Nuclear Reactions Generated From Intense Laser-plasma Interactions

Along with the progress of laser technology,interactions between high power laser and matters become a new scheme for nuclear physics.Recently,ion transportation and neutron generation in laser-driven nuclear reactions arouse a lot of concerning.In this thesis,we have discussed increased neutron yields in foam-like internal target struc-tures;and investigated effects of ion stopping power on nuclear reactions in plasma.We also designed an angular-resolved Thomson parabola spectrometer.It was employed to diagnose ion energy loss in plasma,and provide angular-distributed spectra for different ions to help analyze nuclear reaction rates.Meanwhile,we developed a real-time elec-tron spectrometer with an imaging system.It can help monitor laser energy absorption and reveal the physical mechanism for processes as particle acceleration and radiaction generation,in laser-solid interactions.In order to improve neutron yield,we studied neutron production driven by laser pulses in solid,near-critical density(NCD)and foam targets,by 2D particle-in-cell sim-ulations.We found that compared with solid and NCD targets,the neutron production from foam targets is more efficient because of significant enhancement of ion accel-eration and ion-ion collisions.This is due to the ambipolar electrostatic fields among the foam lamellas.The conversion efficiency from the laser energy to the energy of ions inside the foam target is up to 11%,12 times higher than the one in the solid target for the same laser parameters.We also find that a foam target with thinner lamellas and larger pores between the lamellas is more favorable for neutron production due to higher laser energy absorption and longer distance for ion acceleration.The number of the neutrons can reach 107 from a foam target with a thickness of only 30 ?m driven by a 1020 W/cm2 laser pulse.Nuclear reaction dynamics in plasma environment is critically important for re-search of astrophysical reactions and design of inertial confinement fusion(ICF),which is much more complicated compared with matters in ambient temperature and pressure.Corrections of effective reaction cross-sections and ion stopping power in plasma are needed to analyze the reactivity.We have experimentally investigated interactions be-tween deuteron beams and deuterated polystyrene(CD)plasma,driven by two laser pulses respectively.By adjusting the time delay between the two pulses,D-D reactions(D+D?3He+n)in plasma with different density,temperature and ionization degrees can be obtained.The plasma density,angular-resolved deuteron energy spectra and neutron angular distribution have been measured.It is shown that the neutron yields in plasma conditions are enhanced compared with solid states.We find the energy loss of moderate energetic deuterons(less than 2.5 MeV)in the CD plasma reduces,which can lead to higher D-D reaction rate.The experimental scheme can also enable cross-section diagnosis for fusion reactions in plasma environment,by measuring ion stopping power and plasma parameters together with the product yield on a single shot.Ion transportation and stopping power is critically important for nuclear reaction dynamics.In order to study ion stopping in plasma target,a multi-channel Thomson parabola spectrometer was designed and employed to diagnose ion beams driven by intense laser pulses.Angular-resolved energy spectra for different ion species can be measured in a single shot.This novel compact design provides a comprehensive char-acterization for ion beams.Experimental measurement of energy-dependent angular distributions of target normal sheath accelerated proton and deuterons were demon-strated.Hot electrons are the sources for most of the secondary processes in laser-plasma interaction,such as energetic particles and electromagnetic radiations.It is important to monitor hot electrons in experiments to learn about laser focusing conditions and en-ergy coupling efficiency.A real-time electron spectrometer with an imaging system has been designed and employed to diagnose escaping electrons from high-repetition-rate laser-solid interactions.From the image of the signal on the scintillator the electron energy spectrum can be measured on line,saving time costs for image plate scanning.In physical experiment,we found the escaping electrons were affected by efficient laser intensity after self-focusing in the pre-plasma.As a result,the maximum electron tem-perature occurs at some level of positive defocusing(the target towards the laser beam).These results can help as a reference of fast electron adjusting and controlling,to mod-ulate the following ion and radiation sources effected or generated by electrons.