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SPRG Seminars - Archive
November 29, 2005:
Fast magnetic field penetration into almost collisionless plasmas
Dr. Katia Gomberoff, Lawrence Berkeley Laboratory
Fast magnetic field penetration into almost collisionless plasmas can occur due to Hall electric fields. Electron-MHD (EMHD) is used to describe these processes. We describe two main mechanisms for fast penetration. In the first, magnetic field can penetrate as a whistler wave if the plasma is initially magnetized with a longitudinal field. In the second, the field penetrates as a shock wave. The latter is shown in “almost 1-D” models, for cases with either nonuniform density or with magnetic field curvature. In both penetration mechanisms (whistler and shock) the penetration velocity is defined by the large “Hall resistivity” while the shock front depends on the collisional resistivity. We consider the 2-D case of cylindrical plasma with a nonuniform density. We show that the magnetic field penetrates as a shock wave along constant nr 2 lines, while the 1-D model is a particular case. Since the solution is along characteristics it is possible to solve analytically the 2-D case with time varying magnetic field at the boundary. This is used to explain the fast magnetic field penetration observed in a plasma opening switch (POS) experiments done at the Weizmann Institute. We also consider magnetic field penetration into magnetized plasma with nonuniform density. We show that above some threshold for the longitudinal preexisting field, a new family of shock solutions exists. The shock width is then unaffected by the collisional resitivity and has a whistler precursor.