通道靶对超强激光加速质子束的聚焦效应

发散角过大是制约超强激光与固体靶相互作用加速产生高能质子束应用的一个重大物理难题.本文提出了一种结构化的通道靶型,与超强激光相互作用可提高质子束的发散特性,通道壁上产生的横向电荷分离静电场可对质子有效聚焦.采用二维particle-in-cell粒子模拟程序对激光通道靶相互作用过程进行了研究,分析了加速质子束的性能特点.模拟结果表明,与传统平面靶相比,通道靶可以在不过多损失能量的情况下产生具有更好准直性的质子束,尤其当通道靶的直径与激光焦斑尺寸和质子源尺寸相当时,横向静电场能够有效聚焦质子束,并且可保证相对较高的激光能量利用率.

Focusing effect of channel target on ultra-intense laser-accelerated proton beam

In laser proton acceleration, the inevitable transverse divergence of proton beam restricts its applications in many fields. In this paper, a structured target with a properly wide channel attached to the backside of a foil is proposed, and the interaction of the ultra-short laser pulse with the structured channel target is investigated via two-dimensional particle-in-cell simulation. The simulations show that for the structured channel target, electrons on the front surface are heated by the incident high-intensity laser pulse and then the induced hot electrons transport through the target to the rear surface, building an electrostatic field in the longitudinal direction to accelerate the protons to high energies as the typical target normal sheath acceleration scheme. In the case of the structured channel target, the simulation results indicate that a strong transverse electrostatic field is created by charge separation along the inner surface of the channel while hot electrons propagate along the channel side walls under the guidance of self-induced magnetic and electric fields, which can focus the emitted proton beam transversely, leading to a smaller divergence. By comparing the channel target case with the traditional foil target case under the same conditions, it is found that the divergence angle of the proton beam from the channel target is reduced significantly. Protons with energies above 3 MeV have a divergence angle of 5.3° at the time of 500 fs in the channel target case, while the value is 17.1° in the foil case for a laser intensity of 5.4×10¹⁹ W/cm². Additionally, the effect of the channel target on the maximum proton energy is considered. The simulation results of the energy spectra reveal that the maximum proton cut-off energy of the channel target is about 1 MeV lower than that of the foil target. This small energy loss is due to the refluxing of the cold electrons on the channel walls, which suppresses the increasing of the sheath potential. Therefore, it is concluded that the focusing electric field can work on the proton beam effectively, leading to a better collimation with conserving the proton energy by using the proposed channel target. Especially when the inner diameter of the channel target is comparable to the laser focal spot size, the proton beam can be confined to a small divergence, and a relatively higher laser energy conversion efficiency can be ensured as well.