Filaments in the Sheath Evolution of the Dense Plasma Focus as Applied to Intense Auroral Observations

Abstract

This paper investigates the dense plasma focus (DPF) for applications in space-plasma physics. The plasma focus (a variant of Z-pinch) generates plasmas from fast high-voltage electrical discharges on coaxial electrodes suitable for very different studies: from fusion up to plasma-space modeling. In particular, the plasma sheath of the DPF is studied here and measured in some detail as a possible model for auroral observations. Deuterium gas was used in the experiments, and many helpful techniques were used, such as ultrafast photograph and neutron detection from nuclear-fusion reactions. In this paper, we show the different phases of the plasma focus correlated with discharge-current characteristics and neutron and X-ray production. Filamentary formations in the current sheath are shown, and its correlation with neutron production is done. The same number of filaments (about 60) reported in auroral observations are detected in plasma-focus discharges.

Update on the Scientific Status of the Plasma Focus

“…There are many reports [114,[120][121][122][123] on filamentary structures persisting in the radial phase of a plasma focus, as shown in visible radiation short-exposure photographs, besides those reported by the Bostick's group.…”

Effect of length of outer electrode on plasma characteristics in coaxial gun

“…事实上, 在等离子体惯性约束聚变 以及磁约束聚变中的燃料注入等实际应用中, 提高 喷射等离子体的准直性同样至关重要. 例如, 在等 离子体射流驱动的磁化惯性约束聚变 [23] (plasma jet driven magneto-inertial fusion, PJMIF)的 实 现方案中, 最高需要60束等离子体射流同时向中 电子密度, 其相应表达式为 [25] n e = ( 0.5 型 [13,17] ,…”

“…如华盛顿大 学的Golingo等 [9] 将同轴枪作为研究MHD不稳 定性的重要工具, 美国Los Alamos国家实验室利 用同轴枪放电产生的等离子体为尘埃粒子充电加 速 [10] , 美国通用原子能公司将同轴枪作为聚变装置 F z = J r × B θ 的芯部加料手段 [11] , 以及斯坦福大学的Thomas等 [12] 模式也被称为雪犁模式或预填充模式 [13,14] ; 而爆燃 模式的实现则需要快速气阀的配合, 当加速器内被 抽至高真空环境(10 -3 Pa以上)后, 在充气的瞬间 近乎同时开启主回路放电, 此时的工质大多集中在 注气口附近, 还来不及向其他高真空区域扩散, 被 击穿后所产生的等离子体会快速向前传播直至呈 弥散状分布于整个枪内, 就像爆燃时燃烧波的传播 过程一样, 因此该加速模式也被称为爆燃模式 [4,15] . 福大学的Cheng [15] 将同轴枪的外电极改为圆锥形, 使得放电所产生等离子体的速度以及聚集程度都 得到提高; Witherspoon等 [16] 将同轴枪内电极改 为波浪形, 减弱了由磁场梯度引起的等离子体不稳 定性; 美国加州的Hart [17] 通过改变同轴枪内电极 半径发现, 当内电极半径较小时更有利于产生高速 冲击波, 且在高密度气体中效果更为显著; 为了方 便地观察等离子体在枪内的时空演化过程, 布宜诺 斯艾利斯大学的Milanese等 [18] 将同轴枪外电极结 构改为鼠笼型, 发现了当电源提供的能量不足以完 全电离被电流片扫过的中性气体时, 等离子体片将 呈现出细丝状; 美国天主教大学的Lie等 [19] 发现, 当内电极长于外电极时, 可阻止等离子体焦点的形 成. 美国NASA的Michels和Ramins [20] 通过比较 氩气、氮气和氢气三种不同的工作气体, 得出工作 气体为氩气时能量利用效率最高, 氢气的能量利用 效率最低.…”

Preliminary tests and performance estimate of PFMA-1, the first prototype of a plasma focus device for SLR production

“…This interpretation, which is not uncommon [3], [4], [5], might not be in contrast with the possible formation of plasma filaments, instead of a uniform sheath, due to the s.c. filamentation instability and reported to occur at high pressures. The role of filamentation on the plasma focus performances is still much debated; recently it has been reported [10] the existence of an upper pressure limit (UPL), beyond which the neutron yield for D-D nuclear reactions drastically drops, and has been correlated with an insufficient energy available to the system to ionize completely the initial neutral gas. Two-dimensional MHD models have been used for calculating several parameters of the sheath dynamics, among them the energy collected by the sheath per unit mass; when this quantity goes below the threshold of 1.3 MJ/g, just coincident with the deuterium specific ionization energy, the high-pressure limit affects the pinch effectiveness.…”