We report on near-GeV electron beam generation from an all-optical cascaded laser wakefield accelerator (LWFA). Electron injection and acceleration are successfully separated and controlled in different LWFA stages by employing two gas cells filled with a He/O2 mixture and pure He gas, respectively. Electrons with a Maxwellian spectrum, generated from the first LWFA assisted by ionization-induced injection, were seeded into the second LWFA with a 3-mm-thick gas cell and accelerated to be a 0.8-GeV quasimonoenergetic electron beam, corresponding to an acceleration gradient of 187  GV/m. The demonstrated scheme paves the way towards the multi-GeV laser accelerators.

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We demonstrate experimentally the efficient fusion neutron generation from Coulomb explosion ͑CE͒ of laser irradiated large-size heteronuclear deuterated methane clusters. A conversion efficiency of 2.1 ϫ 10 6 neutrons/ J of incident laser energy is obtained with a 120 mJ, 70 fs laser pulse. It is 50 times higher than that of homonuclear deuterium clusters of similar size. This enhancement is attributed to the significant increase in the deuteron kinetic energies by fourfold due to energetic boosting and overrun effects during CE of heteronuclear clusters. The yield of 5.5ϫ 10 6 neutrons per pulse is obtained with a 100 TW, 50 fs driving laser pulse at an intensity of 1.5ϫ 10 19 W / cm 2 . This work may facilitate the development of a high-flux The generation of deuterium-deuterium ͑DD͒ fusion neutrons from Coulomb explosion ͑CE͒ of laser-heated cryogenic deuterium clusters ͑D 2 ͒ N was first demonstrated by Ditmire et al. in 1999 ͓1͔ with a high-repetition-rate tabletop laser; an efficiency of about 10 5 fusion neutrons/ J of incident laser energy was achieved, which was close to the efficiency of large-scale laser-driven fusion experiments ͓2,3͔. This kind of short ͑subnanosecond͒ bursts of monoenergetic fusion neutrons could find wide applications in materials science ͓4͔ such as high spatial resolution neutron radiography and time-resolved study of radiation damage which is of particular importance for developing future fusion energy reactor. However, the conversion efficiency of neutron generation should be improved dramatically to be 10 7-8 neutrons/ J of incident laser energy ͓5͔. Extensive researches have been devoted to investigate the fusion dynamics in laser-cluster interactions and the temporal and spatial characterizations of fusion neutron emission, as well as to search for higher neutron yields ͓6-19͔. The effects of the ͑D 2 ͒ N cluster size, the laser energies, and focusing conditions were studied by Zweiback et al. to optimize fusion neutron yields ͓7͔. However, the average kinetic energies ͑KEs͒ of deuterons from explosion of ͑D 2 ͒ N clusters were reported to be in the range of 2.5-7 keV ͓6,7,13-15͔ which are still much lower than the optimal KEs in the range of 40-100 keV for an efficient DD fusion.Last and Jortner proposed a scheme to enhance the deuterons' KEs by using clusters of heteronuclear deuterium containing molecules, e.g., ͑D 2 O͒ N and ͑CD 4 ͒ N ͓9,10,12,16,17͔. For the Coulomb explosion of the heteronuclear clusters, the light deuterons' KEs can be greatly enhanced due to kinematic overrun effect and the energetic boosting caused by the large ionic charge of the heavy ions inside the cluster ͓12,16,17͔. Grillon et al. used deuterated methane clusters ͑CD 4 ͒ N as a novel target in a table-top nuclear fusion experiment, demonstrating a conversion efficiency of about 1 ϫ 10 4 neutrons/ J of incident laser energy ͓11͔. Meanwhile, an independent theoretical work on ͑CD 4 ͒ N made by Last and Jortner predicts that the neutron yields with the heteronuclear clusters are 3.7ϫ 10 5 neutr...

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We created a sharp plasma pinnacle structure for localized electron injection and controlled acceleration in a laser wakefield accelerator. The formation of this shockwave-based pinnacle structure was investigated using aerodynamic theory. Details and scaling laws for the shockwave angle, shock position, shock width, and density ratio were experimentally and theoretically presented. Such work is crucial to yielding an expected plasma density distribution in a laserplasma experiment but has had little discussion in the literature. Compared with the commonly used shock downramp structure, the particle-in-cell simulations demonstrated that the e beam injected in the created pinnacle structure could be accelerated to higher energy with much smaller root-mean-square relative energy spread. Moreover, this study indicated that the beam charge and transverse emittance can be tuned by the shock angle.

The adaptive optical system of a TW chirped pulse amplification laser based on Ti:Sapphire was optimized considering the wavefront distortion due to the focusing system itself. A much smaller focal spot was obtained experimentally, and the energy concentration was improved. The size of the focal spot decreased from 2.3 times the diffraction limit to 1.17 times, and more than 50% of the energy was concentrated in the Airy disk. After optimizing the adaptive optical system, we generated hot electrons using a 4 TW laser system under a relativistic power intensity condition. The proposed laser is expected to offer a better experimental condition for laser-matter interaction.

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