The plasma sheath is essential for understanding the plasma-material interaction (PMI) since it regulates the plasma particle and energy fluxes to the wall. The key concept in sheath theory is the Bohm criterion that gives the lower bound of the plasma exit flow speed, also known as the Bohm speed. Traditionally, the Bohm speed is evaluated in the asymptotic limit of an infinitely thin sheath and ignores the transport physics in the plasma-sheath transition problem. Whereas in practical applications, the sheath has a finite thickness and the transport in the neighborhood of the sheath entrance is complicated. The focus of this thesis is on performing Bohm speed analysis for different applications that are away from the asymptotic limits, with an emphasis on the critical role of transport physics on the Bohm speed formulation. The classical sheath problem with a wide range of Coulomb collisionality is revisited. Here, we derive an expression for the Bohm speed from a set of anisotropic plasma transport equations. The thermal force, temperature isotropization and heat flux enter into the eval- uation of the Bohm speed. Away from the asymptotic limit, it is shown that there exists a plasma-sheath transition region, rather than a single point at the sheath entrance. In the transition region, the quasineutrality is weakly perturbed and the Bohm speed is predicted for the entire transition region. By comparison with kinetic simulation results, the Bohm speed model in our work is shown to be accurate in the sheath transition region over a broad range of collisionality. The Bohm speed analysis developed above can be applied to plasma-sheath transition prob- lems with more complex transport physics, such as a high recycling divertor in a fusion reactor. In the high recycling regime, the plasma particles hitting on the divertor surface will be recycled through reflection or desorption and return to the plasma in the form of neutrals. The plasma will interact with the recycled neutrals through atomic collisions such as ionization, excitation, or ion charge-exchange collision, complicating the plasma transport in the transition layer. A new Bohm speed model is proposed to account for the effect of the anisotropic transport and atomic collisions in the transition layer. A first principle ki- netic code VPIC with the atomic collision package is used to investigate a 1D self-consistent slab plasma with a high recycling boundary for tungsten and carbon divertors. The results demonstrate the accuracy of the Bohm speed model in predicting the ion exit flow speed in the transition region, as well as the reduction of the Bohm speed due to the ion-neutral friction. / Doctor of Philosophy / Controlled thermal nuclear fusion is a promising candidate for future energy supply. In a fusion reactor, a vast amount of energy is created and confined in the main plasma, while the boundary plasma can carry a certain amount of energy from the main plasma and deposit it on the surface of the plasma-facing component (PFC) of the reactor. The edge plasma and the material surface are strongly coupled through the plasma-material interaction (PMI). It is widely understood that PMI is a critical issue in realizing controlled thermonuclear fusion. The PMI problem involves complex physics phenomena that cover a wide range of spatial and temporal scales, posing a significant challenge to its modeling. This work mainly focuses on physics at the intermediate scale, where sheath/presheath physics dominates. The plasma sheath is a thin, positively charged layer that forms in front of the material surface to equalize the electron and ion fluxes. In classical sheath theory, an idealized point, the sheath entrance, connects the quasineutral plasma and non-neutral sheath. The ions can be accelerated by the presheath electric field and reach the Bohm speed (equal to the sound speed in classical sheath theory) at the sheath entrance. That is the Bohm criterion, a necessary condition for a stable sheath to form. The plasma sheath in a fusion reactor is exposed to a complex environment where the atoms and molecules are abundant and can interact with the plasma inelastically. As a result, many assumptions made in the classical sheath theory may not be valid for practical applications, such as a divertor sheath. The classical sheath theory is derived in the asymptotic limit of an infinitely thin sheath. In a real plasma, a sheath transition layer, rather than a singular sheath entrance, exists, and it connects the plasma and sheath smoothly. In the transition region, the quasineutrality is weakly perturbed, and the plasma transport is significant. Previous evaluation of the Bohm speed invokes drastic simplification of the transport physics, resulting in a Bohm speed equal to the sound speed. Here, we propose a new Bohm speed model that considers the dominating transport phenomena-anisotropic transport and collisional transport. The Bohm speed analysis is performed in two cases:(i) a classical sheath problem with absorbing boundaries and (ii) a high recycling divertor where the plasma-neutral interaction is significant. In the first case, we extend the classical sheath analysis to a regime that is away from the asymptotic limit. The counterpart of important concepts in the two-scale analysis, such as the sheath entrance and Bohm speed, is established and well explained. The transport dependent Bohm speed model is derived from a set of anisotropic transport equations, where the heat flux, thermal force, and Coulomb collisional isotropization are considered. The model can predict the ion exit flow speed in the transition region over a broad range of Coulomb collisionality, as shown by comparison with the kinetic simulation results. The second case is more practical, where the Bohm speed analysis is performed at the edge of a fusion reactor. The plasma transport in the transition region is complicated by the plasma-neutral interactions. As a result, the Bohm speed model includes atomic collisions, such as ionization, excitation, and ion charge-exchange collision. Among all the collision processes, the ion charge-exchange collision has the most significant influence on the Bohm speed. It acts as a significant momentum sink for the ions and makes the Bohm speed subsonic in the transition region.

