Laser-induced Photonuclear Neutron And Plasma Collision Fusion Neutron Production Research

Since the invention of chirped pulse amplification(CPA)technology,the laser power density has been increased prominently,which greatly promotes the development of the field of laser-matter interaction,including subject of the laser-induced photoneutron source.Compared with traditional neutron sources,photoneutron source induced by ultra-short and ultra-intense laser have the characteristics of short pulse(tens of ps),high peak flux(1018?1021n/cm2/s),small size and can be table-top,which may play an important role in the fields of Fast Neutron Resonance Radiography(FNRR),neutron radiation damage and laboratory astro-nuclear physics in the future.In this paper,the generation and characteristics of laser-induced photoneutrons and the generation area of fusion neutrons in colliding plasma are studied in detail.The main work includes the following aspects:1.Using numerical and Monte Carlo(MC)simulation method,we have studied the influence of target material,size and parameter of electron beams on the photoneutron yield and neutron characteristics.Ponderomotive scaling of the hot-electron temperature and semi-empirical model of bremsstrahlung were used to calculate the photonuclear reactions induced in the converter by the transport of laser accelerated hot electrons at different laser intensities.It was found that the neutron yield of 208Pb was higher than that of 9Be when the intensity was higher than 5�1019W/cm2,and the yield tended to be saturated with the increase of intensity.The neutron yield is proportional to the repetition rate and irradiation time of laser.The generation of photoneutrons during the process of laser-solid interaction has been studied using MC code Fluka.The results show that Tungsten is the best material for photoneutron generation and there is different saturation thickness of neutron yield for different effective temperature of electrons.From simulations of different target size,it was found that the peak of neutron generation is located near the front 3 mm of the target and the size of neutron source is determined by the spread angle of electrons and the thickness of target.The ratio of flux in forward and side direction can be enlarged to 5 by increase the radius of the target.Stable energy spectra of photoneutron source can be obtained when electron slope temperature is higher than 4MeV with energy spectrum of Boltzmann distribution.The result of time distribution of neutrons show that the pulse duration is smaller than 30ps.2.In order to obtain higher neutron yield per shot,experiments for enhancing photoneutron production were carried out by using gas-solid hybrid target on the XGIII laser facility of Laser Fusion Research Center(LFRC)in Mianyang.The so-called gas-solid hybrid target was composed of a N2 gas jet generated by 1 mm diameter nozzle and a 2 cm thick Tantalum stack.Picosecond laser pulses first interact with the tenuous gas to increase the yield of energetic electrons through direct laser acceleration(DLA)and other mechanisms.Then through bremsstrahlung and subsequent(y,n)reactions in a Ta converter,photoneutrons were enhanced effectively and the total number of neutrons were optimized by changing the gas density.A maximum neutron yield of 4x107/shot over 4? was achieved which is 200 times higher than that in direct laser-solid interaction shot in our experiment.The spectrum of photoneutrons was measured which is agreement with Monte Carlo(MC)simulation.From the size of the reaction area and the neutron pulse duration inferred from simulation,the corresponding flux was calculated to be 1.2 � 1016n/cm2/s.3.We have carried out the experiment of photoneutron generation using laser wakefield accelerated electrons on high repetition 45TW laser facility.The parameters of laser and gas nozzle are measured offline.Electron beam satisfying the conditions of photoneutron generation is obtained through the laser wakefield electron acceleration experiment using N2.The neutron yields around 1 x105/sr/J is obtained by using a Tungsten target with a diameter of 1.5 cm and a thickness of 1 cm.Fluka simulation shows that the photoneutron yield of monoenergetic electrons increases with the electron energy,but the increasing trend gradually slows down.The conversion efficiency of laser energy to neutron number and neutron yield can be further improved by using higher power lasers.4.Proton imaging technology based on CR39 detector has been studied and established in order to investigate the space distribution of fusion neutrons generated in the interpenetrating of plasma flows.This method has been used in the indirect-drive inertial confinement fusion hohlraums experiment and double layer target experiment.By innovative use of the phenomenological model of track etching and intelligent fitting algorithm,the curve of the track size on CR39 with ion energy was obtained,and the response of CR39 to proton flux was analyzed.By using conical Titanium alloy pinhole plat,the generation region of fusion neutrons under kinetics effect in hohlraums was observed for the first time in experiment,and the neutron yield was given.The distribution information of neutrons is obtained by decomposition of proton distribution using deconvolution algorithm.It shows that neutrons mainly come from a large area near the surface of target sphere,which verifies the neutron generation mechanism.Finally,X-ray and proton pinhole imaging were performed on the fusion neutron generation region of the interpenetrating plasma flows based on biplanar targets.Clear images of self-generated X-rays of plasma and proton distribution were obtained.Neutron distribution larger than 3 mm x 3.8 mm and smaller X-ray region indicate that neutrons originated from the beam-target reaction caused by the interpenetration of plasma flows.The comparison of neutron yields of different target types also verifies this conclusion.