磁化实验室等离子体中存在平行和垂直剪切流时波动的非线性动力学

M. Gilmore, L. Yan, S. Xie, C. Watts, A. Lynn
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摘要

本文描述了在新墨西哥大学的线性HELCAT(螺旋阴极)装置中利用一组同心偏置环来影响速度(流)剪切的实验室实验。HELCAT长4米,直径0.5,重量2.2千克,利用两个等离子体源:设备一端是稀土螺旋,另一端是热离子阴极。随着环偏置相对于真空室壁面的增大,发现轴向和方位角流动剪切的变化幅度很小,但从壁面向内移动到等离子体核心。随着偏置的增加,漂移波的幅度减小并最终被完全抑制,那么开尔文-亥姆霍兹(K-H)模式就会失稳。方位角剪切对漂移模态的抑制起主要作用,而方位角剪切是K-H失稳的主要驱动因素。在任何半径的环上施加偏置都能以几乎相同的效果抑制漂移波动,而在等离子体边缘施加偏置更容易激发K-H模式。随着偏置的增加,波动表现出越来越混乱和间歇性的行为,达到V ~ 10 kTe/e,当混沌消失时,相关维数迅速下降,并且表现出非常突然的行为。此外,在阴极等离子体中观察到分离的边缘“斑点”,但在螺旋放电中似乎不存在,即使在其他操作参数(磁场,背景压力)相同的情况下。给出了实验结果,并与理论进行了比较。
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Nonlinear dynamics of fluctuations in the presence of sheared parallel and perpendicular flows in a magnetized laboratory plasma
Laboratory experiments are described which utilize a set of concentric bias rings to affect the velocity (flow) shear in the linear HELCAT (HELicon-CAThode) device at the University of New Mexico. HELCAT is 4 m long, 0.5 in diameter, with B0 les 2.2 kG, and utilizes two plasma sources: an RE helicon at one end of the device, and a thermionic cathode at the other. With increasing ring bias, relative to the vacuum chamber wall, it is found that both axial and azimuthal flow shear change by only a small amount in magnitude, but move inward to the plasma core from the wall. As bias is increased, drift waves decrease in magnitude and are eventually fully suppressed, then the Kelvin-Helmholtz (K-H) mode is destabilized. It appears that the azimuthal flow shear is mainly responsible for suppression of drift modes, while the azimuthal shear is the primary driver of the K-H instability. While bias applied to rings at any radii suppresses drift fluctuations with nearly equal effectiveness, the K-H mode is more easily excited by biasing at the plasma edge. Fluctuations show increasingly chaotic and intermittent behavior as bias increases, up to V ~ 10 kTe/e, when the chaos disappears, as indicated by a rapid drop in correlation dimension, and very bursty behavior. Additionally, detached edge "blobs" are observed in cathode plasmas, but appear to be absent from helicon discharges, even when other operating parameters (magnetic field, background pressure) are identical. Experimental results and comparisons with theory are described.
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