One major step needed for fusion to become a reliable energy source is the development of materials for the extreme conditions (high temperature, radioactivity and erosion) caused by hot plasmas. The main goal of the present study is to use and optimise ion beam methods (lateral resolution and sensitivity) to characterise the distribution of hydrogen isotopes that act as fuel. Materials from the test reactors JET (Joint European Torus), TEXTOR (Tokamak Experiment for Technology Oriented Research) and Tore Supra have been investigated. Deuterium, beryllium and carbon were measured by elastic recoil detection analysis (ERDA) and nuclear reaction analysis (NRA). To ensure high 3D spatial resolution a nuclear microbeam (spot size <10 µm) was used with 3He and 28Si beams. The release of hydrogen caused by the primary ion beam was monitored and accounted for. Large variations in surface (top 10 µm) deuterium concentrations in carbon fibre composites (CFC) from Tore Supra and TEXTOR was found, pointing out the importance of small pits and local fibre structure in understanding fuel retention. At deeper depths into the CFC limiter tiles from Tore Supra, deuterium rich bands were observed confirming the correlation between the internal material structure and fuel storage in the bulk. Sample cross sections from thick deposits on the JET divertor showed elemental distributions that were dominantly laminar although more complex structures also were observed. Depth profiles of this kind elucidate the plasma-wall interaction and material erosion/deposition processes in the reactor vessel. The information gained in this thesis will improve the knowledge of first wall material for the next generation fusion reactors, concerning the fuel retention and the lifetime of the plasma facing materials which is important for safety as well as economical reasons.

科学研究費補助金 研究種目:一般研究(B) 課題番号:90213339 研究代表者:上杉 喜彦 研究期間:1993-1994年度

Divertors are in-vessel, plasma-facing, components in magnetic-confinement fusion reactors. Their main function is to remove the fusion reaction ash (α-particles), unburned fuel, and eroded particles from the reactor, which adversely affect the quality of the plasma. A significant fraction (~15 %) of the total fusion thermal power is removed by the divertor coolant and must, therefore, be recovered at elevated temperature in order to enhance the overall thermal efficiency. Helium is the leading coolant because of its high thermal conductivity, material compatibility, and suitability as a working fluid for power conversion systems using a closed high temperature Brayton cycle. Peak surface heat fluxes on the order of 10 MW/m^2 are anticipated with surface temperatures in the region of 1,200°C to 1,500°C. Recently, several helium-cooled divertor designs have been proposed, including a modular T-tube design and a modular finger configuration with jet impingement cooling from perforated end caps. Design calculations performed using the FLUENT® CFD software package have shown that these designs can accommodate a peak heat load of 10 MW/m^2. Extremely high heat transfer coefficients (~50,000 W/(m^2 K)) were predicted by these calculations. Since these values of heat transfer coefficient are considered to be outside of the experience base for gas-cooled systems, an experimental investigation has been undertaken to validate the results of the numerical simulations. Attention has been focused on the thermal performance of the T-tube and the finger divertor designs. Experimental and numerical investigations have been performed to support both divertor geometries. Excellent agreement has been obtained between the experimental data and model predictions, thereby confirming the predicted performance of the leading helium-cooled divertor designs for near- and long-term magnetic fusion reactor designs. The results of this investigation provide confidence in the ability of state-of-the-art CFD codes to model gas-cooled high heat flux plasma-facing components such as divertors.

