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.

No abstract

Laser wakefield acceleration of electrons well beyond 1 GeV and optical guiding of ultraintense laser pulses of peak powers up to 160 TW over a 4-cm long ablative capillary discharge plasma channel were experimentally demonstrated. Electron beams, with energies up to 1.8 GeV, were generated by using the 130 TW, 55 fs driving laser pulses. A comparison of oxygen-containing acrylic resin (C:O:H = 4:2:7) capillary and no oxygen-containing polyethylene (C:O:H = 1:0:2) capillary measurements suggests that the injection of electron into the laser wakefield is assisted by the ionization of oxygen K-shell electrons.

Millimeter wave and infrared spectra of the weakly bound dimer (CO)2 have been studied in low-temperature pulsed supersonic jet expansions. Twenty-five new millimeter wave transitions have been observed and assigned, mostly in the 78–107 GHz region. Combined with previous data, they enable the relative energies of most of the known rotational levels (28 out of 31) in the ground vibrational state (vCO=0) of the dimer to be determined with “microwave” accuracy (≲0.1 MHz). Four new subbands in the infrared spectrum have been assigned in terms of two new stacks of rotational levels in the excited vibrational state (vCO=1), one stack with K=0 and the other with K=1. Energies for these levels have been determined with “infrared” accuracy (≲10 MHz). These results contribute significantly to the considerable body of precise experimental information available for a system that is ripe for further theoretical investigation.

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...

No abstract

No abstract

The infrared spectrum of the weakly bound complex CO–N2 has been studied using a pulsed supersonic slit-jet and a rapid-scan tunable diode laser. A mirror system giving 182 passes of the laser through the jet helped to give improved spectra with lower effective rotational temperatures (≈0.5 to 4 K) and less interference by CO dimer transitions. In the case of the CO-paraN2 spin modification, for which only one subband was previously known, over 10 linked subbands were assigned in terms of three ground (vCO=0) state stacks of levels (with K=0 and 1), and 7 excited state (vCO=1) stacks (with K=0, 1, and 2). In the case of the more abundant form, CO-orthoN2, an excited bending state was observed for the first time. The infrared analysis relied on precise ground state energy level differences obtained from microwave data.