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.

Figure 1

Experimental setup. The maximum strength parameter attainable on target is $a_0=5.6$.

Figure 2

(a) Energy spectrum of the observed x-ray emission. The horizontal bars correspond to each spectral bandwidth of the x-ray spectrometer. The vertical error bars correspond to the statistical uncertainty accumulated over five shots. The electron density is $5\times {10}^{19}{\mathrm{c}\mathrm{m}}^{-3}$ and the angle of observation is $\theta =0°$. (b) X-ray intensity as a function of the electron density of the plasma. (c) Angular distribution. The laser beam is focused on the front edge of the gas jet. Solid curves: free electron model depicted in the text.

Figure 3

X-ray intensity as a function of the electron density of the plasma for $a_0=5.6$ and at an angle of observation $\theta =0°$ and $40°$. The spectral bandwidth is centered at 40 eV, the less energetic part of the spectrum. The laser beam is focused on the front edge of the gas jet. The inset shows the x-ray intensity as a function of the electron density of the plasma at an angle of observation $\theta =0°$ and for $a_0<1$.

Figure 4

Spatial distribution of the observed x-ray emission for $a_0=5.6$. The solid line (dotted line) is the numerical result obtained for a Gaussian electron distribution with a temperature of 0.9 MeV and without (with) $5°$ $\overrightarrow{k}$ spread. The inset shows the effect of the initial energy of monoenergetic electrons on the spatial distribution of the nonlinear Thomson scattering.