Controlled thermonuclear fusion may become an attractive future electrical power source. The most promising of all fusion machine concepts is called a tokamak. The fuel, a plasma made of deuterium and tritium, must be confined to enable the fusion process. It is also necessary to protect the wall of tokamaks from erosion by the hot plasma. To increase wall lifetime, the high-Z metal tungsten is foreseen as wall material in future fusion devices due to its very high melting point. This thesis focuses on the following consequences of plasma impact on a high-Z wall: (i) erosion, transport and deposition of high-Z wall materials; (ii) fuel retention in tokamak walls; (iii) long term effects of plasma impact on structural machine parts; (iv) dust production in tokamaks. An extensive study of wall components has been conducted with ion beam analysis after the final shutdown of the TEXTOR tokamak. This unique possibility offered by the shutdown combined with a tracer experiment led to the largest study of high-Z metal migration and fuel retention ever conducted. The most important results are:   - transport is greatly affected by drifts and flows in the plasma edge; - stepwise transport along wall surfaces takes place mainly in the toroidal direction; - fuel retention is highest on slightly retracted wall elements; - fuel retention is highly inhomogeneous.   A broad study on structural parts of a tokamak has been conducted on the TEXTOR liner. The plasma impact does neither degrade mechanical properties nor lead to fuel diffusion into the bulk after 26 years of duty time. Peeling deposition layers on the liner retain fuel in the order of 1g and represent a dust source. Only small amounts of dust are found in TEXTOR with overall low deuterium content. Security risks in future fusion devices due to dust explosions or fuel retention in dust are hence of lesser concern. / <p>QC 20170630</p>

The protection of plasma facing components from heat and particle overloads is paramount to ensure the operability and desired lifetime of magnetic fusion devices. The possibility of using external 3D magnetic perturbations to improve the steady-state heat exhaust in diverted tokamaks has been studied in this thesis. This approach involves producing a controlled stochastic region in the plasma edge without significantly affecting the core of the plasma. Using field line tracing and 3D advection-diffusion heat transport models, the resulting magnetic and heat flux footprints on the divertor have been analyzed. An optimized configuration has been obtained, which reveals the potential of this approach for considerably reducing the peak heat load on the divertor. / Att skydda plasmakomponenter mot höga värmeflöden och snabba partiklar är av största vikt föratt säkerställa funktionsduglighet och önskad livslängd för en magnetisk fusionsreaktor. Möjlighetenatt använda externa 3D-magnetiska störningar för förbättrad statisk värmeavledningeni tokamaker med magnetiska avledare har studerats i denna avhandling. Tillvägagångssättetinnebär att man producerar en kontrollerad stokastisk region i plasmakanten utan att väsentligtpåverka plasmakärnan. Med hjälp av fältlinjespårning och 3D-modellering av värmetransportsom en advektions-diffusionsprocess har de resulterande magnetiska fotspåren och värmeflödetpå avledaren analyserats. En optimerad konfiguration har erhållits, vilket visar potentialen i dettatillvägagångssätt för att avsevärt minska den maximala värmebelastningen på avledaren.

科学研究費補助金 研究種目:一般研究(A) 課題番号:01420044 研究代表者:高村 秀一 研究期間:1989-1991年度

A key challenge facing the nuclear fusion community is how to design a reactor that will operate in environmental conditions not easily reproducible in the laboratory for materials testing. Finite element analysis (FEA), commonly used to predict components’ performance, typically uses idealised geometries. An emerging technique shown to have improved accuracy is image based finite element modelling (IBFEM). This involves converting a three dimensional image (such as from X ray tomography) into an FEA mesh. A main advantage of IBFEM is that models include micro structural and non idealised manufacturing features. The aim of this work was to investigate the thermal performance of a CFC Cu divertor monoblock, a carbon fibre composite (CFC) tile joined through its centre to a CuCrZr pipe with a Cu interlayer. As a plasma facing component located where thermal flux in the reactor is at its highest, one of its primary functions is to extract heat by active cooling. Therefore, characterisation of its thermal performance is vital. Investigation of the thermal performance of CFC Cu joining methods by laser flash analysis and X ray tomography showed a strong correlation between micro structures at the material interface and a reduction in thermal conductivity. Therefore, this problem leant itself well to be investigated further by IBFEM. However, because these high resolution models require such large numbers of elements, commercial FEA software could not be used. This served as motivation to develop parallel software capable of performing the necessary transient thermal simulations. The resultant code was shown to scale well with increasing problem sizes and a simulation with 137 million elements was successfully completed using 4096 cores. In comparison with a low resolution IBFEM and traditional FEA simulations it was demonstrated to provide additional accuracy. IBFEM was used to simulate a divertor monoblock mock up, where it was found that a region of delamination existed on the CFC Cu interface. Predictions showed that if this was aligned unfavourably it would increase thermal gradients across the component thus reducing lifespan. As this was a feature introduced in manufacturing it would not have been accounted for without IBFEM.The technique developed in this work has broad engineering applications. It could be used similarly to accurately model components in conditions unfeasible to produce in the laboratory, to assist in research and development of component manufacturing or to verify commercial components against manufacturers’ claims.

科学研究費補助金 研究種目:基盤研究(B)(2) 課題番号:11480113 研究代表者:上杉 喜彦 研究期間:1999-2001年度