Ion and electron heating characteristics of magnetic reconnection in a two flux loop merging experiment

Characteristics of the high-power reconnection heating were measured for the first time directly by two-dimensional measurements of ion and electron temperatures. While electrons are heated mainly inside the current sheet by the Ohmic heating power, ions are heated mainly by fast shock or viscosity damping of the reconnection outflow in the two downstream areas. The magnetic reconnection converts the energy of reconnecting magnetic field B(p) mostly to the ion thermal energy, indicating that the reconnection heating energy is proportional to B(p)(2).     

Laboratory observations of spontaneous magnetic reconnection

Detailed measurements of spontaneous magnetic reconnection are presented. The experimental data, which were obtained in the new closed Versatile Toroidal Facility magnetic configuration, document the profile evolution of the plasma density, magnetic flux function, reconnection rate, and the current density during a spontaneous reconnection event in the presence of a strong guide magnetic field. The reconnection process is at first slow, which allows magnetic stress to build in the system while the current channel becomes increasingly narrow and intense. The onset of a fast reconnection event occurs as the width of the current channel approaches the ion-sound-Larmor radius rho s. During the reconnection event magnetically stored energy is channeled into energetic ion outflows and a rapid increase in the electron temperature.     

Generation of suprathermal electrons during magnetic reconnection at the sawtooth crash and disruption instability in the T-10 tokamak

Evidence for excitation of suprathermal electrons ( E(gamma) approximately 20-100 keV) during magnetic reconnection in the T-10 tokamak is presented through analysis of the x-ray measurements with enhanced spatial and time resolution. A toroidally viewing x-ray imaging system and a fast hard x-ray detector placed inside the tokamak vessel allow identification of bursts of the nonthermal x-ray radiation around X points of the m = 1 and m = 2 magnetic islands during the sawtooth crash and prior to the energy quench at the density limit disruption.     

Laser-induced fluorescence measurement of the ion-energy-distribution function in a collisionless reconnection experiment

Observations in space and laboratory plasmas suggest magnetic reconnection as a mechanism for ion heating and formation of non-Maxwellian ion velocity distribution functions (IVDF). Laser-induced fluorescence measurements of the IVDF parallel to the X line of a periodically driven reconnection experiment are presented. A time-resolved analysis yields the evolution of the IVDF within a reconnection cycle. It is shown that reconnection causes a strong increase of the ion temperature, where the strongest increase is found at the maximum reconnection rate. Monte Carlo simulations demonstrate that ion heating is a consequence of the in-plane electric field that forms around the X line in response to reconnection.     

Catastrophe model for fast magnetic reconnection onset

A catastrophe model for the onset of fast magnetic reconnection is presented that suggests why plasma systems with magnetic free energy remain apparently stable for long times and then suddenly release their energy. For a given set of plasma parameters there are generally two stable reconnection solutions: a slow (Sweet-Parker) solution and a fast (Alfvénic) Hall reconnection solution. Below a critical resistivity the slow solution disappears and fast reconnection dominates. Scaling arguments predicting the two solutions and the critical resistivity are confirmed with two-fluid simulations.     

Energetic electrons associated with magnetic reconnection in the magnetic cloud boundary layer

Here is reported in situ observation of energetic electrons (～100-500 keV) associated with magnetic reconnection in the solar wind by the ACE and Wind spacecraft. The properties of this magnetic cloud driving reconnection and the associated energetic electron acceleration problem are discussed. Further analyses indicate that the electric field acceleration and Fermi-type mechanism are two fundamental elements in the electron acceleration processes and the trapping effect of the specific magnetic field configuration maintains the acceleration status that increases the totally gained energy.     

短脉冲强激光驱动磁重联过程的靶后电势分布特征

超短超强激光因其极端的物理参数范围以及可用于研究相对论等离子体等特征,成为当前激光驱动磁重联物理的研究热点.通常采用两路激光与平面靶相互作用实现激光驱动磁重联,然而在实验诊断中,由于激光等离子体自身的复杂性导致很难辨别磁重联的物理特征.本文对两路短脉冲激光驱动平面靶磁重联进行了数值模拟,重点分析了靶后电势分布特征和磁重联之间的关系.模拟结果显示,靶后电势分布可以直接影响被加速离子在探测面上的空间分布,因此可用来直接诊断短脉冲激光驱动磁重联实验.     

