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16Complex Systems of Charged Particles and their Interactions with Electromagnetic Radiation 2016
NONLINEAR THOMSON SCATTERING OF AN INTENSE TIGHTLY FOCUSED LASER PULSE UP TO THE DIFFRACTION LIMIT
O.E. Vais, S.G. Bochkarev, V.Yu. Bychenkov P.N. Lebedev Physics Institute of RAS, Moscow, Russia, e-mail: hochkar@ sci.lehedev.ru
Current femtosecond petawatt (PW) laser technologies have enabled to reach very high concentration of laser energy in a tight focal spot with peak intensity up to ~1022 W/cm2. Laserplasma interaction with ultra-short sub-PW laser pulses is a source of high energy electrons, ions as well as secondary radiation, including ultrashort X and gamma-ray pulses. One possible scenario of electron acceleration in super strong laser fields is so-called direct or "vacuum" electron acceleration. This mechanism works if the laser pulse interacts with a low-density gas or nano-targets, where effect of laser-induced plasma fields is negligible.
The interaction between the tightly focused laser radiation and electrons is determined by concrete topology of the laser fields in a focal spot, especially in the case of a fundamental diffraction limit. As a result, direct electron acceleration and corresponding non-linear Thomson scattering (NTS) of a laser pulse have energy and spectral characteristics which are different from those for the case of modest focusing, where the laser field components can be described in simplified manner relevant to paraxial approximation. The aim of our paper is to study such characteristics. The work addresses namely the regime where the focal spot size, DF, can be comparable to the laser wavelength, X. For description of EM fields of the tightly focused 30 fs Gaussian laser linearly polarized laser pulse beyond the paraxial approximation we use an exact solution of vector Helmholtz equation applying the angular spectrum representation method and the Stratton-Chu integrals [1,2]. By using a test particle method, the effect of laser pulse tight focusing on energy-angular electron and radiation spectra has been studied depending on peak laser intensity and focal spot size as well as compared to the paraxial model.
The calculation of angular-spectral characteristics have shown that in the case of extremely tight focusing the single attosecond pulse generation is possible on the contrary to the modestly focused laser pulse situation for which secondary radiation consists of attosecond pulse trains. The focal spot diameter decrease and corresponding focal intensity increase lead to the reduction of attosecond pulse number in the train for given laser pulse duration. At the same time, the emission power peak and the photon energy, corresponding to the maximum of spectral function, continue to grow reaching a maximum at DF = 9X, that is a compromise between intensity enhancement and decrease of the interaction volume. We found that for tight focusing the radiation characteristics depend on the initial phase of the laser pulse. Corresponding results with averaging over phases have also been obtained. Our calculations clear demonstrate that for DF < 9X a paraxial approximation overestimates maximum energies of both accelerated electrons and NTS gamma-quanta. The angular distribution of accelerated electrons is wider as compared to a paraxial approximation.
Our results on NTS can be used for diagnostics of the laser pulse as complimentary to those based on the energy-angular spectra of directly accelerated electrons [3]. We also conclude that extremely tight focusing can be not optimal for ion acceleration by TNSA (Target Normal Sheath Acceleration) and may not work perfectly for ion energy increase through tight focusing.
The authors acknowledge the support from the Russian Scientific Foundation (grant # 1412-00194).
References
[1] Popov K.I., Bychenkov V.Yu., Rozmus W., and Sydora R.D., Phys. Plasmas, 2008, 15, 013108.
[2] Vais O.E., Bochkarev S.G., Bychenkov V.Yu., Bulletin of the Lebedev Physics Institute, 2016, 43 (1), 12; Plasma Phys. Rep. 2016, to be appeared.
[3] Borovskiy A. V., Galkin A. L., M. P. Kalashnikov et al., Phys. Plasmas 2015, 22 043107.
