Title: Quasi-interchange instability and the Sawtooth Crash
Speaker: Linda Sugiyama ，Laboratory for Nuclear Science，Massachusetts Institute of Technology， Cambridge MA U.S.A.
Place: Bldg11-413，Yuquan Campus (玉泉校区教11-413室)
Date and time: Tuesday 2pm， 10/29/2019（10月29日周二下午2：00）
Magnetically confined toroidal plasmas such as tokamaks often develop periodic crashes of the central temperature and plasma current. In fusion plasmas, many are caused by magnetic reconnection produced by a resistive internal kink instability with toroidal and poloidal harmonics n=1 and m=1, over the region where the magnetic winding number 1/q < n/m = 1. In other cases, where the central q is near unity and the magnetic field lines are closely aligned, quasi-interchange (QI) modes dominated by plasma flows across the magnetic field are able to interchange the plasma in adjacent magnetic flux tubes with little change of the field. New nonlinear 3D MHD simulations explain many properties of the QI instability and show that it can produce complete central sawtooth crashes. The results reproduce many experimental observations. The QI instability at q=1 is also the first example of a truly nonlinear mode in a toroidal plasma, where multiple harmonics n=1,2, and higher couple together and grow as a single coherent instability. The evolution is nearly independent of the resistivity at the low values typical of fusion plasmas, but magnetic reconnection and magnetic islands still occur. Interchange flow processes, rather than reconnection, may occur in other plasmas that have regions of nearly aligned magnetic field lines, such as solar and space plasmas.