, these sources suffer from a few shortcomings such as incoherent radiation and low-efficiency in radiation generation. The theory of FEL using an optical undulator has been proposed in the 1980s for the first time. However, despite the extensive efforts in the past decades, there exists no operational FEL based on optical or EM undulators. Research efforts are already devoted to explore the main challenges in free-electron lasing of low-energy electrons using the available simulation tools. This contribution argues that the strong space-charge forces between electrons are the main impediment in achieving a coherent gain in the radiation. Afterwards, we aim at establishing a mechanism based on particle beam confinement for relativistic electrons to propose an unconventional approach for tackling the compact coherent x-ray source problem. It is known that charged particles inside a spatially varying field profile are influenced by gradient forces, driving them towards the area of weaker field strengths. This occurs inside an optical cavity or when two counter-propagating twin beams impinge on the electron bunch. It is hypothesized that when the electrons in a beam are transversely confined, many of the existing challenges emanating from transverse motions and repulsive forces are overcome, thereby achieving the coherent-gain regime is alleviated. The full-wave solution of first- principle equations based on finite-difference time-domain and particle in-cell (FDTD/PIC) is performed to simulate inverse-Compton scattering (ICS) off both free and confined electrons. It is shown that by confining the electron beam at the field nodes of an optical cavity, the space-charge effect is compensated, and additionally, the ultrahigh charge density enables high FEL-gain at confinement spots. The full-wave numerical simulations predict enhancement of about three orders of magnitude in the radiation efficiency when ICS is carried out with confined electrons compared to free electrons. These theoretical results show promising potential as a new scheme for implementing a compact coherent x-ray source.