Observation of Relativistic Effects in Collective Thomson Scattering

We observe relativistic modifications to the Thomson scattering spectrum in a traditionally classical regime: vosc/c=eE0/cmω0<<1 and Te<1 keV. The modifications result from scattering off electron-plasma fluctuations with relativistic phase velocities. Normalized phase velocities v/c between 0.03 and 0.12 have been achieved in a N2 gas-jet plasma by varying the plasma density from 3x1018 cm-3 to 7x1019 cm-3 and electron temperature between 85 and 700 eV. For these conditions, the complete temporally resolved Thomson scattering spectrum including the electron and ion features has been measured. A relativistic treatment of the Thomson scattering form factor shows excellent agreement with the experimental data.