Particle Simulation Of Energy Deposition And Focusing Effects For Ion Beams In Plasmas

The interaction of charged particles with plasmas has been an interesting topic during the past decades due to its many important applications, such as the neutral beam injection (NBI) in the magnetically confined fusion plasmas, the inertial confinement fusion (ICF) driven by heavy ion beams and high-energy-density matter (HEDM). The NBI has been proven to be a highly successful means of plasma heating, in which fast injected neutral particles are converted into fast ions within the plasma after impact ionizations and charge exchanges, and has been widely used in large tokamak devices. The energy deposition and dynamic behavior of these produced fast ions are very important for plasma heating. In addition, the ion beam with high energy is proven to be an efficient tool to produce warm dense matter for HEDM investigations. For these applications, the ion beam should be simultaneously compressed in both transverse and longitudinal directions to small spot size and short pulse, respectively. Recently, simulations and experiments show that background plasma tends to neutralize the strong repulsive space charge force of beam ions and provides a good compression, which is important for ICF and HEDM. In this work, a self-consisent particle-in-cell (PIC) simulation method is adopted to investigate the wake field and energy deposition of a single ion and ion clusters (or beams) in magnetized two-component plasmas, taking into account the influences of external magnetic field, laser field, strong coupling effect between the injection ion and plasma, and correlation effects between clutser ions. We also investigate the focusing of low and high energy ion beams in background plasmas using electrostatic and electromagnetic PIC models, respectively.In this work, we first briefly review the research background and recent advances in the interactions of charged particles with plasmas, and then give the research purpose of current work in Chapter1. The rest of this work is presented as follows:In Chapter2, a linearized dielectric theory is adopted to investigate the potential and stopping power of injection ions in magnetized two-component plasmas, taking into account the dynamic polarization effects of both plasma ions and electrons. Our main concern here is the influence of magnetic field on the ion energy loss. It is shown that the energy loss of low energy ion beams and the dynamic polarization effects of plasma ions increase significantly as the magnetic field strength increases. In the case of strong magnetic field, the energy exchanges between low energy ion beam and plasma ions become important. Detailed information about the influences of different plasma parameters on the ion energy loss is also given in this chapter.As the interaction strength between the injection ion and plasma increases, the standard linearized theories fail and the nonlinear effects should be taken into account. In Chapter3, a self-consisent PIC simulation method is adopted to investigate the nonlinear interactions between the injection ions and plasmas. Firstly, the simulation results from a one-dimensional (1D) PIC model are compared to that of the linearized theory to show the nonlinear effects on the ion stopping power. The nonlinear effects are found to enhance the energy loss of low energy ion beams, while can be neglected for high energy ion beams. Further, the1D PIC model is extended to the two-dimensional (2D) case to investigate the influence of magnetic field on the wake fields excited by the injection ions. Simulation results show that, in the case of weak magnetic fields, the wake field exhibits the typical V-shaped cone structures and the cone opening angle is seen to decrease with the increasing ion velocity. In strong magnetic field case, the wake field profile becomes highly asymmetrical and loses their typical V-shaped cone structures. In addition, the stopping power calculated from the2D model is also compared with the results from1D and full three-dimensional cases.For large ion clusters or intense ion beams, in addition to the interactions between the beam ions and plasmas, one has to consider the interactions between the beam ions themselves, i.e., the correlation effects. In Chapter4, the time evolution and energy deposition of ion clusters in magnetized two-component plasmas are studied using2D PIC simulation method, with the main concentration on the influence of magnetic field. Numerical results show that the correlated effects, which are closely related to the distances between the beam ions, are found to enhance the energy loss of ion clusters significantly during the initial stage of cluster travelling through the plasma. In the case of weak magnetic fields, the distances between the beam ions increase rapidly due to the strong Coulomb force and the ion beam tends to deposite its energy near the initial injection position. On contrast, the ion cluster tends to deposite its energy more smoothly along its travel path in the strong magnetic field case, due to the slowly increase of distances between the beam ions.In Chapter5, the influence of laser field on the ion stopping power and plasma polarization is considered. Simulation results show that, as the laser frequency approaches the plasma frequency, the stopping power of injection ions reduces significantly, mainly due to the significant increase of electron temperature through energy absorption from the laser field.In Chapter6, a2D electrostatic PIC model is firstly adopted to study the modulation effects of a continuous ion beam with low energy in background plasmas. In the longitudinal direction, an oscillation wake field can be excited by the continuous ion beam and this oscillation wake field can further modulate the ion beam into periodic short beam pulses. The distance between theses pulses is shown to be determined by the plasma density and ion beam velocity. Finally, a2D electromagnetic PIC model is adopted to investigate the focusing of ion beam with high energy in background plasmas.