Self- Focusing and Channeling of Relativistic Laser Pulses in Underdense Plasmas
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Self-focusing
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We report optical investigations of the interaction of femtosecond Ti:sapphire laser pulses with underdense plasmas created from high density gas jet targets. Time-resolved shadowgraphy using a 2ω probe pulse and images of the transmitted radiation are presented for nitrogen and hydrogen. For the laser power available, the experimental results and their analysis based on a simple numerical Gaussian beam model show that ionization induced refraction dominates the interaction process for all gases except hydrogen. The numerical modeling also shows that for a given laser power there exists only a narrow density range in which self-focusing can be expected to occur. In the case of hydrogen for electron densities greater than ∼1020 cm−3, the onset of channeling expected at the critical power for relativistic self-focusing is experimentally observed.
Shadowgraphy
Self-focusing
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Proton emission
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Relativistic plasma
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Filamentation
Self-focusing
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Relativistic self-focusing and channelling of intense laser pulses have been studied in underdense plasma using two-dimensional particle-in-cell (PIC) simulations, for different laser powers and plasma densities. Analytical solutions for the stationary evacuated channels have been recovered in PIC simulations. It is shown that otherwise stable channels can accelerate electrons due to surface waves on the walls of the channels. Relativistic filaments with finite electron density are unstable to transverse modulations which lead in the nonlinear stage to the breakup of laser pulses into independent filaments. Different regimes of relativistic self-focusing and channelling, including electron heating, transverse instability, and break-up of the filaments, have been discussed and characterized using plasma density and laser power.
Channelling
Weibel instability
Relativistic plasma
Self-focusing
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We report the first clear experimental demonstration of relativistic self-channeling of a multiterawatt laser pulse interacting with an underdense plasma. We show via Thomson scattering observations that for laser power above the critical power for self-focusing the beam remains trapped and guided over the plasma length. These observations are in fairly good agreement with a numerical model describing the laser propagation and taking into account the plasma response to the ponderomotive force.
Ponderomotive force
Self-focusing
Thomson scattering
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Energetic electron beam can be generated through the directlaser acceleration (DLA) mechanism when high power picosecond laser propagates in underdense plasma, and the electron yield can reach several hundred nC, which has a great application in driving secondary radiations, such as bremsstrahlung radiation and betatron radiation. When a linearly polarized laser is used, the beam divergence is always larger in the laser polarization direction. What is more, the forked spectral-spatial distribution is observed in the experiments driven by femtosecond laser where DLA is combined with the laser wakefield acceleration (LWFA). The forked distribution is regarded as an important feature of DLA. However, an analytical explanation for both the bigger divergence and the forked spectral-spatial distribution is still lacking. Two-dimensional (2D) particle-in-cell simulations of picosecond laser propagating in underdense plasma are conducted in this paper to show how the fork is formed in DLA. The fork structure is a reflection of the distribution of electron transverse velocity. We find that when electrons are accelerated longitudinally, the transverse oscillation energy in the laser polarization direction increases correspondingly. If the electron energy is high enough, the transverse oscillation energy will increase linearly with the electron energy. As a result, the most energetic electrons will have an equal amplitude of <i>v<sub>y</sub></i>, where <i>v<sub>y</sub></i> denotes the velocity in the laser polarization direction. For a single electron, the distribution of its transverse velocity over a long period <inline-formula><tex-math id="M1">\begin{document}$\dfrac{{{\rm d}P}}{{{\rm d}{v_y}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20191106_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20191106_M1.png"/></alternatives></inline-formula>, will peak at ±<i>v<sub>m</sub></i> (<i>v<sub>m</sub></i> denotes the amplitude of <i>v<sub>y</sub></i>). If all the electrons have the same <i>v<sub>m</sub></i>, the distribution of <i>v<sub>y</sub></i> at a given time will be the same as <inline-formula><tex-math id="M2">\begin{document}$\dfrac{{{\rm d}P}}{{{\rm d}{v_y}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20191106_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20191106_M2.png"/></alternatives></inline-formula>. That means they will split transversely, leading to a forked spectral-spatial distribution. By using a simplified model, the analytical expression of <i>v<sub>m</sub></i> is derived, showing good agreement with <i>v<sub>m</sub></i> in the PIC simulation. However, the oscillation energy in the direction perpendicular to polarization will decrease when electrons are accelerated longitudinally (acceleration damping). As a consequence, the divergence perpendicular to the polarization direction will be smaller. Our research gives a quantitative explanation for the transverse distribution of electrons generated by DLA. With some modification, it can also be used in DLA combined LWFA to better control the dephasing length.
Betatron
Picosecond
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Relativistic plasma
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Channelling
Intensity
Relativistic plasma
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