Laser–solid interactions have been widely investigated in recent decades because of their wide applications, such as charged particle acceleration[
High Power Laser Science and Engineering, Volume. 7, Issue 2, 02000e36(2019)
Enhancement of the surface emission at the fundamental frequency and the transmitted high-order harmonics by pre-structured targets
Laser interaction with an ultra-thin pre-structured target is investigated with the help of both two-dimensional and three-dimensional particle-in-cell simulations. With the existence of a periodic structure on the target surface, the laser seems to penetrate through the target at its fundamental frequency even if the plasma density of the target is much higher than the laser’s relativistically critical density. The particle-in-cell simulations show that the transmitted laser energy behind the pre-structured target is increased by about two orders of magnitude compared to that behind the flat target. Theoretical analyses show that the transmitted energy behind the pre-structured target is actually re-emitted by electron ‘islands’ formed by the surface plasma waves on the target surfaces. In other words, the radiation with the fundamental frequency is actually ‘surface emission’ on the target rear surface. Besides the intensity of the component with the fundamental frequency, the intensity of the high-order harmonics behind the pre-structured target is also much enhanced compared to that behind the flat target. The enhancement of the high-order harmonics is also related to the surface plasma waves generated on the target surfaces.
1 Introduction
Laser–solid interactions have been widely investigated in recent decades because of their wide applications, such as charged particle acceleration[
In this paper, with the help of two-dimensional (2D) and three-dimensional (3D) particle-in-cell (PIC) simulations, we investigate the interactions between normally incident intense laser pulses and ultra-thin plasma sheets with densities much larger than the relativistically critical density
2 Enhancement of the transmitted laser energy and the transmitted high-order harmonics
The pre-structured targets used in our work have been applied in many previous works[
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Figure
To understand how the pre-structured target enhances the transmitted laser energy, we give more details of the propagation properties of
As it is known from electrodynamics that the source of an electromagnetic field is the current, we need to further discuss the motion of the electron ‘islands’. For the transmitted
To investigate the enhancement of the higher-order harmonics, we continue to increase the laser normalized vector potential to
Simulations with different target thicknesses were also performed, but the results are not shown in the paper. The simulation results show that as the target becomes thicker, the intensity of the transmitted laser decreases, and may even disappear. What restricts the target thickness is the so-called ‘return current’. When an electron bunch propagates in a plasma, the background cold electrons can go back to generate a so-called ‘return current’. The return current will generate magnetic fields that can change the propagation direction of the electron bunch. When the target is thick enough, the propagation directions of these electrons with lower energy can even change by more than
3 Conclusion
In conclusion, we investigate the interaction between an intense laser and an overdense plasma slab with the help of 2D and 3D PIC simulations. It is shown that, even if the plasma density is far higher than the relativistically critical density and the thickness is also much greater than the skin depth, considerable laser energy penetrates the plasma slab when the target is pre-structured. Compared to the flat target, the transmitted laser energy behind the pre-structured is increased by about two orders of magnitude. Detailed analyses show that the ‘transmitted’ radiation behind the pre-structured target is actually radiated by the electron ‘islands’ on the target back surface. Also these electron ‘islands’ are generated by the SPW excited on the target front surface. When the SPW is excited on the target front surface, more hot electrons with higher energies will be generated, and some of these hot electrons will get to the target rear surface to generate electron bunches with the help of the SPW. It is also shown that the transmitted radiation contains high-order harmonics, the intensity of which behind the pre-structured target is also greatly enhanced compared to behind the flat target. The enhancement of the higher-order harmonics is also related to the excitation of the SPWs, because the SPWs will increase both the number and energy of the electrons that radiate the high-order harmonics. It is also shown that the pre-plasma will have a negative influence on the SPW excitation; as a result, the intensity of the transmitted radiation will be decreased when the SPW generation is suppressed by the pre-plasma. To avoid or weaken the influence of the pre-plasma, a laser with higher contrast is needed.
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K. Q. Pan, D. Yang, L. Guo, Z. C. Li, S. W. Li, C. Y. Zheng, S. E. Jiang, B. H. Zhang, X. T. He. Enhancement of the surface emission at the fundamental frequency and the transmitted high-order harmonics by pre-structured targets[J]. High Power Laser Science and Engineering, 2019, 7(2): 02000e36
Category: Research Articles
Received: Nov. 22, 2018
Accepted: Apr. 22, 2019
Posted: Apr. 22, 2019
Published Online: Jul. 1, 2019
The Author Email: X. T. He (xthe@iapcm.ac.cn)