Research projects

磁場閉じ込めプラズマ核融合炉の実現のためには，炉心プラズマから周辺領域に流出する膨大な粒子流および熱流を制御し，直接プラズマと接触するプラズマ対向材(ダイバータ板)の損傷を低減することが最重要課題の一つとなっている。熱負荷低減法として，非接触再結合プラズマ（Detached Recombining Plasmas）形成による熱流制御法の確立の重要性が指摘されている。本研究では，直線型高密道プラズマ発生装置を用いた実験と流体コード，中性粒子輸送コードをカップリングした統合コードによる解析により，非接触再結合プラズマの生成機構と制御手法を確立し，核融合発電の早期実現に資する。

本研究は、核融合・高エネルギー加速器に不可欠な高周波(RF)負イオン源の根本的性能限界である悪いビーム収束性の原因を解明し、その革新的改善策を提示することを目的とする。RF負イオン源でビーム収束性が悪い理由として、RF電磁場による引出界面の振動や高い負イオン温度が指摘されている。そこで本研究では、RF負イオン源のプラズマ生成部から負イオン引出・加速領域を総括的にモデル化し、シースや引出界面の形成過程と負イオンビーム引出領域における負イオン輸送過程を解析する。RF負イオン源でビームの収束性を制限する物理的要因を突き止め、高性能RF負イオン源の設計指針を提示する。

内部コイルを用いてダイバータ配位の環状プラズマを生成し、その境界層におけるプラズマ流や不純物イオンの流速などを詳細に計測する。三角形度などプラズマ断面形状を様々に変えた実験を実施し、ダイバータプラズマシミュレーションコードの結果と比較する。これにより、プラズマ流に対するイオンドリフトの効果を明らかにするとともに実験的な裏付けのある不純物輸送モデルを提案する。

To understand the dynamic characteristics of the detached divertor plasmas in fusion reactors, an integrated model has been developed by coupling a particle-in-cell (PIC) model solving for the plasma transport with a 0-dimensional model solving for the time evolution of the space-averaged atomic and molecular density. The integrated model tentatively reproduced the detached divertor plasma. The time evolution of the divertor plasma after the high energy pulse injection and the energy loss due to the interaction with neutral particles were analysed. The basis of a simulation model was established to clarify the dynamic characteristics of the detached divertor plasma.

In this study, the cause of the increase in electron current, which is one of the isotope effects in a deuterium negative ion source, was investigated using both an analysis with a zero-dimensional model that takes into account the atomic and molecular collision processes between hydrogen and deuterium, and a numerical simulation for electron transport analysis (KEIO-MARC code). First, for the zero-dimensional model, the results shows that the plasma density is approximately 1.7 times higher for deuterium due to differences in the effective reaction rate coefficients for ionization of hydrogen and deuterium and the transport loss time constant of positive ions. Similar results are also obtained with the KEIO-MARC code. From the above, it is made clear that the increase in electron current due to deuteration is caused by differences in the effective reaction rate coefficients for ionization and the effective confinement time of positive ions.

プラズマプロセッシングや核融合研究において重要な課題となっている気相中で消えるプラズマの物性を明らかにする。電子・イオン再結合（三体再結合，放射再結合）が主要な過程となる再結合プラズマ中の非熱平衡性，電位構造に関連した熱・粒子輸送過程を，直線型高密度プラズマ発生装置を用いた基礎実験とシミュレーションとの相補的な研究により明らかにする。また，振動励起水素分子により駆動される再結合（分子駆動再結合）プラズマ生成に関して，固体壁で生成される振動・回転励起水素分子の影響と同位体効果を明らかにする。さらに，再結合プラズマのパルス応答を高時間分解で調べ，時空間変化を決定する物理・化学過程を明らかにする。

Divertor design appropriate for a DEMO reactor is a crucial for handling the large power exhaust. Advanced divertors, which modify the magnetic configuration by applying additional divertor coils, has been investigated both from the physics and engineering viewpoints. Calculation code for the advanced divertor configurations was developed. The concept of superconductor divertor coils installed inside the toroidal field coil was proposed, and their locations and currents were determined. Divertor plasma simulation for the advanced divertor has been performed under the DEMO conditions; the divertor plasma temperature became low (1 eV), producing fully detached plasmas efficiently, and the peak heat load was reduced by 30-50% of the conventional divertor. Future issues are clarified; technology and engineering for the interlink-coil and their fabrication are crucial. Improvements of magnetic and geometrical configurations and accuracy of the simulation such as diffusion are necessary.

The final goal of this study is to develop a new kinetic simulation tool of plasmas, neutrals, and impurities in fusion edge plasmas. Taking into account the non-equilibrium and non-steady characteristics of electron energy distribution function (EEDF), we have done a dynamic simulation of the ELM (Edge Localized Mode). The effect of the ELM on the plasma detachment has been studied. Especially. the effects of the ELM on the MAR (Molecular Activated Recombination) has been analyzed. The results show that the vibrational excited molecules, which play a key for MAR, are effectively produced during the ELM heat pulse. Due to this enhancement, MAR rate is possibly enhanced after the ELM pulse. In addition, a new kinetic impurity transport model has been successfully developed. The model includes neo-classical transport effects of impurities, which becomes important under the steep density/temperature gradient in the core pedestal region before and after the ELM pulse.

The detached divertor plasma, which is important for the heat and particle control in the divertor, has been analyzed by using the integrated divertor code in order to understand the physics mechanism of the divertor detachment. Based on the analysis, the characteristics of the divertor detachment in the simulation have been improved. Control of the detachment under the DEMO condition by the impurity gas puff, the divertor geometry and the machine specifications, has been also investigated.

Control of the detached divertor plasma and the resultant reduction of the divertor heat load are one of the most critical issues on the fusion DEMO design study. In this study, understanding of the detached divertor plasma has progressed by analysis of the experimental data and development of the divertor simulation code has progressed. Control of the detached divertor plasma and reduction of the divertor heat load by using impurity seeding has been also investigated.