Z箍缩等离子体磁重联现象

分析了磁重联对晕等离子体加速和能量平衡过程的影响。分析表明晕等离子体向轴心的加速过程分为两个阶段:第一阶段晕等离子体在磁压或热压(依赖于丝数)作用下向轴心运动;第二阶段晕等离子体由于磁重联过程被加速到阿尔芬速度,并到达轴心形成先驱等离子体。估算表明重联层的厚度与离子的惯性长度具有相同的数量级,电流片内电子运动和离子的运动是解耦合的。在内爆滞止阶段电荷分离产生强的径向电场,这个电场将磁能转化为等离子体轴向(z方向)动能,内爆动能和轴向动能共同转化为X射线辐射能,以此解释了X射线能量大于内爆动能这一观测结果。分析了磁重联电磁脉冲的性质,对于1 MA驱动条件,电磁脉冲的总能量可达kJ量级。     

超高速激光纹影技术测量脉冲功率驱动的磁重联现象

用超高速激光纹影技术测量了Z箍缩等离子体磁重联现象。实验采用超高速光电分幅相机,配合激光纹影技术,测量了XP-1装置上两根金属丝产生的等离子体分布,论证了超高速激光纹影技术研究Z箍缩磁重联现象的可行性。双钨丝实验结果表明,电流加载约10ns后金属丝已有明显膨胀,线性拟合得到平均膨胀速度约8km/s,金属丝内外两侧出现了规则的极有可能是垂直磁场的电热不稳定性扰动,并沿角向高度关联。铝丝负载的实验结果表明,早期的不稳定性波长为0.4mm,电流峰值之后金属丝初始位置仍有大量等离子体,后期的不稳定性波长约1.5mm。这些现象揭示了不稳定性发展的一个主要特征:短波模式受抑制,长波模式将占主导。     

Three-dimensional evolution of a relativistic current sheet: triggering of magnetic reconnection by the guide field

The linear and nonlinear evolution of a relativistic current sheet of pair (e(+/-)) plasmas is investigated by three-dimensional particle-in-cell simulations. In a Harris configuration, it is obtained that the magnetic energy is fast dissipated by the relativistic drift kink instability (RDKI). However, when a current-aligned magnetic field (the so-called "guide field") is introduced, the RDKI is stabilized by the magnetic tension force and it separates into two obliquely propagating modes, which we call the relativistic drift-kink-tearing instability. These two waves deform the current sheet so that they trigger relativistic magnetic reconnection at a crossover thinning point. Since relativistic reconnection produces a lot of nonthermal particles, the guide field is of critical importance to study the energetics of a relativistic current sheet.     

Cause of sudden magnetic reconnection in a laboratory plasma

The cause for sudden reconnection in reversed field pinch plasmas is determined experimentally for two cases: large reconnection events (the sawtooth crash) and small reconnection events during improved confinement. We measure the term in the MHD equations which represents the driving (or damping) of edge tearing modes due to the axisymmetric magnetic field. The term is negative for large reconnection events (the modes are stable, implying that reconnection may be driven by nonlinear coupling to other modes) and positive for small reconnection events (modes are unstable, reconnection is spontaneous).     

Effect of the magnetization parameter on electron acceleration during relativistic magnetic reconnection in ultra-intense laser-produced plasma

Relativistic magnetic reconnection (MR) driven by two ultra-intense lasers with different spot separation distances is simulated by a three-dimensional (3D) kinetic relativistic particle-in-cell (PIC) code. We find that changing the separation distance between two laser spots can lead to different magnetization parameters of the laser plasma environment. As the separation distance becomes larger, the magnetization parameter σ becomes smaller. The electrons are accelerated in these MR processes and their energy spectra can be fitted with double power-law spectra whose index will increase with increasing separation distance. Moreover, the collisionless shocks' contribution to energetic electrons is close to the magnetic reconnection contribution with σ decreasing, which results in a steeper electron energy spectrum. Basing on the 3D outflow momentum configuration, the energetic electron spectra are recounted and their spectrum index is close to 1 in these three cases because the magnetization parameter σ is very high in the 3D outflow area.     

Fast resistive reconnection regime in the nonlinear evolution of double tearing modes

Phases of nonlinear double tearing modes are studied numerically. The first two phases lead to the formation and growth of magnetic islands and are followed by a fast reconnection phase to complete the process, driven by a process of neighboring magnetic separatrices merging and magnetic islands coupling. The fast growth can be understood as a result of the island interaction equivalent to a steadily inward flux boundary driven. Resistivity dependences for various phases are studied and shown by scaling analysis for the first time. It is found that after an early Sweet-Parker phase with a eta(1/2)-scale, a slow nonlinear phase in a Rutherford regime with a eta(1)-scale is followed by the fast reconnection phase with a eta(1/5)-scale.     

Driven reconnection about a magnetic X-line with strong guide component

A two-dimensional, two-fluid model is used to investigate driven magnetic reconnection in collisionless or semicollisional plasmas. The reconnection is driven by externally induced plasma flows in a background magnetic configuration that has a hyperbolic null component in the reconnection plane and a strong component, the so-called guide component, perpendicular to that plane. A dynamic solution is obtained in which the reconnection proceeds in two phases: an initial one whose characteristic rate is a fraction of the Alfvén frequency, and a later one whose rate is determined by the electron collision frequency.     

