The two stage electron acceleration model [arXiv: 1512.02411 and arXiv: 1512.07546] is extended to the study of laser magnetized-plasmas interactions at relativistic intensities and in the presence of large-scale preformed plasmas. It is shown that the cut-off electron kinetic energy is controllable by the external magnetic field strength and directions. Further studies indicate that for a right-hand circularly polarized laser (RH-CP) of intensity 10 20 W/cm 2 and pre-plasma scale length 10 µm, the cut-off electron kinetic energy can be as high as 500 MeV, when a homogeneous external magnetic field of exceeding 10000 T (or B = ωc/ω0 > 1) is loaded along the laser propagation direction, which is a significant increase compared with that 120 MeV without external magnetic field. A laser front sharpening mechanism is identified at relativistic laser magnetized-plasmas interactions with B = ωc/ω0 > 1, which is responsible for these super-high energetic electrons.

The propagation of left-hand (LH-) and right-hand (RH-) circularly polarized (CP) lasers and the accompanying generation of fast electrons in a magnetized cone-target with pre-formed plasmas are investigated. In this work, the strength of external magnetic field is comparable to that of the incident laser. Theoretical analyses indicate that the cut-off density of LH-CP laser is larger than that without an external magnetic field. When the external magnetic field normalized by the laser magnetic field is larger than the relativistic factor, the RH-CP laser will keep on propagating till the laser energy is depleted. The theoretical predictions are confirmed by two-dimensional particle-in-cell simulations. Simulation results show that in the presence of external longitudinal magnetic field, the energies and yields of fast electrons are greatly enhanced for RH-CP laser. Besides, the coupling efficiency of laser energy to energetic electrons for RH-CP laser is much higher than that for LH-CP laser and without external magnetic field. Furthermore, detailed simulation results perform an enhancement of the incident laser absorption with increasing external magnetic field.

Two-dimensional particle-in-cell (PIC) simulations have been used to investigate the interaction between a laser pulse and a foil exposed to an external strong longitudinal magnetic field. Compared with that in the absence of the external magnetic field, the divergence of proton with the magnetic field in radiation pressure acceleration (RPA) regimes has improved remarkably due to the restriction of the electron transverse expansion. During the RPA process, the foil develops into a typical bubble-like shape resulting from the combined action of transversal ponderomotive force and instabilities. However, the foil prefers to be in a cone-like shape by using the magnetic field. The dependence of proton divergence on the strength of magnetic field has been studied, and an optimal magnetic field of nearly 60 kT is achieved in these simulations.

Shi-Xiang(彭士香) a) † , Zhang Ai-Lin(张艾霖) a)b) , Ren Hai-Tao(任海涛) a) , Zhang Tao(张 滔) a) , Xu Yuan(徐 源) a) , Zhang Jing-Feng(张景丰) a) , Gong Jian-Hua(龚建华) a) , Guo Zhi-Yu(郭之虞) a) , and Chen Jia-Er(陈佳洱) a)b) a) SKLNST& IHIP