Experimental Studies On Nuclear Astrophysical Reactions In Plasmas Driven By High-intensity Lasers

With the rapid development of high-intensity laser technologies,laser nuclear physics has received considerable attention as a new research area.A focused peak intensity of over 1022 W/cm2 can be achieved in nowadays laboratory and the record is still being renewed.The high-intensity lasers bring various new physics phenomena including ac-celerated particle beams and nuclear reactions when they interact with matter.The com-bination of laser plasma physics with nuclear physics benefits not only the fundamental scientific research with novel ideas and methods,but also wide physical application fields.With high-intensity lasers,extreme physical conditions similar to those inside stars and supernovae can be created,which makes it possible to investigate various astrophysical processes in earth-based laboratories.Especially it can remove the uncertainty due to the electron screening effect in the measurement of cross-sections at low-energies.It is prac-tically the only known method for scientists to study nuclear reactions in astronomical environments.In this thesis,a novel method,wlaser plasma collider for nuclear studies" is reported for the first time.The concept of this method differs from existing schemes such as con-fined thermal fusion,Coloumb explosion,and two-beam lasers.Ions with a quasi-Maxwell-Boltzmann distribution are generated,and the effective Gamow energy can be adjusted in a certain range.The center-of-mass energy of nucleus is boosted in a head-on collision compared with the beam-target scenario.Most reactions have been found taking place in the low-density part of the plasmas,which are a close approximation to those ubiquitous collisionless plasmas in the cosmos.Several experimental campaigns were carried out at the Shenguang-? laser facility.In the first run,ns lasers with an intensity of 6 x 1015 W/cm2 were focused onto two CD targets.Neutrons with an energy of 2.45 MeV from 2H(d,n)3He reactions were observed by using time-of-flight technique,and the typical neutron yield is on the order of 105.Good repeatability has been found on a shot-by-shot basis under the same configuration.The spa-tial distribution of plasmas at a certain time point was measured by interferometry,whose results consist with the hydro-dynamics simulation.We confirm that the nuclear reactions were mostly contributed by collisions between deuterium ion pairs with center-of-mass energies of 27±10 keV from two plasma streams,while thermal fusions and reactions from cold targets could be neglected.Also,we have found a neutron yield enhancement by us-ing a K-shaped target compared with a plane-shaped one,since it can generate plasma jets with a higher density.We build a simplified numerical model to calculate the nuclear re-action rate,in which measured distributions of the plasma density and velocity were input as well as a set of D-D cross-section data from the evaluated nuclear database.The results show that the calculated neutron yield is higher than the experimental observation,and the astrophysical S factor is confined by one order of magnitude.The disagreement hints that the nuclear reactions in plasmas could be significantly modulated by their self-generated magnetic fields.D-Li fusions were also investigated by similar procedures.Monoenergetic neutrons with an energy of 13.3 MeV were produced in the experiment with a CD/LiF dual target.In another experimental run,we used a dual target filled with LiD powder to introduce D-D and D-Li reactions at the same time.Some of the systematic uncertainties have been eliminated by this relative measurement.The results have shown stable yield ratio of two reactions.With the ion velocity distributions derived from interferograms,the astrophysi-cal S factor of 7Li(d,n)8Be reaction around 128 keV is determined,which is the first result of this reaction measured in full plasma environment.This may lead to a better understanding of the problem of 6Li/7Li abundances in stellar nucleosynthesis with further investigations towards lower energies and other related reactions with enhanced precision.We also report a variation of standard neutron time-of-flight technique in which fast neutrons and delayed gamma-ray signals were both recorded in a millisecond time window in the harsh environment induced by the high-intensity laser.Delayed gamma signals,arriving far later than the original fast neutron and often being ignored previously,were identified to be the results of radiative captures of thermalized neutrons in the peripheral materials.The linear correlation between the gamma photon number and the fast neutron yield shows that these delayed gamma events can be employed for neutron yield estimation after calibration procedures.This method can reduce the detecting efficiency deterioration problem caused by the gamma bursts and provides a new way for neutron diagnostics in high-intensity laser-plasma interaction experiments.