Thomson scattering with terawatt laser generates femtosecond x-ray pulses
By focusing the pulsed output of a terawatt infrared laser onto a relativistic beam of electrons, Robert Schoenlein and collaborators at Lawrence Berkeley National Laboratory (LBNL; Berkeley, CA) have generated femtosecond x-ray pulses at 4 nm through 90° Thomson scattering. In systems generating x-ray bursts by scattering between optical pulses and relativistic electrons, pulse duration is driven by the interaction length of the two beams. Previous experiments used counterpropagating beams, so the length of the electron bunch emitted by the accelerator limited the x-ray pulse durations to picoseconds. The LBNL configuration places the laser beam orthogonal to the waist of the electron beam so that the pulse duration is driven by the beam-waist diameter and femtosecond x-ray bursts can be achieved.
Using a Ti:sapphire laser and chirped pulse amplification, the grou¥generated 50-fs, 60-mJ pulses at 800 nm, which interacted with a 90-µm diameter, 50-MeV electron beam produced by the linear accelerator at the Advanced Light Source (Berkeley, CA). The flux of the resultant 300-fs x-ray pulses was approximately 5 ¥ 104 photons per pulse; a more tightly focused beam waist could lead to pulses as short as 50 fs, allowing scientists to study atomic structure and motion occurring on a 100-fs time scale.