Compact ultrafast neutron sources via bulk acceleration of deuteron ions in an optical trap

A scheme for a quasi-monoenergetic high-flux neutron source with femtosecond duration and highly anisotropic angular distribution is proposed.This scheme is based on bulk acceleration of deuteron ions in an optical trap or density grating formed by two counter-propagating laser pulses at an intensity of-10^(16)W~cm^(2)in a near-critical-density plasma.The deuterons are first pre-accelerated to an energy of tens of keV in the ambipolar fields formed in the optical trap.Their energy is boosted to the MeV level by another one or two laser pulses at an intensity of-10^(20)W~cm^(2),enabling fusion reactions to be triggered with high efficiency.In contrast to previously proposed pitcher–catcher configurations,our scheme can provide spatially periodic acceleration structures and effective collisions between deuterons inside the whole target volume.Subsequently,neutrons are generated directly inside the optical trap.Our simulations show that neutron pulses with energy 2–8 MeV,yield 10^(18)–10^(19)n/s,and total number 106–107 in a duration-400 fs can be obtained with a 25μm target.Moreover,the neutron pulses exhibit unique angularly dependent energy spectra and flux distributions,predominantly along the axis of the energy-boosting lasers.Such microsize femtosecond neutron pulses may find many applications,such as high-resolution fast neutron imaging and nuclear physics research.

POD Analysis and Low-Dimensional Model Based on POD-Galerkin for Two-Dimensional Rayleigh-Bénard Convection

Direct numerical simulation based on OpenFOAM is carried out for two-dimensional RayleighBénard( RB) convection in a square domain at high Rayleigh number of 107 and Pr = 0.71. Proper orthogonal decomposition( POD) is used to analyze the flow and temperature characteristics from POD energy spectrum and eigenmodes. The results show that the energy spectrum converges fast and the scale of vortex structures captured by eigenmodes becomes smaller as the eigenmode order increases. Meanwhile,a low-dimensional model( LDM) for RB convection is derived based on POD eigenmodes used as a basis of Galerkin project of Navier-Stokes-Boussinesq equations. LDM is built based on different number of eigenmodes and through the analysis of phase portraits,streamline and isothermal predicted by LDM,it is suggested that the error between LDM and DNS is still large.