Observation of the Second Harmonic in Thomson Scattering from Relativistic Electrons

A free relativistic electron in an electromagnetic field is a pure case of a light-matter interaction. In the laboratory environment, this interaction can be realized by colliding laser pulses with electron beams produced from particle accelerators. The process of single photon absorption and reemission by the electron, so-called linear Thomson scattering, results in radiation that is Doppler shifted into the x-ray and $\gamma$-ray regions. At elevated laser intensity, nonlinear effects should come into play when the transverse motion of the electrons induced by the laser beam is relativistic. In the present experiment, we achieved this condition and characterized the second harmonic of Thomson x-ray scattering using the counterpropagation of a 60 MeV electron beam and a subterawatt ${\mathrm{CO}}_2$ laser beam.

Figure 1

A diagram of the experiment.

Figure 2

Simulated energy spectra of Thomson x rays. Solid lines: (black line) at the interaction point; (red line) on the detector after attenuation in the Be window and air; (blue line) filtered by a $10\mu \mathrm{m}$ Ag foil. Dashed lines show combined spectral transmission of the Be window with air (red line) and Ag foil (blue line). A green line shows the high-energy edge (6.5 keV) for the linear Thomson scattering.

Figure 3

X-ray images observed on a luminescent screen: (a) without the Ag foil, and (b) with the $10\mu \mathrm{m}$ Ag foil filter.

Figure 4

Polar distributions of the scattered x-ray intensity in the plane of laser polarization. Dashed line and triangles: correspondingly simulation and experimental points for unfiltered x-ray signal; solid line and circles with error bars: correspondingly simulation and experimental points obtained with the $10\mu \mathrm{m}$ Ag foil filter.

Figure 5

Azimuthal distribution of the x-ray intensity in the second harmonic of Thomson scattering filtered out after passing through Be window and Ag foil. Crosses and triangles: experimental points; lines: simulations. (a) Obtained with a linear polarized laser (crosses and triangles correspond to two orthogonal directions of the laser polarization). (b) Obtained with a circular polarized laser.