In this paper, fusion neutron yields from the Coulomb explosion of (CD4)N clusters under the irradiation of an intense femtosecond laser pulse is optimized by controlling the propagation of the laser pulse in a cluster jet. A correlated study of fusion neutron yields, kinetic energies of deuterons, together with the plasma channels diagnosed by a pump–probe interferometer, is performed. It has been found that by controlling the focal position related to the cluster jet, the plasma defocusing effect can balance with the tight focusing effect of the laser pulse induced by an off-axis parabolic mirror, and results in a cylindrical-shaped and relatively narrow plasma channel crossing the gas jet. The most energetic deuterons and the maximum yields of fusion neutrons are produced at the same time. For better understanding of the experimental results, numerical simulations for the nonlinear propagation of the femtosecond laser pulse in (CD4)N gas-cluster jets are performed. Simulated results show that plasma defocusing and laser attenuation play dominant roles in this case of optimization.
The Amazon is one of the few continental regions where atmospheric aerosol particles and their effects on climate are not dominated by anthropogenic sources. During the wet season, the ambient conditions approach those of the pristine pre-industrial era. We show that the fine submicrometer particles accounting for most cloud condensation nuclei are predominantly composed of secondary organic material formed by oxidation of gaseous biogenic precursors. Supermicrometer particles, which are relevant as ice nuclei, consist mostly of primary biological material directly released from rainforest biota. The Amazon Basin appears to be a biogeochemical reactor, in which the biosphere and atmospheric photochemistry produce nuclei for clouds and precipitation sustaining the hydrological cycle. The prevailing regime of aerosol-cloud interactions in this natural environment is distinctly different from polluted regions.