Nonlinear Thomson Scattering for Plasmaand Laser Characterization

Abstract

Summary form only given. One of the most powerful methods of diagnosing plasma is detecting the scattering of electromagnetic radiation from the plasma. The process is described by the Thomson scattering (TS) in which the electron radiates with the same frequency of the incident field. The radiation signal is proportional to the number of the electrons but the electron motion shifts the frequency by Doppler effects. Thomson scattering then yields detailed information of both the electron density and speed distribution (temperature). TS typically has a very small cross-section, therefore it is advantageous to use more intense lasers. It is known for years that the electron acquires an additional energy in the high fields, known as the ponderomotive energy. Recent discussion shows that the electron also acquires a counterpart ponderomotive momentum. The ponderomotive momentum causes the electrons to drift along the laser propagation, which results in additional spectral shift. The effect is known as the nonlinear Thomson scattering. Since the drift velocity is proportional to the laser intensity, a spread of the laser intensity results in spectral broadening. We have calculated detailed spectra of the Thomson scattering from intense laser fields by solving the electron motion inside the intense laser field nonperturbatively and subsequently calculating the radiation spectra from electrodynamics theory. The spectral shift and broadening correlate with the laser properties and can exceed the Doppler shifts introduced by the electron speed, which suggests that deconvolution is necessary to remove the intensity effects from the laser and recover the true electron properties for the high intensity Thomson scattering. Concurrently, this basic interaction between the plasma and high intensity laser opens up the possibility of using the nonlinear Thomson scattering to characterize the laser fields, e.g. measure the absolute laser intensities in-situ and optically. The role of the electron in this case is thus changed from being a target to functioning as a probe