X-Ray Radiation from Nonlinear Thomson Scattering of an Intense Femtosecond Laser on Relativistic Electrons in a Helium Plasma

We have generated x-ray radiation from the nonlinear Thomson scattering of a $30\mathrm{f}\mathrm{s}/1.5\mathrm{J}$ laser beam on plasma electrons. A collimated x-ray radiation with a broad continuous spectrum peaked at 0.15 keV with a significant tail up to 2 keV has been observed. These characteristics are found to depend strongly on the laser strength parameter $a_0$. This radiative process is dominant for $a_0$ greater than unity at which point the relativistic scattering of the laser light originates from MeV energy electrons inside the plasma.