Magnetohydrodynamic theories of magnetic field reconnection

This article is concerned with a review of the prominent magnetohydrodynamic theories proposed to date to explain magnetic field reconnection. These theories fall into three categories: (i) resistive tearing-mode instability, (ii) steady externally driven processes, (iii) nonsteady externally driven processes. The purpose of this article is to give on the analytical side - (i) a detailed discussion including a critical appraisal of the existing pr ominent theories of magnetic reconnection, (ii) a further elaboration and more correct versions and extensions of some of the existing theories of magnetic reconnection, and a review of the laboratory and computational work on the problem. The controversies that surround the application of these theories to problems involving explosive releases of magnetic energy are discussed.     

不可压缩等离子体的2维磁场重联模型

提出了一种2维磁场重联模型。磁场重联过程中的电荷分离在等离子体中产生静电场,等离子体在电场中的漂移运动可以解释阿尔芬速度量级的出流。该磁场重联模型给出如下结论:Sweet-Parker模型描述的重联率强烈地依赖于电子质量与离子质量之比;反常电阻率正比于离子惯性长度和电流片宽度比值的平方; 相对论效应和高温等离子体中电子-正电子对的产生可以提高重联率; 电磁波的激发对于磁能的损耗是必要的。     

Self-feeding turbulent magnetic reconnection on macroscopic scales

Within a MHD approach we find magnetic reconnection to progress in two entirely different ways. The first is well known: the laminar Sweet-Parker process. But a second, completely different and chaotic reconnection process is possible. This regime has properties of immediate practical relevance: (i) it is much faster, developing on scales of the order of the Alfvén time, and (ii) the areas of reconnection become distributed chaotically over a macroscopic region. The onset of the faster process is the formation of closed-circulation patterns where the jets going out of the reconnection regions turn around and force their way back in, carrying along copious amounts of magnetic flux.     

The solar wind interaction with the Earth's magnetosphere: atutorial

The size of the terrestrial magnetosphere is determined by the balance between the solar wind dynamic pressure and the pressure exerted by the magnetosphere, principally that of its magnetic field. The shape of the magnetosphere is additionally influenced by the drag of the solar wind, or tangential stress, on the magnetosphere. This drag is predominantly caused by the mechanism known as reconnection in which the magnetic field of the solar wind links with the magnetic field of the magnetosphere. The factors that control the rate of reconnection of the two fields are not understood completely, but a southward direction of the interplanetary field is critical to enabling reconnection with the dayside low-latitude magnetosphere, resulting in magnetic flux transfer to the magnetotail. Numerical simulations suggest that the conductivity of the ionosphere controls the rate of reconnection, but this has not been verified observationally. Although solar wind properties ultimately control the interaction, the properties of the plasma that make direct contact with the magnetosphere are different than those of the solar wind, having been altered by a standing bow shock wave. This standing shock is necessitated by the fact that the flow velocity of the solar wind far exceeds the velocity of the compressional wave that diverts the solar wind around the Earth. The upper atmosphere is the final recipient of all the energy and momentum that enters the magnetosphere. Coupling takes place along the magnetic field Lines principally in the polar and auroral region via current systems that close across the magnetic field both at low and high altitudes and flow parallel to the magnetic field between high and low altitudes     

Magnetic field annihilation and reconnection driven by femtosecond lasers in inhomogeneous plasma

The process of fast magnetic reconnection driven by intense ultra-short laser pulses in underdense plasma is investigated by particle-in-cell simulations. In the wakefield of such laser pulses, quasi-static magnetic fields at a few mega-Gauss are generated due to nonvanishing cross product ▽(n/) × p. Excited in an inhomogeneous plasma of decreasing density, the quasi-static magnetic field structure is shown to drift quickly both in lateral and longitudinal directions. When two parallel-propagating laser pulses with close focal spot separation are used, such field drifts can develop into magnetic reconnection(annihilation) in their overlapping region, resulting in the conversion of magnetic energy to kinetic energy of particles. The reconnection rate is found to be much higher than the value obtained in the Hall magnetic reconnection model. Our work proposes a potential way to study magnetic reconnection-related physics with short-pulse lasers of terawatt peak power only.     

激光驱动磁重联过程中的喷流演化和电子能谱测量

利用神光Ⅱ激光器和日本大阪大学Gekko激光器构建了激光驱动等离子体磁重联过程. 在垂直于磁重联平面方向发现了高速喷流, 从不同观测方向实验证实了该喷流的存在并测量了喷流的流体力学演化过程, 对其中的电子能谱进行了诊断分析.