MHD Simulation Of The Effect Of Shear Flow On DTM At High Magnetic Reynolds Number

Multi-current sheets can be widely observed in space and experimental system,such as solar wind,corona and the tokamak experiments with the reversed magnetic shear configuration.They can trigger double tearing mode instability.After the double tearing mode undergoes the two nonlinear evolution phases,in which two single tearing modes develop independently,the magnetic islands on the two rational surfaces will drive each other,a more intense magnetic field reconnection will happen and the energy of magnetic field will be quickly converted to the kinetic energy of the plasma.This instability greatly affects the stability of the experimental device.At high magnetic Reynolds numbers,the current sheet in the reconnection area becomes very long and thin due to the mutual interaction of two rational-surface magnetic islands.When the aspect ratio of the current sheet is greater than a certain value,the current sheet becomes unstable and generates secondary magnetic islands.The generation of secondary magnetic islands affects the topology of the magnetic field and also accelerates the release of magnetic field energy.It becomes crucial to develop a high-precision numerical simulation algorithm for studying the evolution of double tearing modes at high magnetic Reynolds numbers.We derived the perturbed MHD equation which based on the conserved MHD equation.By casting the numerical approach in the finite difference framework using a flux vector splitting method and a mapped Weighted Essentially Non-Oscillatory reconstruction scheme,we developed a high-precise MHD simulation algorithm.This algorithm can be used to simulate the evolution of MHD instability in linear and nonlinear phases.In order to make the program run efficiently on the computer in parallel,we developed a parallel program with the libraries of MPI and OpenMP,which is based on master-slave structure.The program uses MPI to communicate asynchronously,and uses OpenMP to parallelize calculations internally.The computing load is balanced according to the computing capacity of each computer.It can reduce the time that each computer waits for synchronization and improve the efficiency of parallel computing.Through MHD instabilities benchmark tests,we demonstrated the high accuracy of the program for simulating the large and small perturbations.The program was run on multiple nodes in the computing cluster,and its parallel acceleration effect was testedShear flow is very common in space and experimental plasma,which has an important effect on magnetic field reconnection.We added an anti-symmetrically distributed poloidal shear flow,and used numerical simulation to investigate the effect of shear flow on the evolution of double tearing mode,especially the secondary magnetic island process,at high magnetic Reynolds number.It is found that the weak polar shear flow has a stabilizing effect on both linear and nonlinear evolution of the double tearing mode,and the stabilizing effects increase with the flow velocity as well as flow shearing rate.When secondary magnetic island instability occurs,the shear flow will inhibit the inflow drive of the magnetic reconnection area,so that the number of secondary magnetic islands formed decreases with the increase of the shear flow velocity and the shearing rate.When the shear flow velocity reaches a threshold value,secondary magnetic islands instability can be stabilized.Therefore,introducing shear flow to the tokamak experiments with the reversed magnetic shear configuration can suppress the development of double tearing mode instability.