以氢质量比下的半碰撞动力学模拟重新审视MRX电子电流片宽度

中文摘要 AI 辅助

十八年多来，磁重联实验（Magnetic Reconnection Experiment, MRX）中测得的电子电流片宽度比类MRX动力学模拟的预测值宽2到5倍，且测得的电子力平衡无法仅通过经典项闭合。因此，导致冻结条件破缺的主导非理想项一直未得到解释。本研究在代表MRX的圆柱形几何结构中，采用真实氢质量比$m_i/m_e = 1836$和MRX相关碰撞度，开展了含二元库仑碰撞的二维动力学模拟。这些模拟首次重现了测得的电子电流片半宽度。模拟得到的数值$δ_{BT} = 0.744 \u00b1 0.054$ cm（即$6.26 \u00b1 0.45$个电子趋肤深度$d_e$）处于5.5–7.5 $d_e$的实验范围内。电子力平衡仅通过经典通道即可闭合：压强张量散度支撑76%的非理想电场，碰撞摩擦支撑剩余部分。仅当以实验的3 cm流出分辨率对模拟层采样时，历史上的力平衡赤字才会重现，这表明该赤字反映的是探针分辨率问题而非反常耗散。除此次重现外，本研究还建立了一个层宽度解析模型，该模型依据电子轨道受碰撞影响的相干性对蜿蜒运动的电子进行排序。在该模型中，Dreicer比$E_D/|E_y|$筛选出载流运动得以保留的电子，由此得到的宽度预测值在大范围碰撞度扫描中涵盖了实测值。但以局地电子回旋半径$ρ_e$归一化的宽度仍存在差异：其实测值比模型和模拟结果都高3到6倍。

英文摘要

For over eighteen years, the electron current sheet measured in the Magnetic Reconnection Experiment (MRX) has stood a factor of 2--5 wider than predicted by MRX-like kinetic simulations, and the measured electron force balance has not closed with the classical terms alone. As a consequence, the dominant nonideal terms responsible for breaking the frozen-in condition have remained unexplained. Here, two-dimensional kinetic simulations with binary Coulomb collisions are performed in a cylindrical geometry representative of MRX, at the realistic hydrogen mass ratio $m_i/m_e = 1836$ and at MRX-relevant collisionality. For the first time, these simulations reproduce the measured electron current sheet half-width. The simulated value, $δ_{BT} = 0.744 \pm 0.054$~cm or $6.26 \pm 0.45$ electron skin depths ($d_e$), lies within the experimental range of 5.5--7.5~$d_e$. The electron force balance closes through the classical channels alone: the pressure-tensor divergence supports 76\% of the nonideal electric field and collisional friction the remainder. The historical force-balance deficit reappears only when the simulated layer is sampled at the experimental 3~cm outflow resolution, suggesting that the deficit reflects probe resolution rather than anomalous dissipation. Beyond this reproduction, an analytic model of the layer width is developed that orders the meandering electrons by the coherence of their orbits against collisions. In this model, the Dreicer ratio $E_D/|E_y|$ selects the electrons whose current-carrying motion survives, and the resulting width prediction brackets the measured values across a wide collisionality scan. A discrepancy remains in the width normalized to the local electron gyroradius ($ρ_e$), whose measured value lies a factor of 3--6 above both the model and the simulations.

发表机构

• Princeton University（普林斯顿大学）
• Princeton Plasma Physics Laboratory（普林斯顿等离子体物理实验室）
• Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）
• Korea Institute of Fusion Energy（韩国聚变能源研究所）

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