Characterization of bright betatron radiation generated by direct laser acceleration of electrons in plasma of near critical density

Directed x-rays produced in the interaction of sub-pico second laser pulses of moderate relativistic intensity with plasma of near-critical density are investigated.Synchrotron-like（betatron） radiation occurs in the process of direct laser acceleration（DLA） of electrons in a relativistic laser channel when the electrons undergo transverse betatron oscillations in self-generated quasi-static electric and magnetic fields In an experiment at the PHELIX laser system,high-current directed beams of DLA electrons with a mean energy ten times higher than the ponderomotive potential and maximum energy up to 100 MeV were measured at 1019 W/cm2 laser intensity.The spectrum of directed x-rays in the range of 5-60 keV was evaluated using two sets of Ross filter placed at 0° and 10° to the laser pulse propagation axis.The differential x-ray absorption method allowed for absolute measurements of the angular-dependent photon fluence We report 1013 photons/sr with energies>5 keV measured at 0° to the laser axis and a brilliance of 1021 photons s-1 mm-2 mrad-2（0.1%BW）-1.The angular distribution of the emission has an FWHM of 14°-16°.Thanks to the ultra-high photon fluence point-like radiation source,and ultra-short emission time,DLA-based keV backlighters are promising for various applications in high-energy-density research with kilojoule petawatt-class laser facilities.