反场构型平面薄膜电爆炸等离子体电流通道

Current channel in plasma of inverse exploding planar foils

Z箍缩负载初始化电流通道的形成建立过程、电流密度分布模式及其演化特征对研究等离子体动力学发展、分析等离子体不稳定性模式、开展磁流体数值模拟工作等具有重要意义.平面薄膜是研究这一问题中具有连续二维结构的理想构型.基于理想无限长平板假设下的反场薄膜-回流柱负载模型,通过理论对电感主导模式下的电流通道建立和演化过程、薄膜平面磁场分布特征和薄膜各部分受力特征进行了计算分析.通过开展对称型与非对称型下的平面薄膜电爆炸实验,以可见光分幅相机记录薄膜等离子体自辐射光分布为主要诊断手段开展了实验验证.结果显示,早期电流的建立与分布符合电感模型预期,薄膜等离子体表现出电流密度边缘聚集和钳状型等离子体发光形态.中后期图像显示发光强区会由薄膜边界向中心位置转移并导致峰状凸起型等离子体发光形态的演化,表明电流分配受等离子体发展影响,边界融蚀等离子体向心汇聚导致电流随之转移,最终造成电流通道的快速切换.

In the research of Z-pinch,the initial establishment of current in load,current density distribution mode and current evolution characteristics in different stages are of great significance in studying the development of plasma dynamics,analyzing plasma instability mode,and carrying out the MHD numerical simulation.Thin planar foil is an ideal configuration with continuous two-dimensional structure to study the problem.Based on an ideal model that consists of foil-backpost configuration under the assumption of infinite length in reverse exploding condition,the establishment,distribution and evolution of the current channel,magnetic field distribution characteristics and the force state of foil plasma by the inductive mode are calculated and analyzed theoretically.Relevant experiments are carried out on the QG-1 facility with about 1.4 MA peak current and 100 ns rise time to verify the calculated results.The self-emission graphs of exploding foils are recorded by visible-light frame cameras from side-on view and end-on view.Different load configurations are utilized including symmetric and asymmetric condition both assembled with two 20-μm-thick aluminum foils.The results show that the establishment of the initial current in foil(0–70 ns)accord with the expectation of the inductance model.The inductance feature determines the distribution of the current in this stage,which causes the current to aggregate in foil edges and then the edges will suffer much higher magnetic field and J×B force than the center.This results in the formation of a clamp plasma morphology.The later images(70–120 ns)show that the intense emission region will transfer from the foil edges to the center,which proves a transfer of current channel.So the magnetic field and J×B force in the foil center increase and a peak convex plasma morphology forms then.It shows that the current distribution will be affected by the plasma evolution process.The convergence process of ablated plasma with current finally leads to rapidly switch the current channel(in 60–80 ns).