Electron plasmas: Confinement and mode structure in a small aspect ratio toroidal experiment
Electron plasmas: Confinement and mode structure in a small aspect ratio toroidal experiment
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DOI：
10.1063/1.2345584
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发表时间：
2006-09-01
影响因子：
2.2
通讯作者：
John, P. I.
中科院分区：
文献类型：
--
作者：
Pahari, S.;Ramachandran, H. S.;John, P. I.
环形电子等离子体由于其较差的约束特性而仍然较少被探索。因此，它们的平衡、稳定性和限制性质还没有完全被理解，并且仍然是正在进行的激烈研究的主题。大的纵横比理论表明，在环形器件的约束差，可以克服的径向电场的应用程序，这已被成功地验证了在最近的一些实验。在本文中，我们报告了最长的约束时间没有这些外力。增加环形度有助于我们从本质上产生这些力。为此，一个陷阱，以限制电子等离子体已被创建在一个小的纵横比（约1.6）环。电子从电子源注入后，被看作是保持限制在一个纯粹的环形磁场。约束时间远大于已知的单粒子漂移时间尺度。重要的是，它是在没有任何外部电场，额外的旋转变换，和/或磁场，虽然不是必需的，在原则上，可能出现必不可少的，特别是由于它们的作用，在最近的一些大纵横比陷阱改善限制。成功的限制在小的纵横比的限制，也导致了几个有趣的观察：封闭等离子体的演变是由一个有趣的非线性（大振幅），静电波活动的标志。相干的，周期性的，双峰振荡的结果从一个低频率ExB运动的环形涡在等离子体中，紧密地靠在内壁。多达16个高度相位相干的谐波与占主导地位的权力在m=2表明，该模式不仅仅是一个中心的电荷运动。相反，模式的强耦合导致一种新的非线性状态。主要的能量存在于电子云（m=2）的成形中，而不是在大纵横比陷阱中看到的电荷中心（m=1）的位移中。没有任何幂律尾表明没有任何湍流，至少在时间尺度上长于壁探测分辨率（40 ns）。频率（在200 G时约为100 kHz）显示出不寻常的时间剪切：它随着模式的演变而降低，但随后随着模式的消失而增加。(c)2006年，美国物理学会。
Toroidal electron plasmas have remained less explored due to their poor confinement properties. Their equilibrium, stability, and confinement properties are therefore not entirely understood and continue to remain a topic of intense ongoing research. Large aspect-ratio theory suggests poor confinement in toroidal devices can be overcome by the application of a radial electric field; this has been verified successfully in some of the recent experiments. In the present paper, we report the longest confinement time without these external forces. Increasing the toroidicity has helped us to generate these forces intrinsically. To this end, a trap to confine electron plasmas has been created in a small aspect-ratio (approximate to 1.6) torus. Electrons after being injected from a thermionic source are seen to remain confined with a purely toroidal magnetic field. The confinement time is far more than known single particle drift time scales. Importantly, it is in the absence of any external electric field, additional rotational transform, and/or magnetic fields, which, although not required, in principle, may appear essential particularly due to their role in improving confinement in some of the recent large aspect-ratio traps. The successful confinement in the small aspect-ratio limit has also led to several interesting observations: the evolution of the confined plasma is marked by an interesting nonlinear (large amplitude), electrostatic wave activity. Coherent, periodic, double peak oscillations result from a low-frequency ExB motion of a toroidal vortex in a plasma that closely leans against the inner wall. As many as 16 highly phase-coherent harmonics with dominant power in m=2 suggest that the mode is not merely a center-of-charge motion. Rather, a strong coupling of modes leads to a novel nonlinear state. The predominant energy is present in the shaping of the electron cloud (m=2) and not in the displacement of the center of charge (m=1) seen in large aspect-ratio traps. The absence of any power-law tail suggests absence of any turbulence, at least on time scales longer than the wall-probe resolution (40 ns). The frequency, (around 100 kHz at 200 G) shows an unusual shear in time: it reduces as the mode evolves, but later increases as the mode dies. (c) 2006 American Institute of Physics.