11月11日 Quasi-interchange instabilities and the sawtooth crash

　　报告时间: 11月11日14:00

　　地点：EAST大厅二楼会议室

　　报告题目：

　　Quasi-interchange instabilities and the sawtooth crash

　　报告人简介：

　　Linda E. Sugiyama

　　Laboratory for Nuclear Science

　　Massachusetts Institute of Technology, Cambridge MA USA

　　Dr. Linda E. Sugiyama received a Ph.D. in Applied Mathematics from the Massachusetts Institute of Technology in 1980, working with Bruno Coppi and C.C. Lin. She was a post-doc and then research scientist at MIT in plasma physics, working on a wide variety of subjects, with an emphasis on magnetically confined plasmas for fusion energy. Her research combines numerical simulation and theoretical interpretation with an interest in the description of complexity using fluid and kinetic plasma models. She proposed the importance of nonlinear two-fluid effects for magnetically confined plasmas and developed the first numerical model in the M3D code. She has worked for many years at the forefront of large-scale numerical simulation with extended MHD models and their application to the interpretation of experimental observations.

　　报告摘要：

　　Sawtooth crashes that flatten the central temperature and current are common in toroidal magnetically confined fusion plasmas with central magnetic safety factor q near unity. In cases with significant magnetic shear, where qo on the magnetic axis falls well below unity, magnetic reconnection is the dominant process, driven by the resistive m/n=1/1 internal kink mode. When qo is close to unity, the magnetic field lines across q≤1 are closely aligned and quasi-interchange (QI) instabilities dominated by plasma flows that transport plasma across the magnetic field line can drive a different type of sawtooth where the temperature and current can be flattened with minimal magnetic reconnection. Nonlinear numerical simulation with the extended MHD code M3D demonstrates for the first time that QI instabilities with qo<1 can produce a complete sawtooth crash that resembles experimental observations and reproduces the characteristic differences observed between QI and internal kink crashes. Central flux surface shaping (ellipticity with weak triangularity) and higher beta increase instability and help overcome the pre-crash saturation predicted by previous theories and simulation. The instability has many unique properties, including true nonlinearity, where multiple toroidal harmonics n=1,2, and higher can grow together coherently at small amplitude. The crash is nearly independent of resistivity, but can still develop magnetic islands. The QI flows drive a low level of magnetic stochasticity. The results suggest that QI-type instabilities should also be considered in other cases with low magnetic shear, as an alternative to magnetic reconnection.

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