2006 Fiscal Year Final Research Report Summary
Statistical Picture of hysteresis characteristics of transition phenomena in steady state current drive plasma
| Project/Area Number |
15206105
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| Research Category |
Grant-in-Aid for Scientific Research (A)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Nuclear fusion studies
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| Research Institution | KYUSHU UNIVERCITY |
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Principal Investigator |
ZUSHI H. KYUSHU UNIVERCITY, Research Institute for Applied Mechanics, Professor, 応用力学研究所, 教授 (20127096)
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| Co-Investigator(Kenkyū-buntansha) |
HANADA K. KYUSHU UNIVERCITY, Research Institute for Applied Mechanics, Professor, 応用力学研究所, 教授 (30222219)
NAKAMURA K. KYUSHU UNIVERCITY, Research Institute for Applied Mechanics, Professor, 応用力学研究所, 教授 (30117189)
SAKAMOTO M. KYUSHU UNIVERCITY, Research Institute for Applied Mechanics, Associate Professor, 応用力学研究所, 助教授 (30235189)
IDEI H. KYUSHU UNIVERCITY, Research Institute for Applied Mechanics, Associate Professor, 応用力学研究所, 助教授 (70260049)
YAGI M. KYUSHU UNIVERCITY, Research Institute for Applied Mechanics, Professor, 応用力学研究所, 教授 (70274537)
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| Project Period (FY) |
2003 – 2006
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| Keywords | Transition Phenomenon / Hysteresis / current drive / Threshold power / PWI driven relaxation oscillations / Electron cyclotron current drive |
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| Research Abstract |
The summary of this research is as follows.
In the current drive plasma, various transition phenomena have been investigated. Firstly, the transition for the confinement (∝ plasma stored energy/input power) and current drive efficiency (∝ current^★density^★major radius/power) is studied with view point of hysteresis. It is found that there exists a power threshold above which a transition is triggered. At the transition, both ion and electron temperature are increased and the density is also increased. Since driven current is raised, current drive efficiency is enhanced. The transition probability is also studied. The driven power is varied with a time, much longer than the confinement time, and the transition characteristics below the threshold power to above it, called forward-transition. Backward- transition is also studied from the "Enhanced current drive" mode achieved above the threshold down to the L-mode below the threshold power. The transition probability is defined as a delay
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time with which a transition is delayed with respect to a change in power. This transition probability is studied for both forward and backward transitions, and it is found that there is a critical difference between then and this difference leads to a hysteresis.
Secondly, a transition in current drive property is studied for bi-directional lower hybrid current drive operation. In this experiments, the forward LHW and backward LHW are superposed and the directivity of the driven current is studied. Namely, the forward current is driven by FW LHW, but the backward current is driven by the BW LHW. The experiments show that a transition of the current directivity from the BW direction to FW direction as a function of the BW/FW power ratio. This transition is triggered when this power ratio exceeds a critical value of 〜 0.8. It is found that the current density profile becomes narrow when the BW-LHW is superposed and the power ratio exceeds above the critical one, and then the transition of the current profile changes from the centrally peaked to a broad one suddenly. A hysteresis is not taken for this experiments.
Thirdly, a transition is studied in a long pulse discharge from a view points of plasma wall interaction. Oscillations at the very low frequency are found for the first time. This is called ULF events. In the long pulse operation, there are several processes which dominate discharge characteristics. The input rf power is launched from the out board side via a tunneling process in front of the rf antenna. The rf coupling to the plasma depends strongly on the density gradient in this region. During the long pulse discharge, the heat load from the plasma and escaped energetic electrons are deposited on the wall and antenna. The surface temperature rise changes the wall pumping property of the plasma facing components. Thus the edge or SOL density may vary in the discharge. Then this perturbation changes the reduction of the coupling or increment in the reflected power. A transition occurs during the ULF event, and then the five hour discharge is terminated. The heat load and particle perturbation grow rapidly from the dominant PFC (rail limiter) around the torus and finally they collapse the discharge. The related events are hot spot formation which may caused by the escaped energetic electrons. The local temperature on the hot spot is measured with the IR spectrometer. The hydrogen Balmer and Faulcher band are studied as a function of the surface temperature. It is found that both emission lines start to increase non-linearly as the hot spot temperature exceeds a critical values of 〜2100 K and they are sharply increased with increasing temperature and turns over even when the temperature still increases, and finally they decay to the previous level at the lower temperature. Less
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