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内容記述 |
Understanding of the heat and particle transport processes is highly important since it determines the feasibility of controlled fusion. The particle transport in plasma can be analyzed by observing the motion of impurity ions introduced artificially into plasma, which are different species from the main plasma ions. This can be done by impurity pellet injection when the source profile of the impurity is created along t path of the pellet ablation. The pellet should be made of a loZwmaterial such as lithium in order to provide a fast and complete ionization of the injected impurity. This essentially simplifies the calculations as the source term in the particle balance equation can be neglected.<br /> Experiments with Li pellet injection into Heliotron E plasmas have brought important information about main characteristics of Li ions during the pellet ablation and also about the transport properties of Li ions after the injection. The transport coefficients have been calculated by simulating the experimental data by means of the impurity transport code,taking into account the impurity puffing from the wall and the impurity recycling. However, because of the non-local initial source of lithium ions, there is no clear transient process to analyze and the transport task requires an account of a variety of factors.<br /> As an accurate diagnostic for local particle transport, a new method of TECPEL(tracer-encapsulated cryogenic pellet) injection is proposed, which provides reliable information about transport in plasmas without the need for complex measurements of all the plasma parameters. With this method, the local transport is measured directly by observing evolution of a small particle source as tracer, which is initially localized within a limited plasma volume. The tracer particles are deposited locally by the TECPEL, which consists of a small core as the tracer of a light atom and the major outer layer of a frozen hydrogen isotope. When such a pellet enters the plasma, the outer layer is ablated first, which allows the tracer core to reach the central plasma region with a higher temperature. This results in a more intensive ablation of the core providing the necessary localization of the deposited tracer ions. It is shown that in this case, the diffusion coefficient can be calculated analytically from experimentally observed evolution of the impurity density.<br /> A device for production and acceleration of tracer-encapsulated cryogenic pellet has been constructed and tested experimentally. For a technical demonstration of the device operation, a 240μm diameter carbon sphere is encapsulated in a φ3×3mm cylindrical hydrogen pellet. The accelerated TECPEL is photographed from two perpendicular directions simultaneously. From the obtained images, the 3D geometry of TECPEL has been reconstructed and found to be consistent with the projected dimensions. Thus, the proof-of-principle of the device operation has been successfully demonstrated.<br /> As application for the existing fusion devices with a medium size plasma, an alternative configuration of the pellet made of non-cryogenic materials has bee proposed. The idea of the tracer-encapsulated solid pellet (TESPEL) injection will be still advantageous if the emission of the core ions can be observed on the background emission of the major pellet ions. From various TESPEL configurations tested, the most appropriate one has been selected in the form of a polystyrene shell (300μm diameter and 50μm wall thickness) contain in galithium hydride core (50μm diameter). This configuration has been experimentally real ized and the produced TESPELs have been successfully accelerated with conservation of the TESPEL's integrity. Calculation of the TESPEL ablation rate has demonstrated that the obtained TESPEL dimensions and achieved pellet velocities are appropriate for injection into a medium size plasma.<br /> For proving the essential concept of the new diagnostic method, a series of experiments with TESPEL injection has been carried out for the case of CHS plasmas. The light emission from the ablating pellet is registered in Hα and Li I (or Li II) lines with high time resolution, and the spatial resolution is provided by the CCD photography. From these measurements, the information is obtained about the exact location and width of the deposition of tracer material, and this confirmed that a good localization of the tracer has been achieved. The experimentally measured TESPEL ablation rate is compared with the calculation based on the impurity pellet ablation model, and a good agreement has been found.<br /> Behavior of the tracer ions deposited locally in the core plasma region was observed by CXRS method using the heating neutral beam as a source of neutral atoms for the charge-exchange reaction with Li3+ ions. The observed diffusion was found to be consistent with the theoretical calculations, and the experimentaldata have been simulated by means of the impurity transport code for obtaining the transport coefficients. The difference in the diffusion coefficient for various plasma configurations is discussed.<br /> Thus, a new diagnostic method for particle transport study with TESPEL has been experimentally implemented for the first time. The results from CHS have proved the new diagnostic concept from the both viewpoints of the production method of a tracer-encapsulated pellet and observation of the transport properties of the tracer particles. |
要旨・審査要旨 / Abstract, Screening Result (407.4 kB)
