Accurate prediction of the sawtooth cycle[1]is an important test for nonlinear MHD codes.The sawtooth cycle in the CDX-U tokamak[2],chosen because its small size and low temperature allow simulation using actual devic...Accurate prediction of the sawtooth cycle[1]is an important test for nonlinear MHD codes.The sawtooth cycle in the CDX-U tokamak[2],chosen because its small size and low temperature allow simulation using actual device parameters,has been an important benchmark for the comparison of theM3D[3]and NIMROD[5]codes for the last several years.Successive comparisons have led to improvements and refinements in both codes.The most recent comparisons show impressive agreement between the two codes both on the linear instability and on the details of nonlinear cyclical behavior.These tests are somewhat idealized and do not yet agree quantitatively with the experimentally observed sawtooth period.We expect a second generation of CDX-U sawtooth benchmarks based on an analytically specified equilibrium,with source terms that show greater fidelity to the physical device,to produce better agreement.展开更多
A two-dimensional compressible MHD code has been used to numerically study the asymmetric driven reconnection processes in the vicinity of the magnetopause. The initial magnetic field configuration is assumed to be in...A two-dimensional compressible MHD code has been used to numerically study the asymmetric driven reconnection processes in the vicinity of the magnetopause. The initial magnetic field configuration is assumed to be in a mechanical equilibrium state. The cases with identical temperatures (T m0/T s0=1.0) and four different ratios of magnetic field strength (Q=B m0/B s0=1.0, 1.5, 2.0, 2.5), and the case withT m0/T s0=2.0 andQ=1.5 are investigated (B m0,T m0 andB s0,T s0 are the initial magnetic strength and temperature outside the current sheet on the magnetosphere and the magnetosheath, respectively). When the magnetic field on the magnetosheath side is set as southward, a recurrent formation of multiple magnetic bubbles with various scales occurs under the action of the inward plasma flow imposed at the left and right boundaries. In the simulation, some bubbles coalesce into a bigger one and then it is convected out of the simulation domain; the others are convected through the top boundary all alone. Thus, the plasmoid events with different scales and different time intervals take place intermittently and the impulsive features of magnetic reconnection are clearly shown. The multiple magnetic islands are all high-temperature and large-density regions in comparison with the ambient environment. The bipolar signatures or fluctuant variations of normal magnetic field component are generated by the formation of multiple magnetic islands. This result is similar to the FTEs signature.展开更多
基金Cal-culations were performed at the National Energy Research Scientific Computing Center(NERSC)which is supported by the Office of Science of the U.S.Department of Energy under Contract No.DE-AC02-05CH11231,and at the National Center for Computational Sciences(NCCS)This work was performed under Grants No.DE-AC02-76CH03073 and DE-FC02-02ER54668 with the U.S.Department of Energy.
文摘Accurate prediction of the sawtooth cycle[1]is an important test for nonlinear MHD codes.The sawtooth cycle in the CDX-U tokamak[2],chosen because its small size and low temperature allow simulation using actual device parameters,has been an important benchmark for the comparison of theM3D[3]and NIMROD[5]codes for the last several years.Successive comparisons have led to improvements and refinements in both codes.The most recent comparisons show impressive agreement between the two codes both on the linear instability and on the details of nonlinear cyclical behavior.These tests are somewhat idealized and do not yet agree quantitatively with the experimentally observed sawtooth period.We expect a second generation of CDX-U sawtooth benchmarks based on an analytically specified equilibrium,with source terms that show greater fidelity to the physical device,to produce better agreement.
文摘A two-dimensional compressible MHD code has been used to numerically study the asymmetric driven reconnection processes in the vicinity of the magnetopause. The initial magnetic field configuration is assumed to be in a mechanical equilibrium state. The cases with identical temperatures (T m0/T s0=1.0) and four different ratios of magnetic field strength (Q=B m0/B s0=1.0, 1.5, 2.0, 2.5), and the case withT m0/T s0=2.0 andQ=1.5 are investigated (B m0,T m0 andB s0,T s0 are the initial magnetic strength and temperature outside the current sheet on the magnetosphere and the magnetosheath, respectively). When the magnetic field on the magnetosheath side is set as southward, a recurrent formation of multiple magnetic bubbles with various scales occurs under the action of the inward plasma flow imposed at the left and right boundaries. In the simulation, some bubbles coalesce into a bigger one and then it is convected out of the simulation domain; the others are convected through the top boundary all alone. Thus, the plasmoid events with different scales and different time intervals take place intermittently and the impulsive features of magnetic reconnection are clearly shown. The multiple magnetic islands are all high-temperature and large-density regions in comparison with the ambient environment. The bipolar signatures or fluctuant variations of normal magnetic field component are generated by the formation of multiple magnetic islands. This result is similar to the FTEs signature.
