Suppression of turbulence and subcritical fluctuations in differentially rotating gyrokinetic plasmas
Suppression of turbulence and subcritical fluctuations in differentially rotating gyrokinetic plasmas
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差分旋转回旋等离子体中湍流和亚临界波动的抑制
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发表时间：
2011
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通讯作者：
S. Cowley
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作者：
A. Schekochihin;E. Highcock;S. Cowley
已知微分旋转可以抑制聚变等离子体中的线性不稳定性。然而，数值实验表明，即使在没有增长的本征模，亚临界波动增长瞬态可能导致持续的湍流，限制了速度剪切抑制异常运输的能力。本文考虑了在垂直（E × B）速度切变存在下，由平行速度梯度（PVG）和离子温度梯度（ITG）驱动的静电涨落的瞬态增长。最大限度地简化（但最有前途的运输减少）的情况下，零磁剪切处理的局部剪切盒近似的框架。在这种情况下，没有线性增长的本征模，所以所有的激励都是瞬态的。在PVG主导区，最大放大因子e与N q/（安全因子/纵横比）有关，垂直于磁场方向和平行于磁场方向的最大放大波数与kyρi <$（/q）k vthi/S有关，其中ρi是离子的拉莫尔半径，vthi是离子的热速度，S是E × B剪切。在ITG主导的制度，N是独立的波数和N vthi/（LTS），其中LT是离子温度标度长度。还分析了中间ITG-PVG状态，并计算了N作为q/q、LT和S的函数。分析结果证实和补充线性陀螺动力学数值试验。用N . 1对于所有波数都是可能的。7在我们的模型）;离子尺度的湍流预计将完全抑制在这样的制度。当它不被抑制的情况下，一个基本的启发式理论的亚临界PVG湍流导致相关的离子热通量的缩放与q，，S和LT的建议，有人认为，运输是少得多的“僵硬”比在ITG制度。PACS编号：52.30.Gz、52.35.Qz、52.35.Ra、52.55.Fa旋转等离子体中的亚临界波动2
Differential rotation is known to suppress linear instabilities in fusion plasmas. However, numerical experiments show that even in the absence of growing eigenmodes, subcritical fluctuations that grow transiently can lead to sustained turbulence, limiting the ability of the velocity shear to suppress anomalous transport. Here transient growth of electrostatic fluctuations driven by the parallel velocity gradient (PVG) and the ion temperature gradient (ITG) in the presence of a perpendicular (E × B) velocity shear is considered. The maximally simplified (but most promising for transport reduction) case of zero magnetic shear is treated in the framework of a local shearing box approximation. In this case there are no linearly growing eigenmodes, so all excitations are transient. In the PVG-dominated regime, the maximum amplification factor is found to be e with N ∝ q/ǫ (safety factor/aspect ratio), the maximally amplified wavenumbers perpendicular and parallel to the magnetic field are related by kyρi ≈ (ǫ/q)k‖vthi/S, where ρi is the ion Larmor radius, vthi the ion thermal speed and S the E × B shear. In the ITGdominated regime, N is independent of wavenumber and N ∝ vthi/(LTS), where LT is the ion-temperature scale length. Intermediate ITG-PVG regimes are also analysed and N is calculated as a function of q/ǫ, LT and S. Analytical results are corroborated and supplemented by linear gyrokinetic numerical tests. Regimes with N . 1 for all wavenumbers are possible for sufficiently low values of q/ǫ (. 7 in our model); ion-scale turbulence is expected to be fully suppressed in such regimes. For cases when it is not suppressed, an elementary heuristic theory of subcritical PVG turbulence leading to a scaling of the associated ion heat flux with q, ǫ, S and LT is proposed; it is argued that the transport is much less “stiff” than in the ITG regime. PACS numbers: 52.30.Gz, 52.35.Qz, 52.35.Ra, 52.55.Fa Subcritical fluctuations in rotating plasmas 2