The aim of the thesis is to examine plasma-wall interaction using computer modeling. Tokamak- relevant plasma conditions are simulated using the particle-in-cell model family SPICE working in three or two dimensions. SPICE model was upgraded with a parallel Poisson equation solver and a heat equation solver module. Plasma simulation aimed at synthetic Langmuir probe measurements were performed. First set considered a flush-mounted probe and the effect of variable magnetic field angle was studied with aim to compare existing probe data evaluation techniques and assess their operational space, in which the plasma parameters estimation via fit to the current-voltage characteristic is accurate. Second simulation set studied a protruding probe pin. Effective collecting area of such probe was investigated with intentions of density measurement collection. This area was found to be influenced by a combination of two factors. First, the density dampening inside the magnetic pre-sheath of the probe head, and the second, the extension of the area caused by Larmor rotation. A comparison with experimental results obtained at COMPASS tokamak was was performed, confirming these results. Keywords Langmuir probe, simulation, particle-in-cell, tokamak, Poisson equation, COMPASS 1

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第21069号 / 工博第4433号 / 新制||工||1689(附属図書館) / 京都大学大学院工学研究科電気工学専攻 / (主査)教授 山川 宏, 教授 松尾 哲司, 准教授 海老原 祐輔 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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Axisymmetric radial Bernstein modes are known to exist in non-neutral plasmas and have been studied theoretically and computationally in 1D, but detection of these modes has still proven to be difficult due to self-shielding. To help advance the work on this front we created a 2D particle-in-cell (PIC) code that simulates a non-neutral plasma in a Malmberg-Penning trap. A detailed description of the PIC code itself has been included that highlights the benefits of using an $r^2$--$z$ grid and how it can be tested. The focus of the PIC simulation was to discover how best to drive and detect these modes. While it is improbable that radial Bernstein modes will be detected in long plasmas, we show that it may be a possible due to the axial nodal structure in the potential and electric field generated by confining plasmas of any finite-length. Additionally, we find that for a short plasma the strongest detection signal along the trap wall occurs at the plasma's midpoint rather than near the ends. Results show that oscillating the confinement potentials is sufficient to excite the fundamental radial Bernstein mode, but not any of the higher order modes. The higher order modes can be seen in the simulation, however, by sinusoidally driving the radial electric field. Unfortunately, the individual modes are difficult to isolate which we suspect is due to mode mixing. Finally, we report frequencies and mode shapes for the fundamental mode and the (lower) first higher order mode.

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Solar wind-magnetosphere-ionosphere (SW-M-I) coupling is investigated with three different computational models that characterize space plasma dynamics on distinct spatial/temporal scales. These models are used to explore three important aspects of SW-M-I coupling. A particle-in-cell (PIC) model has been developed to explore the kinetic scale dynamics associated with the magnetotail dipolarization front (DF), which is generated as a result of magnetotail reconnection. The PIC study demonstrates that the electron-ion hybrid (EIH) instability could relax the velocity shear within the DF via emitting lower hybrid waves. The velocity inhomogeneity driven instability is highlighted as an important mechanism for energy conversion and wave emission during the solar wind-magnetosphere coupling, which has been long neglected before. The Lyon-Fedder-Mobbary (LFM) global magnetohydrodynamic (MHD) model is used to explore the fluid scale electrodynamic response of the magnetosphere-ionosphere to the interplanetary electric field (IEF). It is found that the cross polar cap potential (CPCP) varies linearly with very large IEF if the solar wind density is high enough. With controlled experiments of global MHD modeling driven by observed parameters, the linearity was interpreted as a result of the magnetosheath force balance theory. This study highlights the role of solar wind density in the electrodynamic SW-M-I coupling under extreme driving conditions. The LFM-TIEGCM-RCM (LTR) model, which is the Coupled-Magnetosphere-Ionosphere-Thermosphere (CMIT) model with Ring Current extension, is used to explore the integrated SW-M-I system. The LTR simulation study focuses on the subauroral polarization streams (SAPS), which involve both MHD and non-MHD processes and three-way coupling in the SW-M-I system. The global structure and dynamic evolution of SAPS are illustrated with state-of-the-art first-principle models for the first time. This study has successfully utilized multiscale models to characterize the forefront issues in the space plasma dynamics, which is required by the facts that plasmas have both particle and fluid featured properties and those properties are vastly different across geospace regions. It is highlighted that SW-M-I coupling could be significantly influenced by both microscopic and macroscopic processes. In order for a comprehensive understanding of the SW-M-I coupling, multiscale models and integrated framework of their combinations are critical. / Doctor of Philosophy / Three numerical models are used to explore the processes occurring in the Earth’s space environment from an altitude of ∼ 100 km to 10s Earth radii (R_E). This environment is mainly filled with plasma, the gaseous state of charged particles that collectively behave like a fluid and are also subject to complex electromagnetic interactions. The intrinsic features of plasma determine that the physics on the scale of charged particles and that on the scale of fluids are both very important. On the other hand, considering the vast differences in the plasma properties throughout space, different regions need to be represented by different physically-based models. This dissertation study addresses the processes on three distinct spatial/temporal scales with different models. A particle model that treats plasma as a group of charged particles is used to explore wave generation in the magnetotail (10s R_E in the nightside). It is found that inhomogeneous plasma flow in the sharp boundary layer at the magnetotail (called “dipolarization front”) can excite plasma waves to dissipate the energy originating from the solar wind (high speed plasma ejected from the sun). A magnetohydrodynamic (MHD) model that treats the plasma as a magnetized fluid is used to explore the efficiency of electric field mapping from the solar wind (10s R_E) to the ionosphere (∼ 100 km altitude). The electric field in the ionosphere usually linearly increases with solar wind electric field until it is too strong. An observational event showed that their relationship remains linear for very large driving field. MHD modeling experiments demonstrate that the linearity at large driving field is due to the high solar wind density, which is explained with force balance theory. An integrated model framework is used to explore the system level response of geospace by investigating the enhanced plasma flow in the subauroral ionosphere (called the subauroral polarization streams, SAPS). The generation of SAPS involves driving and feedback processes in different regions (magnetosphere, ring current, ionosphere) that can not be simulated with any individual model. The global structures and dynamic evolution of SAPS have never been explored before with first-principle characterization of the effects from the solar wind to geospace. This integrated modeling represents a state-of-the-art model framework to explore processes in coupled geospace. These studies illustrate that different models are necessary to explore fundamental physics on small and large scales and the coupling processes between different space regions. It is also suggested that incorporating the different models into an integrated framework is necessary to get a comprehensive understanding of the dynamics in geospace.