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【云台学术报告】1月17日(周二)上午10:00-11:00 Y12会议室
2017-01-09  |  作者:  |  【  】 【打印】 【关闭

【学术报告】
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报告题目:Plasmoid instabilities in double current layers

报告人:Dr. M.J.Nemati

报告人单位:大连理工大学

报告时间:2017年1月17日(周二)上午 10:00-11:00

报告地点:Y12会议室

报告语言:英文

报告摘要: 

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  Magnetic reconnection, a topological rearrangement of magnetic field lines, is a fundamental process in plasmas. The typical time scales of reconnection in solar dynamic events demonstrate the existence of fast reconnection. In recent years, there have been significant new developments in reconnection theory that provide alternative and more convincing mechanisms for fast reconnection. One of them is the plasmoid instability. This super-Alfvénic instability occurs in an extended current sheet, when the Lundquist number \[S\] exceeds a critical value. The large-aspect-ratio current sheet is fragmented by born, growing, coalescing and ejection of plasmoids that this phenomenon has been proposed as a possible mechanism for fast reconnection scenario. Recently, a wide variety of plasmoids observational have been presented, ranging from space and astrophysical phenomenon to magnetically confined laboratory plasmas, where there are many evidences of observational plasmoid-like features supported by direct numerical simulations.

   However, most previous numerical studies in plasmoid dominated reconnection, have been limited in single current sheet systems where the reconnection rate usually decreases with increasing \[S\] and is independent of \[S\]. In fact, in most real applications of reconnection, in space plasma and tokamaks multiple current systems are mainly generated. Interestingly, the interplay of adjacent current layers makes new physical regimes with different temporal and spatial scales in the whole system, which has different features from those characterized by single current sheet systems.

  The surprising scalings of the double tearing mode (DTM) has been presented, within the framework of a reduced magnetohydrodynamic model in the linear regime for a high Lundquist number (\[S \ge {10^5}\]), leading to onset of fast reconnection. Analytical analysis shows that if the separation of double current sheets is sufficiently small [\[\kappa {x_s}\],\[{\kappa ^{2/9}}S_L^{1/3}\] ], the growth rate of DPMs scales as  \[{\kappa ^{2/3}}S_L^0\] in the non-constant-\[\psi \] regime, where \[\kappa = kL_{CS}/2\] is the wave vector measured by the half length of the system \[L_{CS}^{}/2\] , \[2{x_s}\]  is the separation between two resonant surfaces, and \[{S_L} = {L_{cs}}{V_A}/2\eta \] is Lundquist number with \[{V_A}\] and \[\eta \] being Alfven velocity and resistivity, respectively. If the separation is very large [[\[\kappa {x_s}\],\[{\kappa ^{2/9}}S_L^{1/3}\] ]], the growth rate scales as \[{\kappa ^{2/5}}S_L^{2/5}\] in the constant-\[\psi \] regime. Furthermore, it is also analytically found that the maximum wave number scales as \[x_s^{ - 9/7}S_L^{3/7}\] at the transition position between these two regimes, and the corresponding maximum growth rate scales as \[x_s^{ - 6/7}S_L^{2/7}\] there. The analytically predicted scalings are verified in some limits through direct numerical calculations.

  Furthermore, in a fully nonlinear regime a new physical process for the formation of plasmoids in a double current system has been reported based on a set of 2D resistive magnetohydrodynamics (MHD) simulations. This new physical process is essentially different from the result obtained directly from a single large-aspect-ratio SP current sheet. The nonlinear simulations show that due to the interaction of adjacent current layers, the DTM reconnection current layer can self-consistently change into the elongated current sheet, which is unstable to the plasmoid instability. Interestingly, it is observed that as the flux drive on the current sheet becomes very strong, the resultant growth rate of the dominant plasmoid increases with increasing Lundquist number , thus leading to a very fast reconnection.

  It is shown that as the system size is increased, the secondary current sheets become so long as to produce more plasmoids. It is demonstrated that dependence of the number of plasmoids on resistivity \[\eta \] is changed from no clear scaling \[ \sim \eta _{}^{ - 1}\]for small system size to the scaling for large system size. Moreover, increasing the current length of the system weakens the negative dependence of early growth rate of the monster plasmoid on \[\eta \]. This is qualitatively different from the reconnection rate for a single current sheet, where it usually has a positive dependence on \[\eta \] or is independent of \[\eta \].

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1月9日

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