Measurement of Bright DLA-based Betatron Radiation on PHELIX

In experiments involving the interaction of sub-ps PHELIX pulses with an intensity of 10$^{19}$ W/cm$^{2}$ with pre-ionized, low-density foams, a high-current beam of directly laser-accelerated electrons (DLA) with energies of up to 100 MeV is observed. A directed betatron radiation is emitted due to transverse oscillations of relativistic DLA electrons confined in the ion channel.

The initial measurements of DLA-based betatron radiation were conducted at PHELIX in 2021 using Ross filters and X-ray diodes. Several issues were identified during the experiment, including spatial distribution of multiple X-ray sources and proton input into IP signal. To further develop diagnostic methods, a modified magnetic spectrometer was constructed, successfully addressing the challenges encountered in the pilot experiment. The device was tested and utilized in a series of subsequent experiments at PHELIX in the years 2022-2023. As a result, betatron and bremsstrahlung sources were spatially separated from each other. Additionally, measurements of X-ray radiation from various target combinations were carried out for comparison.

As a result, forward-directed DLA-based betatron radiation with a divergence half-angle of 6-10° and a photon number of ~ 3·10$^{10}$ at energies between 10-30 keV were recorded. The estimated brilliance is approximately 2·10$^{20}$ photons/s/mm$^{2}$/mrad$^{2}$/(0.1% BW) at 10 keV. There is a good agreement between experiment and 3D PIC simulations.