A study of the Thomson scattering of radiation by a relativistic electron of a tightly-focused, co-propagating femtosecond laser beam

The paraxial approximation to a laser field is no longer valid when the beam waist of the laser is of the order of its wavelength. In such a case, a laser beam propagating even in free space is not purely transverse but contains a significant longitudinal field. The present study reveals that the characteristics of the Thomson scattering by a relativistic electron (80 MeV) of a tightly-focused, co-propagating laser field is sensitive to the electron's initial transverse position and phase with respect to the laser field: the electron radiates more strongly when it is initially located off the laser axis by about the beam waist than when on the laser axis. An enhancement of about 2000 times in the radiation power is noticed for the focused (beam waist w ₀ of 5 μm) laser intensity of 5 × 10¹⁸ W cm ^-2 compared to a paraxial Gaussian beam case.