Tunable Narrowband Thomson Source based on a Laser-Plasma Accelerator

Abstract

A compact, high-energy, tunable, ultra-short pulse duration, narrowband X-ray source with high brilliance could unlock novel developments in a wide range of scientific and medical X-ray applications, such as X-ray Fluorescence Imaging (XFI) with gold nanoparticles (GNPs). Compared to light sources driven by large-scale radiofrequency-based accelerators, Thomson Scattering (TS) sources based on electrons from a compact laser-plasma accelerator are a promising alternative for providing high-energy and high-quality X-ray beams. These could provide the necessary compactness for a future transition into clinical environments for many applications relying on X-ray radiation. Even though X-ray sources based on the combination of TS and Laser-Plasma Acceleration (LPA) have been demonstrated before, the X-ray spectral bandwidth was insufficiently narrow for XFI. LPAs driven by 10-TW class lasers typically produce electron bunches with multi-percent-level bandwidths and milliradians divergences. As a result, Thomson X-ray beam generation suffers from severe spectral broadening, resulting in X-ray beams with many tens of percent bandwidths that are impractical for most X-ray applications.

In a proof-of-principle experiment designed to mitigate these restriction, an Active Plasma Lens (APL) was utilised for focusing of the electron beam, allowing for tunable X-ray beams with reduced bandwidths to be produced. Without any changes to the electron bunch properties, only the focusing strength of the APL was varied to tune the X-ray beam energy between 34 keV and 81 keV. Beam imaging reduced the electron beam divergence and acted as a chromatic filter for the electron beam. Although an electron bunch with an initial FWHM energy spread of more than 100 % was used in the TS interaction, the average FWHM bandwidth of the generated X-ray beam was measured to be (25.6 +- 2.5) %. While the total bandwidth measured in this proof-ofprinciple experiment exceeds the typical requirements for novel X-ray applications, this bandwidth is no longer dominated by electron beam properties, as is usually the case for LPA-based TS sources. With further optimisation of the scattering laser in the TS interaction, the total bandwidth can be significantly reduced. The demonstrated bandwidth filtering Thomson source based on a laser-plasma accelerator with APL focusing is a key development that has the potential to be beneficial in a variety of compact X-ray applications and to enable their implementation outside of research environments.