2012 Fiscal Year Final Research Report
Intermittent events dominated by meso-scale physics and control of a macro system in steady state spherical tokamak
| Project/Area Number |
21246139
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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 University |
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Principal Investigator |
ZUSHI Hideki 九州大学, 応用力学研究所, 教授 (20127096)
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| Co-Investigator(Kenkyū-buntansha) |
SHIKAMA Taiichi 京都大学, 工学研究科, 講師 (80456152)
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| Project Period (FY) |
2009 – 2012
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| Keywords | メゾスケール / blob / fast camera / PDP |
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| Research Abstract |
This research is aiming at the subject associated with "Intermittent events dominated by meso-scale physics and control of a macro system in steady state spherical tokamak". Mainly, structure formation and its controllability under a process from an open magnetic field configuration to the closed one, contact to two closed magnetic surfaces, and open field region outside the closed one are of interest.During the research period, 1) current start-up in tokamak using non-inductive method and 2) blob propagation crossing the SOL region, which extends from the closed flux surfaces to the open ones. These phenomena can be recognized as intermitted or off-normal events against the steady state operation of the tokamak plasma. This research aims at pointing out the characteristics of the meso-scale physics for the macroscopic structure formation under the microscopic turbulence. The evaluation for the coupling coefficient between the meso-scopic and macroscopic systems will be investigated by
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evaluation of a phase relaxation time. Finally we would like to establish control methods for steady state tokamak operation. During H21-23 experiments for rf driven non-inductive plasma production and sustainment have been performed in QUEST and CPD. The followings have been achieved.
(1) Using 8.2 GHz ECWs at ~ 100 kW, non-inductive current start-up and current sustainment has been successfully demonstrated at 35 kA, which is the world record. High poloidal equilibrium at ・・p =1.5 and ・<1(triangles) has been obtained. The maximum duration of 300 sec is also performed for ST configuration. During the formation phase from the open to closed surfaces, several diagnostic tools have been applied and then the following physical picture could be drawn. The chamber aspect ratio is small enough for several harmonics to exist inside the plasma, and it has been found that the MOMFC(mirror ratio depending on the Bzcurvature) and the amplitude of Bz/Bt contribute to the fast ramp-up of the current, because ofthe initially born energetic electrons of 10 keV could be well confined in the open magnetic fields. The high ・・p~ 1 plasma scenario has been demonstrated at high MOMFC=2 and Bz/Btof 10 %. Ipof 16 kA at the high current ramp-up rate of 0.3 - 0.5 MA/s was achieved non-inductively. A natural inboard poloidal field null appeared as MOMFCincreased and this equilibrium could be sustained for ~ 10 seconds. Based on the fluctuation measurements it has been observed that the long correlation length along the magnetic surfaces has been measured when a closed surface is formed. Thisindicates that the phenomenon dominated by a mesoscopic scale occurred during the topological change.
(2)The second aspect addressed in this research isthe statistical approach of the SOL turbulence, plasma transport, blob propagation.This is aimed at identification ofthe generation region of the blob by charactering the pdf of the fluctuations, two dimensional structure of pdf and statistical aspect, blob propagation and acceleration, and flow pattern in the SOL and its impact on the turbulent transport. These observations have been carried out by using the wide viewing fast camera imaging and local probe measurements. Statistical aspects of the convective transport with respect to the variation in magnetic field pitch are dwelled upon. Progressive enhancement of fluctuations and consequent blob generation and propagation are observed with the increase in Bzat a constant Bt. Amplitude and waiting time of the blobs attains a maximum for highest Bz/Bt(=7.8%). 2D statistical analysis of the images enables us to identify blobformation location precisely at the steep density gradient region. Accelerated radial propagation was observed for large sized blobs. The effect of mirror ratio on turbulence is studied with the change in poloidal field curvature. Fluctuation characteristics are quite different for magnetic shears. The region characterized by the deviation of pdf from the Gaussian to gamma can be considered as a typical scale, with which the mesoscopic phenomenon dominates. A new diagnostic and analytical method have been developed to evaluate the plasma flow using the fluctuations. Less
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Research Products
(45 results)
