High-harmonic generation in plasmas by relativistic Thomson scattering
Results are presented on the generation of high harmonics due to the scattering of relativistic electrons from high-intensity laser light. The experiments are carried out with a Nd:glass laser system with a peak intensity of 1019Wcm-2 in underdense He, N2 and Ar plasma. It is shown that, at high intensities when the normalized electric field approaches unity, in addition to the conventional atomic harmonics from bound electrons, there is significant contribution to the harmonic spectrum from free electrons. The characteristic signatures of this are found to be the emission of even-order harmonics, a linear dependence on the electron density, a significant amount of harmonics even with circular polarization and a much smaller spatial region over which these harmonics are produced as compared to the atomic case. Imaging of the harmonic beam shows that it is emitted in a narrow cone with a divergence of 2-3°. The measured spatial profile of the harmonics is shown to be in excellent agreement with calculations, which assume that electrons with relativistic energies play a significant part in the scattering process.
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