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内容記述 |
The majority of known universe consists of the plasma. Stars, stellar and extragalactic jets and the interstellar medium are examples of astrophysical plasmas. Moreover, a wide variety of plasma experiments have been performed in the laboratory. Since the particle dynamics in a plasma is governed by internal fields due to the nature and motion of particles themselves and by externally applied fields, it is very difficult to describe dynamics and predict behavior of real plasma systems. Complexity in plasmas is predominately characterized by the nonlinear excitation of an enormous variety of collective dynamical modes. In a plasma system there can be a rich interplay and coupling of these modes. It is possible to have modes with growing amplitudes, as a result of instabilities. Instability phenomena are important in a wide variety of physical situations involving dynamical processes in a plasma.<br /><br /> During a large amplitude electromagnetic (EM) wave propagation through a plasma, a large number of nonlinear processes may occur. Some applications are based on processes that occur during intense EM wave-plasma interaction, on the other hand, for certain applications many of these processes play a destructive role. In both cases, however, there is an interest to control and optimize responses of plasmas. There exist considerable interest in the study of relativistic laser plasma interaction relevant to a number of potential applications. These include: inertial confinement fusion (ICF), the simulation of astrophysical processes in laboratory, X-ray sources, nuclear reactions, properties of matter at very high density and pressure, acceleration of particles (relativistic electrons, MeV protons), RF plasma heating and current drive in tokamaks, etc. Baser fusion represents one of the most challenging goals in current energy research. An intense laser beam which penetrates a plasma is a source of free energy, and a plasma is able to efficiently convert laser energy into its modes. Some of the processes in a plasma can seriously affect laser beam and change the performance of laser-fusion target. Since fusion pellets are surrounded by large regions of coronal plasma, a general issue in laser fusion that has been of considerable interest in past decades is growth of instabilities in underdense plasmas. Experiments, theory and computer simulations agree on a possible complex interplay between various laser-plasma instabilties.<br /><br /> The study of parametric instabilities in laser plasmas is of vital importance. Most of these instabilities represent the resonant coupling of the intense EM wave to two other waves, in particular scattered EM wave and electrostatic (ES) wave (scattering instabilities). The excitation of scattering instabilities is through a positive feedback loop by which the beating between the EM field of the Baser and scattered light matches the frequency of longitudinal ES mode of the plasma. These instabilities are nonlinear wave-conversion mechanisms and can be very efficient in a plasma. Stimulated Raman and stimulated Brillouin scattering (SRS and SBS, respectively) are known as major processes that can bring about high reflectivity and undesirable target preheat to prevent efficient compression of the fuel. Quantitative prediction of the onset and saturation of these instabilities under given laser and plasma conditions is an important goal of laser-plasma research. However, although much effort has been devoted to this subject, observations and theoretical models are rarely in good agreement. Not surprisingly, any process that occurs in a plasma can alter substantially the dynamics of the nonlinear system and the behavior of instabilities.<br /><br /> In efforts to obtain deeper insight into nonlinear processes, computer simulations serve as a powerful research methodology and the present-day supercomputers offer a good base for investigating complexity from first principles. By creating mathematical models and running computer simulations one is able to explore a large number of possible initial conditions and to analyze the common features of the results. Although simulation systems are on space and time scales smaller than real ones, it can be sufficient to discover important plasma properties and to come to conclusions applicable to real systems.<br /><br /> Motivated by plasma complexity in the laser-fusion research, the goal of this thesis is to examine properties of a large-amplitude wave propagation in an underdense plasma by using relativistic theory and computer simulations.<br/ ><br /> As an illustration of laser parametric instabilities in underdense plasmas a standard model of large amplitude EM wave propagation through an unmagnetized cold electron plasma has been analytically considered. The relativistic hybrid dispersion equation that couples laser wave with two sidebands has been derived and solved for real values of the wave number in a broad range of electron densities and laser strengths. The dispersion equation predicts forward and backward stimulated Raman scattering instabilities (F-SRS and B-SRS, respectively) and relativistic modulational instability (RMI). In general RMI has lower growth rate than B-SRS and F-SRS instabilities. In contrast to SRS instability, RMI does not have a density restriction, i.e. it can appear in a whole underdense plasma. However, for large amplitude waves, Raman instability can be relativistically shifted to plasma densities beyond 0.25n cr (here n cr is the critical (EM wave cutoff) density). For high intensity EM waves RMI, B-SRS and F-SRS instability branches can merge.<br /><br /> Further, one-dimensional EM relativistic particle-in-cell simulations have been performed to study high intensity laser wave-plasma in a bounded plasma. The homogeneous plasma layers which are overcritical for SRS instability, placed in vacuum and driven by an intense laser light have shown some unexpected features. Namely, a strong reflection near the electron plasma frequency has been observed. It has been shown that the intense reflection and heating of the plasma layer are due to the novel electronic instability (Stimulated Electron-Acoustic Scatterings (SEAS) instability) that excites an electron-acoustic ES wave. The key features of SEAS can be summarized as follows: <br /><br /> ･ SEAS can be described as a three-wave parametric decay of the laser light (ω0,<br /> k0) into a scattered light (ωs,k s) and an electron-acoustic ES wave (ω a,k a).<br /><br /> ･ The scattered wave is driven near critical, i.e. ωs 〓 ωp which implies k s 〓 0 and<br /> Vs 〓 0 ( Vs 〓 0 is the group velocity). Therefore, the scattered wave is a <br /> slowly propagating (almost standing) EM wave. The dominant propagation of<br /> scattered EM waves is observed to be in backward direction.<br /><br /> ･ Since ωa 〓 ωb, the frequency and wave number of the electron-acoustic wave<br /> are ωa = ωo-ωs〓 ωo-ωp and ka = ko + ks 〓 ko, respectively. Therefore<br /> the phase velocity of this wave is υph〓 ωo-ωp / ko.<br /><br /> There is an evidence that SRS instability can assist (mediate) SEAS instability excitation and growth. Near the threshold intensity for SEAS instability, high electron temperatures can be essential for the instability growth. There is an optimum temperature when resonant SEAS instability gives a large reflectivity response. In general, SEAS instability produces strong heating and high reflectivities that exceed SRS reflectivities. A possibility of exciting intense trapped electron-acoustic ES waves in large inertial fusion experiment targets deserves more attention.<br /><br /> To further investigate SEAS features and clarify background of this instability, deeper considerations of relativistic nonlinear properties of electron-acoustic waves as well as nonlinear effects of EM wave condensation at the plasma frequency are expected. The slowly propagating backscattered wave which serves as an "attractor" to this instability, as it has been shown, seems to be a key factor for SEAS growth as an absolute instability. |
要旨・審査要旨 / Abstract, Screening Result (498.6 kB)
